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Author Topic:   AGM Battery Voltage and Longevity Between Charges
lurkynot posted 11-03-2013 04:03 PM ET (US)   Profile for lurkynot   Send Email to lurkynot  
What has been your experience with [absorbed glass mat] batteries regarding recharging and longevity [perhaps means either self-discharge, stored charged, or the time interval] between charges?

I have are two Sears Diehard Platinum batteries. One will only reach 12.35-Volts and the other 12.5-Volts after extended charging. They are less than two years old [and] have not been abused or neglected. Everything I read tells me these batteries should top off at higher voltages.

K Albus posted 11-03-2013 08:48 PM ET (US)     Profile for K Albus  Send Email to K Albus     
I have two Group 27 Die Hard Platinum AGM baterries that are two years old. One charges up to 12.81 volts, and the other to 12.65 volts. Those readings are taken after fully charging the batteries and then waiting a day or two before testing.
tmann45 posted 11-03-2013 10:12 PM ET (US)     Profile for tmann45  Send Email to tmann45     
Mine are reading 12.99 and 12.95 and have been off charge for about four days. They are 14-months old, manufactured date of August 2012.
jimh posted 11-04-2013 08:57 AM ET (US)     Profile for jimh  Send Email to jimh     
An absorbed glass mat battery is just a lead-acid battery. It charges and discharges like any lead-acid battery. There are six cells in the battery, and you can figure that at full charge each cell will be around 2.1-Volts. This means at full charge the battery should read about 12.6-Volts. Depending on the temperature, this could easily be higher, perhaps to a maximum of 12.9-Volts There is a good article in the reference section that lists the battery voltage as a function of state of charge. See

A battery that can only reach 12.35-Volts of charge is not being fully charged. One has to be careful with these measurements; use a very accurate voltmeter. A tenth of a volt is a big error in these measurements.

As for "longevity between charges," I am not clear what you intend with that expression. It seems a bit imprecise. Let me answer on three possible interpretations: self-discharge, stored charged, and recharge interval.

It is often claimed, without any real basis, that all AGM batteries will have superior self-discharge characteristics, that is, they will show very low rates of self-discharge. The rate of self-discharge has nothing to do with the AGM construction. Low self-discharge results when a battery uses very high purity lead. It is true that some AGM batteries will use very high purity lead, and they will have superior self-discharge rates. Flooded cell batteries tend to use an alloy of lead with some intentional impurities to give the lead plates more strength, so as a group they often will show a slightly worse self-discharge rate. But, better quality flooded cell batteries may be using purer lead and will also show excellent self-discharge rates.

The actual difference in self-discharge between a typical flooded cell battery and an AGM battery is often described by nebulous and vague terms like "better". It is just about impossible to find any real data on the difference in self-discharge rate between AGM construction and flooded cell construction lead-acid batteries. I performed an experiment to test the self-discharge rates of two batteries, one an AGM and the other a flooded cell. I found the AGM had a lower rate of self-discharge, but it was not particularly hugely better.

You might find it interesting to read this prior discussion at

Once the brand boosters get out of the discussion and the topic turns to the actual electro-chemistry of a battery, there are some interesting observations made about batteries.

The amount of charge a battery can hold is proportional to the surface area of the plates, and is influenced by the condition of those surfaces. The condition of the surfaces is affected by the prior use the battery has seen, how well it has been charged, and the number of discharge--recharge cycles it has endured. The treatment of the battery by its charger plays an important role in determining the condition of the plates.

In particular for AGM batteries, it is important to observe a limit to the charging voltage, particularly the float voltage, that is, the voltage applied as the battery terminal voltage rises toward its maximum charge. If too much voltage is applied to any lead-acid battery, the result will be to drive hydrogen gas from the electrolyte. In flooded-cell vented batteries a lot of over-voltage float charging can be tolerated. Any electrolyte lost to gas escape can be replaced with new liquid. In a valve-regulated battery--all AGM batteries are sealed and valve-regulated--loss of electrolyte is a big problem. It cannot be replaced. Loss of electrolyte from chronic over-charging is a common problem. In boater parlance, it is possible to "fry"" an AGM battery if it is chronically over-charged.

The time interval between re-charging any battery should be short. Battery chemistry does not work well if allowed to sit in a discharged state for any duration. Therefore it is always best to immediately recharge a battery to full charge as soon as possible after it has been discharged.

lurkynot posted 11-04-2013 08:58 AM ET (US)     Profile for lurkynot  Send Email to lurkynot     
The batteries were 12.7ish when I purchased them in April of 2012. Used them the duration of that season sparingly at best charging them after each use most of the time. After heavy use I made sure they always went on the charger. After light use I would charge them most of the time depending on spot I had on the seawall since the boat was rack stored.
jimh posted 11-04-2013 09:14 AM ET (US)     Profile for jimh  Send Email to jimh     
AGM construction sometimes allows for rather large capacity in a battery of modest size. It also sometimes produces very low internal resistance. Generally a battery with a low internal resistance can absorb charge current faster. I think this plays a roll in some applications to outboard engines where AGM batteries are deemed mandatory. Certain outboard engines, notably the Brunswick Mercury-brand VERADO, insist on an AGM battery. I believe that is an outgrowth of the AGM being able to absorb high charging current. It is the nature of the charging current available on the VERADO that tends to favor a battery that can absorb high charging current. The VERADO does not produce much charging current at low engine speeds, and it is therefore advantageous to be able to jam a lot of current into the battery when it is available from the engine charging system.
jimh posted 11-04-2013 10:14 AM ET (US)     Profile for jimh  Send Email to jimh     
From your last comment I make the inference that the AGM batteries are not connected to the outboard engine, and they are charged by a 120-VAC-operated charger. Is that right?
lurkynot posted 11-04-2013 10:18 AM ET (US)     Profile for lurkynot  Send Email to lurkynot     
That is correct. They are used exclusively for the electric trolling motor and are charger with an on board charger.
lurkynot posted 11-04-2013 10:29 AM ET (US)     Profile for lurkynot  Send Email to lurkynot     
I should add they work quite well or did the last time out however the voltage level after charging makes me wonder if the batteries are defective or this is just their nature. The previous batteries I used for my electric troll motor were Sears wet cell deep cycles which lasted about 3 years .

The Platinums seem to hold a charge longer from what I remember about the previous batteries. It is just the voltage readings that give me a bad feeling. I really hoped these batteries for their extra cost would have been able to take more of a charge at this point.

tmann45 posted 11-04-2013 12:38 PM ET (US)     Profile for tmann45  Send Email to tmann45     
Have you looked at your charger for the source of the problem?
lurkynot posted 11-04-2013 01:48 PM ET (US)     Profile for lurkynot  Send Email to lurkynot     
Think I am going to take them up to Sears and ask if they can charge them to see if the results are the same.
jimh posted 11-05-2013 10:08 AM ET (US)     Profile for jimh  Send Email to jimh     
Do you know what the charger output voltage is when the charger enters the float-stage of its charging, if the charger is a multi-stage charger, or if the charger is just a simple charger, what the charger output voltage tapers to as the battery begins to draw minimal charging current?
lurkynot posted 11-06-2013 10:09 AM ET (US)     Profile for lurkynot  Send Email to lurkynot     
The manual states that LED’s 1 – 5 indicate:
5 RED LED’s – Initial charging
4 RED LED’s – Above 13.0 volts
3 RED LED’s – Above 14 volts
2 RED LED’s Battery 80% charged
1 RED LED final Charge stage
1 GREEN LED – Charge Current Off Monitoring Stage
jimh posted 11-06-2013 10:15 AM ET (US)     Profile for jimh  Send Email to jimh     
It seems you do not know the float voltage of the charger. You should measure the battery terminal voltage when your battery charger is indicating it is in the phase denoted by one green indicator lamp.
lurkynot posted 11-06-2013 10:41 AM ET (US)     Profile for lurkynot  Send Email to lurkynot     
I will check on that.

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