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Author Topic:   Self-discharge of Storage Batteries
jimh posted 12-19-2008 01:54 PM ET (US)   Profile for jimh   Send Email to jimh  
I have read many references to the self-discharge rate of lead-acid storage batteries, and generally there is a mention that a sealed valve-regulated lead-acid (SVRLA) battery using an absorbent glass mat construction (AGM) will have a lower rate of self-discharge than a conventional vented flooded cell lead acid battery. However, in spite of the many mentions of this phenomena--to the point where it seems to have become a well-accepted matter of fact--I don't recall seeing an explanation of how the advantage is obtained.

What aspect of a SVRLA AGM provides the lower self-discharge rate, if indeed it exists?

tmann45 posted 12-19-2008 03:47 PM ET (US)     Profile for tmann45  Send Email to tmann45     
Different composition of the lead plates.
jimh posted 12-19-2008 04:30 PM ET (US)     Profile for jimh  Send Email to jimh     
I am skeptical on three points:

--is there really a difference in the lead composition of an AGM compared to a flooded-cell?

--if there is a difference, what keeps a flooded-cell from using that composition?

--how does the composition influence self-discharge?

glen e posted 12-19-2008 04:44 PM ET (US)     Profile for glen e  Send Email to glen e     
Sorry Jim, but it's true - try page 10 or so...

jimh posted 12-19-2008 06:52 PM ET (US)     Profile for jimh  Send Email to jimh     
Apparently Glen is citing this sentence fragment:

"...deep cycle flooded lead acid batteries contain antimony in the grid alloy which causes a high rate of self discharge..."

That mentions only deep-cycle flooded-cell batteries. All flooded-cell batteries are not deep-cycle batteries, so this does not provide an exclusion of all flooded-cell batteries, for example a cranking battery. If it is the purity of the lead that engenders low self-discharge, then that same property would obtain in a flooded cell as well as in an AGM, as long as the same lead alloy were used.

Sorry, Glen, but it may not be true (that only an SVRLA AGM battery can have low self-discharge)--nothing substantive shown yet.

Here is another battery manufacturer's reference to what engenders the low self-discharge rate:

5. Why the low rate of self-discharge?
Mostly this is due to the purity of the lead in the... plates.


This manufacturer makes no claim that it is the AGM construction that provides low self-discharge--it's the lead that is used. The logical inference is if a flooded cell battery uses the same purity of lead, it will have the same low self-discharge rate.

tmann45 posted 12-19-2008 07:40 PM ET (US)     Profile for tmann45  Send Email to tmann45

The third criterion of battery runtime is state-of-charge (SoC). The battery capacity is always measured on a fully charged battery and the most simplistic method of estimating SoC is reading the open terminal voltage (OTV). This approach is accurate if the battery has rested for at least four hours after charge or after applying a load. The rather long rest period is the required recovery time to pacify a battery when disturbed. The reader should also be aware that different plates composition alter the OTV reading. Calcium raises the voltage by 5-8%, affecting SoC estimation. Calcium is an additive that helps in making the battery maintenance-free.

jimh posted 12-20-2008 12:17 AM ET (US)     Profile for jimh  Send Email to jimh     
I don't see the significance of the above quoted passage (which focuses on the open terminal voltage). The quoted passage does not mention self-discharge considerations.

Nothing mentioned so far makes any link between AGM construction and low self-discharge rate. Apparently AGM has nothing to do with creating a low self-discharge rate. Low self discharge rate seems to come from the lead alloy composition. I think this is an important point to observe. What it means is that buying an AGM battery is not an automatic guarantee of a low self-discharge rate.

glen e posted 12-20-2008 12:25 AM ET (US)     Profile for glen e  Send Email to glen e     
I do not know why you persist in trying to tear down AGM's...the simple fact is an AGM that sits all winter in a boat without a charge will be at a much higher voltage than a standard batt in the spring. That is a irrefutable fact. I don't know why and don't care.
jimh posted 12-20-2008 09:10 AM ET (US)     Profile for jimh  Send Email to jimh     
Glen--You are really a strange duck. You think that any inquiry into a subject on which you already hold an opinion represents blasphemy. I am sorry to tell you that just because you have an opinion about a subject it does not foreclose the rest of us from making inquiries into the subject. You act as if once ol' Glen's made up his mind, further investigation is pure folly. You're welcome to think that, but you'll just have to indulge the rest of us with a bit more intellectual curiosity.

It is entirely reasonable that an AGM battery will have a low self-discharge rate, but it seems that the underlying basis for that is misunderstood. All that's been established so far is that a low-self discharge rate occurs because of the purity of the lead alloy that is used. There is no reasonable basis to proclaim that every AGM battery will have a low self-discharge rate--only the AGM batteries that use very high purity lead will have a low self-discharge rate. There is nothing that guarantees that every battery that calls itself an AGM battery will intrinsically be made of high purity lead. The acronym AGM refers to the construction technique using absorbent glass mat, not to intrinsic use of high purity lead.

tmann45 posted 12-20-2008 09:45 AM ET (US)     Profile for tmann45  Send Email to tmann45     
Sorry the quote was not directly to the point, the reference was to different materials are added to the lead in batteries for different purposes. Calcium to make them gas less for maintenance free batteries, antimony to strengthen the lead for ?rough service?, etc.

Battery manufacturing is a fairly simple science, all of the manufacturers have the basics down pretty well, they know what properties are affected by what additions they make to the lead. Hence they all know that adding calcium helps to make a battery "maintenance free" by reducing gassing, thus reducing water loss.

Therefore one, with an open deductive mind, could infer that all AGM batteries are constructed similarly, thin plates made out of similar composition lead, and thus generally they are all low self discharge batteries compared to other types of batteries.

If you want to know why and what composition lead, I suggest you ask a battery manufacturer or Google.

tmann45 posted 12-20-2008 10:16 AM ET (US)     Profile for tmann45  Send Email to tmann45     
No, this does not answer your question, but a lot of good battery info.

glen e posted 12-20-2008 10:21 AM ET (US)     Profile for glen e  Send Email to glen e     
Opinion Jim, is not fact. I really don't care if you don't believe me, but everything I have ever read says it's true and my own eyes have seen it: EVERYTIME. I have an AGM and a Std interstate battery in my garage and the AGM holds the voltage MUCH better over time than the std battery. It's not me Jim, but it seems that in your world, if anyone can't give you total explanation of something, it can't be true. Why don't you put down the 1947 Cabernet, take of the ascot, turn off the Bach and call a AGM mfr and ask them how it's done? I just know it is....
glen e posted 12-20-2008 10:33 AM ET (US)     Profile for glen e  Send Email to glen e     
And Jim, I'd be happy to email you the Mercury service bulletin that outlines their reasons for going AGM, if you would believe that. Or is that not good enough either as proof of the advantages of the AGM? And BTW, AGM's can take up to 14.7 charging, acc to Mercury.
jimh posted 12-20-2008 11:08 AM ET (US)     Profile for jimh  Send Email to jimh     
ASIDE to tmann45--Thank you for making the suggestion of using GOOGLE to search for information, however, that already occurred to me, and so far my search using GOOGLE has not provided any particularly wonderful yield of information on this topic. Therefore, I would encourage you, or anyone else, who has information or a cite to other on-line sources to feel free to mention them or give a hyperlink to them. A further irony is that shortly this discussion may become a result of a on-line search on this topic, so suggesting that on-line searching is the way to an answer is going to become self-referential. Now back to our discussion of self-discharge of storage batteries.

In regard to the rate of self-discharge of a lead-acid storage battery, it seems to be well known that temperature is a significant influence, and the rate of self-discharge is directly proportional to temperature. It would be interesting to know exactly how temperature influences self-discharge as compared to lead alloy composition. So far only the most general and imprecise characterizations have been ascribed to the influence of lead alloy, that is, it is only said to produce "low" self-discharge rates. I don't recall any actual data or figure of merit being given.

Temperature influence on self-discharge is obviously an influence on the reaction rate of the lead-acid chemistry. One would assume that this behavior would be predictable and would be universal, that is, the presence of a brightly colored orange case instead of a dull black case for the battery would not influence the basic chemistry and chemical reaction and all batteries exhibit lower self-discharge rates with lower temperature.

I have collected some data about the self-discharge rate of a conventional flooded cell lead acid battery and presented it in another article. See for details. My investigation showed that over a period of about four and a half months of storage at near room temperature (the floor of a closet), a flooded cell lead acid battery experienced about a ten percent loss of charge. The experiment involved three batteries of various brands and age. The loss of only ten percent of the charge was not considered to be significant, as the lost charge could be restored in a short time by recharging.

In northern climates when the outdoor temperature declines, a battery stored in lower temperatures should show even less self-discharge, and it would be reasonable to expect that a flooded cell lead-acid battery stored for four months would exhibit less than a ten percent decline in stored charge.

In this regard, concern about the self-discharge rate of a storage battery does not seem to be foremost in the list of important criteria when selecting a battery. If offered the choice of a battery that costs $75 or one that costs three times that much but has a somewhat lower self-discharge rate, I would be unlikely to be persuaded to triple my battery expense to obtain lower self-discharge.

I have observed that some of the most vocal proponents of the virtue of using a battery with a low self-discharge rate happen to live in a region with a tropical climate, and this coincidence makes me curious if there is a relationship there. Perhaps self-discharge is a greater concern in areas where the ambient temperature is much higher than my boating environs, as high temperatures drive the self-discharge rate higher. Self-discharge may be something to worry about at 95-degrees-F, more so than at 20-degrees-F.

glen e posted 12-20-2008 12:50 PM ET (US)     Profile for glen e  Send Email to glen e     
Jim writes:

"The loss of only ten percent of the charge was not considered to be significant, as the lost charge could be restored in a short time by recharging."

If a battery (per the book "12V bible") is fully charged at 12.7V and a 10% drop is 1.27 volts, that leaves the battery at 11.43 which is 85% discharged or DEAD in my terms.

An AGM, AT MAX, drops 3% to 12.3 which is only 30% depleted, and is still in good shape.

How is that not significant?Especially when a depleted lead acid battery is never the same when coming back from a large depletion. Damage has been done long term.

And as far as tropical advantages, quite the opposite is true. The simple fact that AGM's do not require winter charging and don't freeze and crack, means you can save your back (ache)and eliminate hauling and maintaining your batteries in the winter climates up north. Mercury went to AGM's because they know most boaters do not maintain their batteries and then get into problems.
AGM's provide a cushion for battery abuse and neglect.

Is all this worth it? Certainly not to some, that's a personal decision. If I was a lake boater and had my boat on a trailer, I probably would not opt for one. I can charge and add water at home. But when I am offshore or on vacation, I don't want to worry about batteries that were depleted once.

As I said before, to think nothing of upgrading your GPS to the 10 inch model and spending an extra $500, and then whining that AGM's are $150 dollars more is plain stupid.

jimh posted 12-20-2008 06:10 PM ET (US)     Profile for jimh  Send Email to jimh     
Glen--You should actually read my article before you begin to dismiss it. I said that the charge dropped ten percent. You mistakenly reinvented that to be the open terminal voltage dropped ten percent. I did not say that the open terminal voltage dropped ten percent. The open terminal voltage only dropped about 0.1-volts.

I don't remove the batteries from my boat in the winter. Bringing the batteries into a warmer environment would only accelerate their self-discharge rate. I leave the batteries in the boat, in the cold environment.

In his latest posting Glen has invented a new property of the AGM--it does not freeze. I am afraid that I don't have any understanding of how the freezing point of a hydrochloric acid changes because it is in an AGM battery. I don't think it does. As far as I know, a liquid will freeze at a particular temperature. It does not matter if the liquid is in a pool or if it is being held in some absorbent material. Once the temperature falls below its freezing point, a liquid freezes. I will leave it for Glen to explain how hydrochloric acid knows not to freeze because it is inside a battery called "AGM" on the case.

The only basis for an AGM battery surviving colder temperatures than a flooded cell would be if there was a very significant difference in the concentration of the hydrochloric acid, which would of course correspond to the state of charge. It remains to be demonstrated precisely how much difference in state of charge and how cold it would have to get in order for an AGM to avoid being frozen while a flooded cell would freeze.

In terms of comparing costs of batteries with costs of electronic navigation devices, one can generally see a direct correspondence between benefit and cost in the electronics. If I spend three times more on the electronics I can be reasonably certain I will get something in return, say a bigger screen, faster performance, more information. If I spend three times more for a battery, I don't know that I will see a three-fold improvement in the rate of self-discharge. In fact, so far, all that has been alleged is that the rate will be "lower," and no quantitative data has been provided.

Dick E posted 12-20-2008 06:30 PM ET (US)     Profile for Dick E  Send Email to Dick E     
The only way that a battery can freeze is if it is left in a state of partial or complete discharged. As the state of charge in a battery decreases, the electrolyte becomes more like water and the freezing temperature increases. The freezing temperature of the electrolyte in a fully charged battery is -92.0 F. At a 40% state of charge, electrolyte will freeze if the temperature reaches approximately 16.0F.
When you use a hydrometer you are measuring the specific gravity of the electrolyte solution. It will change as the battery becomes more fully charged.
AGM are less likely to self-discharge, thus less chance of their electrolyte will have a lower freezing point.
glen e posted 12-20-2008 06:30 PM ET (US)     Profile for glen e  Send Email to glen e     
Oh boy here we go...a undercharged lead acid battery will freeze and the water will crack it. An AGM will freeze but not get hurt and there is not water. Page 10 of the same lifeline doc I posted above.
glen e posted 12-20-2008 06:45 PM ET (US)     Profile for glen e  Send Email to glen e     
And I would be very interested in why you think Mercury has requested AGM's be used if they are just so much smoke and mirrors. It certainly is not because I like them. There must be something to it. As the other mfrs go to DTS and power steering applications, I would not be surprised if they request them also. They are standard equipment for Yellowfin, SeaVee and Intrepid. 3 very exclusive boat builders, and now because of Mercury, Whaler too! (on verados.)

We've beat this to death and I fear Jim will keep moving the board. Here is the exact wording Mercury published in the service bulletin I spoke of above. You can make you own decision as to what you want to buy.

AGM Advantages:
* Electrolyte is absorbed into the fiberglass mat and will not spill if the battery case is ruptured.
* Maintenance\0x2011free - Water never needs to be added.
* Low self\0x2011discharge rate - Can be left in the boat for winter storage if it is fully charged and a battery cable is disconnected.
* Can be mounted in any orientation - Upside down not recommended.
* Best solution for a dual purpose starting and deep cycle battery.
* Increased vibration resistance and service life - An AGM is typically rated for 200-400 deep\0x2011discharge cycles. A flooded lead\0x2011acid starting battery will only last for 2-4 cycles.
* Can accept a very high charge rate. Battery will not be harmed if it is charged with 70 amp or more current.
* Standard lead\0x2011acid charging equipment can be used as long as the charging voltage does not exceed 14.7 V.
* Sealed to prevent the escape of combustible charging gasses.
* Classified as nonspillable and can be shipped by UPS and FedEx

jimh posted 12-20-2008 10:52 PM ET (US)     Profile for jimh  Send Email to jimh     
Glen--The topic here is not what Mercury recommends for their Verado motor, and while that might be of intense interest to owners of the Verado motor, whatever Mercury has recommended is not binding on all battery topic inquiries, that is, just because Mercury decided to recommend an AGM does not mean that the topic of boat batteries is now closed for discussion.

Also, I don't recognize Mercury has being the final word on chemistry and physics, and while it is interesting to read their literature, Mercury is not really authoritative on the self-discharge rate of a battery. Finally, I have not said here or anywhere else that I disagree with Mercury's recommendation of an AGM for their Verado motor. If Mercury says get an AGM for a Mercury motor, it makes perfect sense to me to follow that recommendation. Although Glen wants to turn this into some sort of Mercury versus jimh argument, it is nothing of the kind. Mercury is completely peripheral to my interest in the self-discharge of a battery and what influences it.

Simply repeating the mantra that an AGM battery has a low discharge rate does not explain what qualities create a low discharge rate in a battery, whether it is an AGM or not.

Glen has again invented two marvelous properties of an AGM battery: he says it is not a lead-acid battery and it doesn't have liquid. These are both wrong. An AGM battery is a lead-acid battery, and the electrolyte in it is in a liquid form. It is hard to imagine how an AGM could be anything but a lead-acid battery since we already learned that the fundamental feature that creates the low self-discharge is the purity of its lead. As for the state of its electrolyte, it obviously is not a solid or a gas, and that leaves only a liquid or a plasma.

As for freezing, the determinant of the freezing point of the electrolyte is its acidity, and this variable is independent of what sort of container holds the electrolyte. To make an analogy, if I take a paper towel and soak it with water, the water still freezes at 32-degrees-F, and when its temperature drops below 32 the water will freeze.

Finally, I don't consider that the topic of battery self-discharge has been "beaten to death," and really quite the opposite is true. The surface has hardly been scratched here. So far we have identified just two influences on battery self-discharge rate: the temperature and the alloy of lead used. Lower temperatures mean lower discharge rates, and higher purity of lead means lower discharge rates.

number9 posted 12-20-2008 10:57 PM ET (US)     Profile for number9  Send Email to number9     
In the short time period AGM marine batteries have been in my life I can attest to their lower discharge rates. Non AGM flooded batteries do discharge at a much faster rate. Sorry don't have the #s, only my observations from somebody who's been accused of being anal more than once. Only down side I see besides initial cost and usually little or no access to check fluid levels and mfg. recommendations not to add electrolyte if low.
jimh posted 12-21-2008 11:44 AM ET (US)     Profile for jimh  Send Email to jimh     
Of all the many virtues cited by Glen or Mercury for an SVRLA AGM battery, perhaps the most significant in contributing to the growth in their use is:

* Classified as nonspillable and can be shipped by UPS and FedEx

The ability to be easily shipped has made it possible for AGM batteries to be sold in ways that flooded cell batteries cannot. AGM batteries can also be shipped already charged. Flooded cell batteries are usually shipped dry, and the electrolyte is added later and the battery charged, usually somewhere much closer to the point of sale.

However, one should not confuse these conveniences in the distribution and sale of AGM batteries as constituting some overwhelming advantage for the end user. As long as we have given in to Glen's relentless efforts to turn this discussion into a recital of Mercury Marine's service bulletin recommending AGM batteries to be used with their Verado motors, I may as well observe that completely absent from that list is any notion of longer life span.

A high-quality flooded cell battery--and here if we want to go on an equal footing with the often recommended AGM batteries we have to allow for a three-fold increase in price, which will get you a rather high-quality flooded cell battery--often will last for ten years or more. Flooded cell batteries from Surrette come with a ten year warranty. The cost of a Surrette battery is higher than the $45 Walmart battery, but as long as we are comparing features, we should also consider costs. Let's compare a $250 AGM to a $250 flooded cell.

HAPPYJIM posted 12-21-2008 01:04 PM ET (US)     Profile for HAPPYJIM  Send Email to HAPPYJIM     
I've never owned an AGM battery but I can see that they have certain advantages. The high cost is probably the biggest reason that I will never own one.

I maintained lead acid batteries while doing Coast Guard time in both shipboard and aircraft use. The Battery locker on the ship was maintained at 72 degrees F. so freezing was not a factor.

On aircraft that traveled North to colder climates, The battery was removed and stored in the motel room until we went flying. If memory serves me the cutoff temp was 0 degrees F. Batteries will freeze and they don't need to be discharged to do so. We eventually changed to Nickel Cadmium (NiCad) battery and the freezing point was not a factor. Thermal run-away was though.

I guess being a cheap skate and changing my batteries every 3 years for offshore fishing safety makes buying an AGM battery not an option. I wouldn't trust any battery past 3 years regardless of cost or warranty.

Dick E posted 12-21-2008 03:49 PM ET (US)     Profile for Dick E  Send Email to Dick E     
They last comment by HAPPY JIM "Batteries will freeze and they don't need to be discharged to do so." is not true.

A fully charged battery will not freeze until the temperature drops to a negative 92 deg F.

In lead acid battery the electrolyte solution CHANGES during discharge. During discharge the sulfuric acid goes thru a chemical reaction creating lead sulfate with the lead, which is in the battery.
During charging this lead sulfate will be change back to sulfuric acid and lead.

glen e posted 12-21-2008 05:50 PM ET (US)     Profile for glen e  Send Email to glen e     
Our fearless leader writes:

"Of all the many virtues cited by Glen or Mercury for an SVRLA AGM battery, perhaps the most significant in contributing to the growth in their use is:
* Classified as nonspillable and can be shipped by UPS and FedEx"


Somebody's got to be on the other end of this massive conspiracy to flood the world with AGM's. saying "I want one"
And all the best boat building companies are part of this sinister plan.I guess...


HAPPYJIM posted 12-21-2008 09:27 PM ET (US)     Profile for HAPPYJIM  Send Email to HAPPYJIM     
We had a spare battery freeze in Thule, Greenland while parked on the ramp for two days. It was in our spare parts "fly away kit" and had been forgotten. I don't know what the temp was but it was a freshly charged battery. It had to be discarded because the sides were bowed out. I saw it with my own eyes. This is not a story that I heard from some one.
John W posted 12-21-2008 10:27 PM ET (US)     Profile for John W  Send Email to John W     

I'll say up front that I have no insights on WHY an AGM battery has a lower discharge rate than "standard" flooded batteries. Hopefully someone will have some information to that effect, other than what is written up in the West Marine catalog on the subject each year.

I have owned numberous marine starting, deep cycle, and/or "dual starting/ deep cycle" flooded batteries. These were not the super expensive Rolls batteries or the Surrette you mention (I'm not familiar with that brand)...but they were decent quality, marine grade (Interstate, etc), and cost around $80 apiece. Compared to these batteries, the four AGM batteries I have owned and used in boats were far superior to any of the flooded batteries I have ever owned, in all of the ways claimed by their manufacurers. They did not discharge in any meaningful way when stored, they recharged more quicly without boiling out their water, they were completely maintenance free, and they lasted longer than any battery I have owned. I sold a boat with the AGM batteries after 8 years, and when I sold the boat they worked as well & held a charge exactly like when they were purchased. My current Whaler has deep cycle "dual purpose" flooded batteries, but when they die I will replace with AGM.

Your comment that the most significant reason for the growth of AGM batteries in the marketplace is FedEx shipping is odd, to say the least. To me, that sounds like you are disregarding all of the advantages claimed of AGM batteries in catalogs like West Marine, by the manufacturers themselves, and by first hand accounts from people who have owned AGM and flooded batteries. Is that what you are saying? That because the reason these $150 batteries might work better is not yet obvious to you, then the manufacturers, the marine resellers ,and the first hand accounts of owners of AGM batteries must be lies, exaggerations, or the misinformed comments of rubes? Or will you grant that based on first hand accounts by users of the performance advantages of these batteries, these advantages probably do exist?

Is it possible that I could have spent as much or more on a Rolls, Surrette, etc., and had a battery that lasted as long as the $150 AGM's? Sure, I suppose. But why would I want to do that? Do you really think that a completely maintenance free, vibration-resistant battery, that will recharge quicker, is not an advantage at all?

I am also somewhat curious as to the "Why's" of all of this, which is what your article is looking for. We may eventually find out that the only reason AGM's have some particular advantage in longevity or self-discharge rate is quality construction & materials, versus an inherent advantage of the AGM design. But obviously, AGM's still have the advantages of no maintenance, vibration resistance, no spill issues in a pitching or heeling boat, etc. The only meaninful disadvantage of AGM's I have heard is initial cost. As such I'll buy AGM's again.

jimh posted 12-22-2008 09:22 AM ET (US)     Profile for jimh  Send Email to jimh     
What I've seen so far is that following the initial reply by tmann, there has been little information added to this discussion about self-discharge rates and how they are affected. Instead, there has been a recitation of claimed advantages for AGM batteries. In that discussion there is a surprising lack of any meaningful description of the discharge rate of an AGM other than it is described as "low," or if compared to some presumed standard battery is it is described to be "lower" or "better." It would be far more interesting if someone had actual data.

Boaters seem to be a skeptical group with regard to most claims about performance. For example, if Mercury Marine produced advertising material that said a Verado motor was "faster" than others, there would be a howl of complaints demanding to see some data or test results. But with regard to these batteries, everyone seems content to accept without question the rather broad descriptors ("low," "lower," and "better") without as much as raising an eye.

As for the "why" of this inquiry, there is nothing more to it than simple curiosity. As was previously noted by other participants, some apparently lack interest in understanding why things behave the way they do. For me, if I am going to triple my cost of owning a battery, I would like to understand what advantage I might get, and how it is obtained.

Bella con23 posted 12-22-2008 11:22 PM ET (US)     Profile for Bella con23  Send Email to Bella con23     
I spent some time this evening studying the differences between the two subject battery types and why the self discharging should differ between the two.

Apparently the AGM type battery boast very low internal resistance boosting its ability to operate at a cooler temperature under load and having the extended shelf life as compared to that of the standard, low maintenance, or maintenance free flooded cell battery.

SVR Absorbed Glass Mat (AGM)
Sealed, Valve-Regulated (SVR) Absorbed Glass Mat (AGM) batteries use special absorbed electrolyte technology. Fine, highly porous micro-fiber glass separators absorb the electrolyte, increasing efficiency by lowering internal resistance and boosting capacity. Lower internal resistance also means that the batteries can be recharged faster than conventional batteries, allowing the user to put them back into operation sooner. In contrast to SVR Gel batteries, the SVR AGM excels in high current, high power applications, as well as auxiliary power for dual-purpose applications.
The completely sealed, valve-regulated AGM battery is vibration resistant, and eliminates gas emissions and acid leakage for longer and safer battery operation. AGM batteries are also completely maintenance-free and must never be opened.

AGM, Absorbed Glass Mat, (Vlies) keeps the acid in place by the separator paper that consists of a fiberglass mat operating like a sponge. The capillary forces in the separator do the trick. The batteries can be constructed with extremely thin separators which keep the internal resistance low. This means that you can leverage high power from a small volume which makes it ideal as a starter battery. AGM has one drawback which is that the amount of acid is limited. All acid must be absorbed by the separator paper and when the small amount of acid has been converted into lead sulphate, this signals that the "petrol tank" is empty. To deal with this problem, AGM batteries often have a somewhat higher acid weight. This means that an AGM battery can, and often should, be charged using a voltage that is a little higher.

The following links will bring you to the sites that I used that explain the reasons consistently. antimony_calcium_selenium_tin_alloying.htm


jimh posted 12-23-2008 01:36 AM ET (US)     Profile for jimh  Send Email to jimh     
Joe--I appreciate your comments about the low internal resistance that can be obtained in an lead-acid battery, but low internal resistance is not the same parameter as self discharge rate. If there is a relationship between low internal resistance and low self discharge rate, I would appreciate it if you could explicitly state the relationship and give us some understanding of it.

I also notice that none of the material you presented cited anything about the discharge rate, and I don't know if that was intentional. Was low discharge rate a quality that was missing in the various resources you cited?

AGM batteries have a few differences from flooded cell batteries, and people seem intent on introducing as many of those as possible. As long as we're mentioning just about everything except the self-discharge rate, I'll throw in the cool colors on the cases. Most of the AGM batteries I see have a cool color case.

I am also beginning a test of the discharge rate of an AGM battery. I have a premium AGM battery which is a few years old, and I just made a careful measurement of its open terminal voltage. I will put that battery in the closet and check it again in about four months. This will give comparative data to the test I conducted some time ago with flooded-cell batteries. Although it will just be one data point, it will at least be some data. That has to be better than just saying an AGM has a "low" self-discharge rate.

Chuck Tribolet posted 12-23-2008 09:14 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
FWIW, NiCd and NiMh batteries have a low internal resistance,
and (at least until the latest generation of NiMh), relatively
high self-discharge rates.


Buckda posted 12-23-2008 09:53 AM ET (US)     Profile for Buckda  Send Email to Buckda     
My intuition tells me the following:

The purity of the lead and amount of lead surface area are probably the most important factors in both AGM and "standard" flooded cell batteries.

The quality of lead in AGM's may generally be higher (as a premium product with a premium price, this just makes sense).

"standard" batteries run the cost/quality gamut - from the inexpensive Wal Mart batteries to the top of the line batteries that JimH mentions above.

It is reasonable to hypothesize that the lead quality and # of plates in a Wal Mart battery will be somewhat less and the price reflects that, as compared to the Surrette.

Is it possible that AGM's, due to their construction, can utilize less lead but take advantage of the manufacturing process to provide greater surface area exposed to the electrolyte and still be vibration resistant? That may address some of the questions at hand.

And note to Glen - An intellectual inquiry into WHY there is a difference starting with the question of IF there is a difference isn't bashing. It is the foundation for sound reason and understanding. You start by questioning all assumptions, and build your rationale from there. It was really frustrating to read this thread because not all participants were on the same page regarding the process of investigating the topic.

I'm sorry JimH called you a strange duck.

Merry Christmas.

jimh posted 12-23-2008 10:46 AM ET (US)     Profile for jimh  Send Email to jimh     
Lead is a material that is recycled to a great extent. Old batteries are typically returned by their owners for a core credit, and the lead they contain is presumably recycled. The recycling of the lead may have an influence on the purity of the lead available and its cost. Here I am assuming that recycled lead may tend to have more impurities, but recycled lead is available at a lower cost than newly refined lead. If this is true, it seems that there would be a higher cost associated with pure lead, and particularly so with a lot of recycled lead available at a lower cost.

The source mentioned by Joe at antimony_calcium_selenium_tin_alloying.htm

is a gold mine (a lead mine?) of information about the influence of impurities on the self discharge rate of a battery. It provides this important observation:

"A pure lead grid structure is not strong enough by itself to stand vertically while supporting the active material."

From that I infer a conventional lead-acid flooded cell battery may not be able to be made with pure lead plates, or at least perhaps not in the usual form factor we are accustomed to seeing, that is, a typical Group-24 battery.

A graphic (in the form of a cartesian coordinate plot but totally without any units or dimensions) shows that a lead alloy with calcium has a low self discharge rate as well as higher strength. No plot is shown for pure lead, however, so we are still left with no information about how much lower the self discharge rate may be when pure lead is used.

At that same website, I found another interesting technical paper:


This monograph notes:

"While there are customers using VRLA batteries, based on space restrictions, ventilation requirements etc, many end users have returned to VLA (flooded) after being disappointed with the performance or operational life of their VRLA solution."

The focus of this paper is on stationary batteries used in standby power applications, so its conclusions are not completely transferable to a marine application.

Let me pose a new question for discussion: To what extent is a low self discharge rate significant for a battery used on a small boat?

Before answering that question, let me observe that a low self-discharge rate may be significant to other users of batteries, and a couple of examples come to mind immediately. A battery retailer will appreciate a low self discharge rate because he will not have to maintain his inventory of batteries on a float charger. The retailer can have batteries on the shelf that will sit for a long time (perhaps a year or more) at nearly their full capacity, and when sold to a customer the battery will be ready for use. Another situation in which low self discharge may be significant is that of a battery user who has a large number of cells in a standby power source (UPS). That user may appreciate a low self discharge rate because it reduces the charging current needed to float the bank of cells.

In the case of a small boat, the most significant instance in which a low self discharge rate would be appreciated seems to be for long term storage of the boat in the off season. In northern climates it is typical that a boat will be laid up in storage for at least four months each year. A low self discharge rate would be beneficial so that a battery can survive the long storage period without being maintained on a float charge.

When a boat is in seasonal use, the interval between charging of the battery is not likely to be more than 30 days. Over a period of 30 days I would not expect that the self discharge of a battery would be significant factor, even when the battery is not of a type characterized here as "low" self discharge rate.

Bella con23 posted 12-23-2008 08:12 PM ET (US)     Profile for Bella con23  Send Email to Bella con23     
Jim you are correct in that the low internal resistance of the AGM battery does not have any effect on self discharge rate. I guess I over read on the subject and got caught up in the moment.

I attached the battery references to help clear up any other thoughts and ideas on the subject such as at what point does the battery electrolyte freeze.

This was the best explanation my extensive research has come up with. I cut this from and article on a car audio forum who in turn cut and pasted it from who knows where. Note that it coincides with Buckda's (and others) take on the subject.

Self Discharge

The self-discharge rate is a measure of how much batteries discharge on their own. The Self-Discharge rate is governed by the construction of the battery and the metallurgy of the lead used inside.

For instance, flooded cells typically use lead alloyed with Antimony to increase their mechanical strength. However, the Antimony also increases the self-discharge rate to 8-40% per month. This is why flooded lead-acid batteries should be in use often or left on a trickle-charger.

The lead found in Gel and AGM batteries does not require a lot of mechanical strength since it is immobilized by the gel or fiberglass. Thus, it is typically alloyed with Calcium to reduce Gassing and Self-Discharge. The self-discharge of Gel and AGM batteries is only 2-10% per month and thus these batteries need less maintenance to keep them happy.

This next one was the funniest of the bunch. -

What is rechargeable battery self discharge?
Primary battery or full charged secondary battery, when put aside for a period, it is capacity will decline or lose, this phenomenon calls self discharge that is electricity flee away. It is decided by the inner electro-chemistry system, similar to water leakage from pond or reservoir.

I spent more time then I care to admit on this subject. I don't no why I became so hell-bent on finding the answer to the self-discharge question and I'm not convinced that I did.
I did learn a lot about various primary and secondary batteries along the way and I thank Jim for the exercise.

I believe that the reason a lot of boaters including myself only are getting two or less seasons out of their marine batteries is because of improper care. We let these batteries run down to zero output because we leave the selector switch on for days at a time only to find out that the recharged battery is degraded and not charging to full capacity any longer ultimately not making it through the winter months.

I was in my office this morning when one of the biomedical engineer staff walked in a started to tell me that his 1999 Chevy Aerostar with the original battery was a little slow to start in the 13 degree temperature this morning. I asked how he was able to have the factory installed battery for nine years of service. He didn't think it was anything special as the engine was always tuned and cared for there by cranking very little to start. He also check the maintenance free battery frequently and added only distilled water. He was tempted to use Reverse Osmosis water but though it might not be a good idea due to this water's property to grab minerals and metal. Hmm, I wonder!

andygere posted 12-24-2008 11:47 AM ET (US)     Profile for andygere  Send Email to andygere     
I think Bella has found the answer, good job. I think this is a good discussion, and I do recall looking into this when I was making my last marine battery purchase. One of my references, perhaps the 12 Volt Bible, has a good chapter on battery types, construction, advantages and the like.

There's no doubt in my mind that AGM batteries self-discharge at a lower rate than flooded cell batteries based on my experience using flooded cell batteries and AGMs in the identical application. For many boaters, the big advantage is simply knowing the boat will crank on the first try, regardless of how long it's been since the boat was last used. I think this benefit goes beyond maintaining voltage through winter storage. For those who keep their Whaler on a mooring, or in a wet slip where there is no AC power available, using a trickle charger to maintain a battery is not an option. Even trailer boaters that want to be able to use their Whaler on a moment's notice without having to worry if the battery is charged can benefit from a battery with a lower rate of self-discharge. Another advantage I discovered when shopping for batteries the last time around is that the Group 24 sized AGMs are typically rated for CCA and MCA comparable to flooded cell batteries in the Group 27 or Group 31 size. In my case, this meant I could move down to smaller battery boxes, gaining space in the boat while reducing weight. These are just a few more of the advantages that in my mind justified the additional cost.

jimh posted 12-24-2008 12:20 PM ET (US)     Profile for jimh  Send Email to jimh     
It should be mentioned that we're discussing the phenomenon of self-discharge. When a boat is not being used there are other mechanisms which can cause a battery discharge when not intended, and these are best described as parasitic drains. Let me mention a few.

In a small boat electrical system there typically is a primary battery distribution switch which can disconnect the battery from all loads. During storage such a primary distribution switch should be moved to the OFF position. If you leave a battery in a small boat with the primary distribution switch in the ON position, you risk having parasitic drains discharge the battery.

An example of a parasitic drain is the reverse-bias current of all the diodes in the rectifier on the motor attached to the battery. Typically these reverse currents are very low, perhaps less than a milliampere or two. However, over the course of several months such a drain will remove some charge from a battery. Other parasitics drains could occur with attached electronic devices which consume a very small current even in their off state.

In some small boats there are loads connected directly to the battery which bypass the primary battery distribution switch. Typically a small sump pump could be connected directly to the battery. If the pump is electronically controlled, it may cycle on and off momentarily at certain intervals, and this load, although intermittent and very short, can cause significant discharge to a battery.

A parasitic drain can also occur across the battery case and terminals, particularly if there is any build up of corrosion or deposits on the battery case. In fresh water applications, this may not be a significant problem, as pure fresh water is not much of an electrical conductor. Saltwater is a better electrical conductor, and current leakage across the case between the battery terminals may be more of a problem in that environment.

As already mentioned, temperature is an influence on the rate of self discharge, so when data is given (and here we have none given so far) it should be annotated with the temperature environment.

The only reasonable way to observe the self-discharge rate of a battery and not be confused by other parasitic discharge rates is to completely isolate the battery. There should be nothing connected to the battery, its case should be completely clean and dry, and the battery should be in a steady temperature environment which is noted and mentioned as part of the data.

I get the impression that among the anecdotal reports provided so far which have overwhelmingly endorsed the AGM battery as having "low" or "lower" or "better" or "far better" self discharge rates as compared to a "standard" battery, that some of these observations may have been influenced by the presence of a parasitic discharge, or at least that the setting in which the observations were made may have the potential for parasitic discharge to occur.

Another parameter that ought to be controlled for is the age of the battery. I don't have data on how the self-discharge rate of a battery varies with the age of the battery, but my suspicion is that there is an influence. To keep a comparison between an AGM battery and a non-AGM battery accurate, the age of the batteries should be similar.

It seems particularly unfair to make a comparison of the discharge rate of a new AGM battery as compared to a flooded cell battery that is several years old or otherwise not in new condition. We don't have data about the age of the other batteries in most of these anecdotal reports of comparisons.

andygere posted 12-24-2008 02:01 PM ET (US)     Profile for andygere  Send Email to andygere     
Jim makes a good point about parasitic drains. These are often numerous and poorly understood on many amateur marine wiring jobs. I suspect that a substantial fraction of the problems most boaters experience with discharged batteries are at least in part due to parasitic drains. It's noteworthy to mention that in my Suburban, there are numerous parasitic drains known to me: The alarm system, the stereo and on-board trip computer memory, the "security lighting" that leaves the headlights and parking lights lit for a period of time when the vehicle is locked, and several others. I have experienced no problems with a discharged battery since installing the AGM. The fairly new (18 month old) flooded cell battery that was installed when I purchased the truck was frequently too weak to start it if the vehicle sat for 2 or 3 weeks of non-operation.

The system on my boat fully isolates each battery from any and all electrical loads via heavy duty switches (one for each battery) very near the batteries themselves. Since I use high quality gold-plated post connectors, I have had no problems with corrosion that might lead to drain across the terminals. I replaced all of the wiring and switchgear on my boat's high current electrical system with new equipment, and used this wiring system with both flooded cell and AGM batteries.

When comparing battery performance, I am comparing my results using new flooded cell batteries with those of new AGM batteries, which I purchased after retiring the lead acid units. I'm also comparing results with my 2-year old AGM batteries, which perform just as they did when they were new. My on-board instrumentation reports battery voltage to the tenth of a volt, so I have a good measurement system that's in front of me all the time. The AGMs invariably show 12.4 volts or more, where the flooded cells regularly dipped below 12 volts (and into the low elevens as they aged) after several weeks of non-operation. There is no question in my mind that the AGMs outperform the previous set of high-quality flooded cell batteries in exactly the same conditions when it comes to self-discharge properties. Accordingly, I have found AGM batteries to be a good investment in terms of reliability and performance.

Perhaps a controlled experiment would be the best way to determine if my experience can be repeated. I suggest the following:

1) Purchase a new AGM marine battery (say the Optima Blue Top). Charge it to a full state of charge and measure voltage across the terminals a few hours after taking it off the charger.

2) Purchase a new flooded cell lead acid battery with a CCA and MCA rating comparable to the AGM. Charge it to a full state of charge and measure voltage across the terminals a few hours after taking it off the charger.

3) Store both batteries side by side at room temperature (approximately 70 degrees F).

4) Measure the voltage across the terminals on a weekly basis and record.

5) After 6 months, publish the results here.

number9 posted 12-24-2008 05:55 PM ET (US)     Profile for number9  Send Email to number9     
The controlled experiment is just going to show what some of the repliers and the battery manufacturers tell us, the AGM will be at a substantially higher voltage at the end of 6 months. Do agree though it's a wonderful idea for one of us to conduct said experiment.
jimh posted 04-05-2009 10:25 AM ET (US)     Profile for jimh  Send Email to jimh     
Following the discussion that took place above, I conducted a small experiment of my own.

I charged an AGM battery using a small float charger for several days, until the battery was at a nominal full-charge state. I then took it off the charger and let the terminal voltage decay for a day or so to its true resting state. Then I measured the terminal voltage and recored the date. I used a FLUKE Model 77 DVM which has an accuracy of 3-percent. Then I covered the battery with a non-conducting cotton towel cloth and let it sit for several months. The battery was in a heated, indoor environment, and had no load attached to it. About three month later, 104 days to be exact, I measured its terminal voltage again, using the same meter:

December 22, 2008 = 12.78-volt
April 5, 2009 = 12.70-volts

The terminal voltage decreased by 0.08 volts.

According to a chart in the REFERENCE article on Battery Charge, the initial state of the battery was indicative of about 10-percent discharge, or a nominal 12.8-volts. A 20-percent discharge state would be 12.65-volts. Interpolating, we see that in the three months or so of self discharge, the AGM battery lost about 6-percent of its charge.

Previously I had conducted a similar experiment using a conventional flooded cell lead-acid battery. In that experiment the discharge period was longer, about four months or 134 days to be exact. The cell voltage started at 12.65 and decreased to 12.50, a loss of 0.15 volts. This represented a loss of about 10-percent of the charge.

The rate of voltage drop per day was follows:

AGM = 0.08 volts/104 day = 0.00076923 volts/day
Flooded Cell = 0.15 volts/ 134 days = 0.0011194 volts/day

The rate of discharge per day was as follows

AGM = 6-percent/104 days = 0.0577-percent/day
Flooded Cell = 10-percent/134 days = 0.0746-percent/day

In both analysis, either voltage drop or charge loss, the AGM showed a lower self discharge rate than the flooded cell. In both cases the battery under test was not a new battery, but both were about the same age, three years or so, and both were in good condition. The charger that was used with the AGM may have higher output, so it was able to raise the AGM terminal voltage a little higher than the flooded cell battery.

The results of this experiment show that the AGM battery tested had a slower rate of self-discharge than the flooded-cell battery tested. The voltage loss rate was in a ratio of 1.4:1. The charge loss rate was in a ratio of 1.29:1.


jimh posted 04-05-2009 11:14 AM ET (US)     Profile for jimh  Send Email to jimh     
As far as I can tell, the information above is perhaps the only published test comparing an AGM and a conventional lead-acid battery self-discharge rate.

I don't know if the results are indicative of the required endorsement that the AGM battery have a "low" or "lower" or "better" or "far better" or "much better" self discharge rate than a conventional lead-acid battery.

As far as I can tell the self-discharge rate of any lead-acid battery is related to the purity of the lead which is used. In some flooded cell lead-acid batteries the lead is alloyed with other materials in order to improve its structural strength, with the result that the self-discharge rate is degraded. In some AGM batteries a very pure lead is used, leading to an improved self-discharge rate.

As far as I can tell, there is nothing inherent in the AGM construction that engenders lower self-discharge. If an AGM battery shows lower self-discharge it is likely due to its use of high-purity lead. It is entirely possible that one could use the AGM construction technique and not employ high-purity lead, and, in that case, I do not believe there would be a significant difference in the self-discharge rate when compared to a conventional battery.

I do not see a basis to make a claim that all AGM batteries have lower self-discharge rates than a flooded-cell battery. Among flooded cell batteries there are likely variations in the self-discharge rate which is reflective of the purity of the lead being used.

It should also be noted that in the comparison above the cost ($285) of the AGM battery was about four times greater than the cost ($70) of the flooded-cell battery. The AGM battery was a premium grade battery and weighed about 80-lbs. The flooded cell battery was just a run-of-the-mill flooded cell battery that was not particularly special; it probably weighed 50-lbs.

It would be interesting to compare a high-purity lead-acid flooded cell battery such as a Surrette brand to an AGM. The Surrette batteries often come with a seven to ten year warranty and their cost is equal or greater than an AGM battery of comparable size. Most AGM batteries, even premium grade ones, are sold with warranty periods of one or two years.

Chuck Tribolet posted 04-05-2009 11:19 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Fluke 77s rule!

What's interesting to me is that it looks like either of those
batteries would have started Everrude after three months.

How old were they at the start of the experiment? I've always
had a hunch that self discharge went up with age.


AZdave posted 04-05-2009 11:54 PM ET (US)     Profile for AZdave  Send Email to AZdave     
Jimh, I'm always impressed by real data. I think your original position was that you doubted the difference in discharge rates claimed by AGM battery manufacturers. Even though your data show a lower discharge rate for the AGM, the small voltage changes might be due to experimental error. If the Fluke meters have an accuracy of 3%, that would translate to 0.4V or so for a 12V battery. That makes up half the difference found for the AGM, and more than 25% of the difference for the flooded cell battery. Again, I applaud the effort to find real data. It's just very hard to get definitive data in reasonable periods of time, and without buying large numbers of batteries. Dave
jimh posted 04-06-2009 09:12 AM ET (US)     Profile for jimh  Send Email to jimh     
My initial position was expressed in my initial article and its only question:

"What aspect of a SVRLA AGM provides the lower self-discharge rate, if indeed it exists?"

At that point, the literature I had seen in which a manufacturer made a claim of lower self-discharge rate failed to provide any reasonable basis for making the claim. Further, I had not, and still have not, seen any actual data from a manufacturer that shows the difference between the self-discharge rate of their AGM and that of a conventional flooded cell battery, other than a cartesian coordinate plot in which there were no dimensions given on either axis. You may call me a skeptic, but I didn't find any of that information to be particularly demonstrative of a proof.

As a result of this discussion, I now believe that use of high-purity lead in a battery can contribute to a lower self-discharge rate. Prior to this discussion, I believed that the length of the manufacturer's warranty coverage was generally indicative of the anticipated useful life of the product. I still hold that belief.

jimh posted 04-07-2009 08:57 AM ET (US)     Profile for jimh  Send Email to jimh     
Regarding the self-discharge rate of flooded-cell batteries using lead-calcium alloys versus absorbent glass mat (AGM) batteries, one manufacturer characterizes it as follows:

Self-discharge rate

Lead-calcium battery = 1% to 5% per month

AGM battery = 1% to 3% per month


We compare to the experimental data I collected:

Non-AGM battery = 0.0746 %/day X 30-days = 2.238 % per month

AGM battery = 0.0577 %/day x 30-days = 1.731 % per month

The experimental data I collected fits precisely in the range of values suggested by the battery manufacturer as being typical of the values expected for the two types of batteries.

This provides us with a basis for understanding the claims which have been made that an AGM battery will have a "better" or "much better" or "lower" or "much lower" rate of self-discharge. We now can appreciate that in the case of comparison of the self-discharge rate of an AGM battery compared to a flooded-cell battery these terms are being used to describe a difference which would be likely to be in the range of a variation of 1- to 2-percent.

A variation of 1- to 2-percent is not a particularly large variation. For example, if we were to apply this range of variation to the price of the battery, we would see that one battery might sell for $100 and the other battery would sell for $101 or $102. Curiously, when we do compare the price of these batteries we see that the variation is very much greater, indeed there is a 300-percent variation (i.e., the AGM battery costs four times as much).

HAPPYJIM posted 04-08-2009 12:28 PM ET (US)     Profile for HAPPYJIM  Send Email to HAPPYJIM     
Good stuff, Jim. I don't know of many people that would go through the trouble and time to do an experiment of this magnitude. To me it shows that the old fashioned wet cell battery is still the best value for the money.

I'm lucky to be in an area where year round boating is possible and the longest lay-up of my boat is maybe 1 month. It's good to know that in that month my batteries should not be a factor to worry about. I sometimes thought to put a topping charge on the batteries before heading out. I won't be using any memory for that and will put more thought into what lures to use.

Thanks again for the time you spend to give us all a great reference for working on and maintaining our Whalers.

John W posted 08-23-2009 01:40 PM ET (US)     Profile for John W  Send Email to John W     
I'm adding to this older posting after jimh redirected it to me from another post.. Just some more written info on self discharge rates of battery types: per the West Marine Advisor, the self discharge rate for flooded batteries is 6-7% per month, versus AGM at 3% per month.

I'm sure jimh will point out that this isn't backed up with hard data, and they don't provide info on where these statistics come from. And obviously, plenty of variables would come into play in how much self discharge a battery will experience, such as age, quality, and useage history of the battery in question, the temperature in which the battery is stored, and who knows what else. Of course, these same variables might come into play in jimh's test, of one used flooded battery & one used AGM battery, the latter of which was not purchased & used previously by jimh. (Was the previous duty cycle of these two batteries similar? Was either allowed to sit for a prolonged period in a discharged state? Was the flooded battery well maintained? What brands & original cost/quality of each were these batteries? etc).

However, in spite of the shortcomings of West Advisor's claims noted above, I have found the West Marine Advisor to generally have very good factual data, and they generally don't push a particular product or brand. There is a lot to dislike about West Marine, most notably their prices, but I think the West Advisor information on various topics to be pretty accurate. It is interesting to note that West Marine does not claim that AGM or Gel Cell batteries will have a longer life than flooded...this fits with jimh's beliefs on lifespan of battery types. I have found AGM batteries to last significantly longer than flooded, but I believe that may be due to some combination of self discharge of my flooded batteries during periods of nonuse, and possibly occasionally less-than-perfect cell water levels if I go too long without topping them off, in flooded batteries I own.

I have also experienced flooded batteries being low after being stored unused for a period of weeks or months, whereas the AGM's I have owned always turned over the motor with no obvious drop in charge after storage. I don't have factual data to back this up, it's simply what I have experienced. This is also what glen e, number9, and andygere all attest to in this article as being their personal experience with AGM batteries as well.

West Advisor's commentary on battery types is here:

Choose the right battery chemistry
Marine batteries are available in three chemical types: flooded, Gel and AGM (Absorbed Glass Mat). Which type you choose is based on your needs (deep cycle vs. starting), the capacity and lifespan you are looking for, and your budget.

Flooded batteries,unlike other types, use a reservoir of liquid sulfuric acid to act as a pathway between positive and negative plates, which produce hydrogen and oxygen when the battery is being charged. Vented wet cell batteries allow the gases to escape into the atmosphere, unlike gel and AGM batteries, which recombine the gases and re-introduce them to the system. Vented hydrogen is an explosive gas, so battery boxes and compartments must be vented to let the gas escape safely outside the boat. Flooded deep-cycle batteries require maintenance-periodic inspection and topping-off with distilled water.

Flooded batteries handle overcharging better than gel and AGM batteries, because of this hydrogen venting and because they are not sealed like the other types. They self-discharge at a higher rate (6 to 7% per month) and thus require off-season charging. Wet cells must be installed in an upright position and do not tolerate high amounts of vibration. Their initial cost is lower than similarly sized AGM or gel batteries. Properly charged and maintained, our premium wet cell deep-cycle batteries are capable of between a few hundred and over a thousand discharge cycles, which can translate to many years of dependable service.

Gel Batteries: The SVR design nearly eliminates gassing, so they are safer to install around people and sensitive electronics (but gel and AGM batteries still need to be vented). Gel batteries are manufactured to very high standards of quality and consistency, since it is not possible to add water or gain access to the interior. The "gel" is a combination of sulfuric acid, fumed silica, pure water and phosphoric acid. After mixing during manufacturing to a thin liquid form, it is sucked into each cell by vacuum pressure up to six times, eliminating voids and air pockets on the plates that would cause dead spots and reduce performance. Once it is in place, the gel becomes quite viscous, which prevents leaks if the battery is inverted or the case is damaged.

Charging causes a small amount of hydrogen and oxygen to be generated at the plates, like a flooded battery, but the pressure inside the cells combines the gasses to create water (so they are called "recombinant" batteries). This keeps the battery from drying out due to charging, but it also requires that the vessel's charging system be very carefully regulated to prevent high voltage over-charging.

AGM batteries: Sealed Valve-Regulated AGM (Absorbed Glass Mat) batteries feature fine, highly porous microfiber glass separators compressed tightly between the battery's positive and negative plates, which are saturated with just enough acid electrolyte to activate the battery. During charging, precision pressure valves allow oxygen produced on the positive plate to migrate to the negative plate and recombine with the hydrogen, producing water. In addition to providing equal saturation across the entire surface of the battery's positive and negative plates, the fibers in the dense glass mats embed themselves into the plates' surface like reinforcing rods in concrete, providing more plate support and better shock and vibration protection than in conventional batteries.

High-density AGM batteries have lower internal resistance, allowing greater starting power and charge acceptance, and quicker recharging than other types of deep cycle batteries. High acceptance means that AGM batteries can accept the highest charging current, up to 40% of the amp hour capacity of the battery, compared to about 25% for the other two battery chemistries. Long life, a low 3% self-discharge rate and outstanding performance make AGM batteries excellent dual-purpose batteries for boaters who require quick starting power and reliable deep cycle ability.

glen e posted 08-23-2009 02:23 PM ET (US)     Profile for glen e  Send Email to glen e     
Here's the advantage from Merc's technical bulletin - but I'm sure it will be dismissed here as heresay. Nevertheless, I publish it for those with a open mind in this discussion. Per Merc's bulletin # 2008-04:

* Electrolyte is absorbed into the fiberglass mat and will not spill if the battery case is
* Maintenance\0x2011free - Water never needs to be added.
* Low self\0x2011discharge rate - Can be left in the boat for winter storage if it is fully charged
and a battery cable is disconnected.
* Can be mounted in any orientation - Upside down not recommended.
* Best solution for a dual purpose starting and deep cycle battery.
* Increased vibration resistance and service life - An AGM is typically rated for 200-400
deep\0x2011discharge cycles. A flooded lead\0x2011acid starting battery will only last for 2-4 cycles.
* Can accept a very high charge rate. Battery will not be harmed if it is charged with 70
amp or more current.
* Standard lead\0x2011acid charging equipment can be used as long as the charging voltage
does not exceed 14.7 V.
* Sealed to prevent the escape of combustible charging gasses.
* Classified as nonspillable and can be shipped by UPS and FedEx.

glen e posted 08-23-2009 02:25 PM ET (US)     Profile for glen e  Send Email to glen e     
and yes, I know I posted this above but since you can't edit, this time I posted the bulletin #.
jimh posted 08-23-2009 03:04 PM ET (US)     Profile for jimh  Send Email to jimh     
Both Glen and WEST Marine are sellers of batteries, and it is natural that a seller will encourage the customer to buy more expensive items.

I am not selling batteries. I buy batteries. My only interest in batteries is to get good value for my money, and to understand what improvement I might get if I increase the cost of my battery by a factor or four or five. With regard to the rate of self discharge it is abundantly clear that the benefit of spending four or five times more for a battery is going to be a decrease in the rate of self discharge of about one to two percent per month.

For situations where a difference in the rate of self discharge of one to two percent per month is a critical difference, perhaps the increase in cost by four to five times can be justified, or at least rationalized.

glen e posted 08-23-2009 03:17 PM ET (US)     Profile for glen e  Send Email to glen e     
I can't believe you would stoop to dismissing what I say because I "sell" batteries. How about the mfr's I listed? What's tehir motive? They can buy any battery in the owrld as std equioement. Why would they choose AGM's? And I don't sell them- we supply them for installations. Everything you don't agree with, you accuse of having an ulterior motive. Are you saying west marine did a full adviser page on AGM's because they want to sell a more expensive battery? When will you quit? Here's another article liking AGM's:

but I assume they have a motive too...

jimh posted 08-23-2009 04:04 PM ET (US)     Profile for jimh  Send Email to jimh     
Glen--I have hardly stooped to any depth. I simply observe that in this fight some of us have a dog. Your dog is the AGM battery, and based on your unrelenting promotion for it, I have to wonder if you are also working for the Chinese manufacturers that make them!

I began this discussion with a simple goal: to learn what influences the self-discharge rate of storage batteries. In return I have been swamped with propaganda for AGM batteries, but, other than my own test data, hardly a single word of credible support for the often repeated claim that an AGM battery has a "low" or "lower" or "much lower" or "better" or "much better" rate of self discharge than a conventional flooded cell battery. About 99-percent of what has been offered in support of AGM is just palaver, and frankly, it is embarrassing that AGM supporters cannot find something more to say than this stunning revelation from Mercury,

"Low self-discharge rate - Can be left in the boat for winter storage if it is fully charged and a battery cable is disconnected,"

or this amazing factoid from WEST Marine:

"... low 3% self-discharge rate..."

Low self-discharge comes from high purity lead, not from AGM construction. It is that simple.

White Bear posted 08-24-2009 02:39 PM ET (US)     Profile for White Bear  Send Email to White Bear     
Lots of interesting food for thought, but not much nurishment as it applies to the real world as opposed to the world of fractional voltages. I've been using conventinal lead acid automobile batteries in boats for years. I leave the batteries in the boats over the Long Island winter and have yet to have a freeze up. I charge the batteries in the springtime and they're good to go. I employ two batteries and buy a new one every three years which equates to a replacement battery life of six years. The AGM and other super batteries may be good for some folks, but there is nothing in my 40+ years of experience that warrants the higher cost.

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