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Author Topic:   Charging Deep-cycle Batteries
David Pendleton posted 08-05-2013 05:43 PM ET (US)   Profile for David Pendleton   Send Email to David Pendleton  
I took my batteries (both identical deep-cycle) out of the boat last fall and stored them on a shelf in my garage. Once a month I would put them on a charger at 2A.

I have a pretty-good quality charger, and it used to indicate that the charge was complete via a little green light. The past three months, the light no longer comes on, yet when I remove the clamps and check the voltage, it is usually 13-14V for both batteries.

Just today, one battery read 14.1V when I took it off. Is this too high? While the batteries are identical, one is three years old, the other is new this season. They have never been discharged below 12V.

Also, I check both batteries weekly using a multi-meter, and it seems they discharge about 1/10 volt every two weeks. What is normal for a battery that isn't "doing" anything?


tmann45 posted 08-05-2013 07:56 PM ET (US)     Profile for tmann45  Send Email to tmann45     
Flooded deep cycle batteries should not be discharged below 50% for long life, which is about 12.1 volts. They should be recharged as soon as possible or loss of capacity due to sulfation can occur, the voltage will show normal.

14.1 volts is high for float charge, normally it is about 13.5 volts. This indicates battery or charger problem to me if your charger is a multi-stage charger (bulk, absorption, and float).

Resting voltage (no charging or discharging for +/- 12-hours) for flooded battery is around 12.6 - 12.8 volts. This is the best voltage test for battery condition. Let them rest over night and check voltage in the morning.

From the Solar-Electric.com website: The self discharge of batteries with Lead-Antimony plates can be high - as much as 1% per day on an older battery.

kwik_wurk posted 08-05-2013 08:03 PM ET (US)     Profile for kwik_wurk  Send Email to kwik_wurk     
From what you state you are doing seems to be the correct actions. And 13.8 VDC typically is floating charge for most battery chargers, so 14.1 is a little high. For the most part what you are getting for readings match for regular lead/acid batteries with distilled water or AGM's (not gel cells). With three exceptions:

-Charger type: There are a lot of chargers out there with different charging profiles, and if you can be more specific, that would be helpful. (Like trickle charger, three stage, brand/model)
-Old and new battery. Ideally they should be of same vintage because the cells will age (different discussion as to what actually happens). Thus you will get different operating and charging profiles, and this will lead to slightly adverse performance of the system as a whom, because the newer battery will be drawn down to the lower battery when (or if) electrically paralleled.
-Check water level (lead/acid type only). Depending on usage, you should check the water levels and fill with distilled water at least once per year.

As for the light. Let the batteries sit, and naturally discharge, then when you plug the charger, take occasional voltage readings. (Take several in the first hour or so, as the charge cycle is a bit more active to say.) And this will tell you whether it is charging or not. The fact that you get 14.1VDC indicates you are getting a charge from the charger.

kwik_wurk posted 08-05-2013 08:04 PM ET (US)     Profile for kwik_wurk  Send Email to kwik_wurk     
correction: [I applied the changes to the original--jimh]
jimh posted 08-06-2013 08:08 PM ET (US)     Profile for jimh  Send Email to jimh     
Let me clarifiy a few things, which, in the preceding comments, seem to me to have been presented in a way that might cause confusion:

--AGM or absorbent glass mat batteries are lead-acid batteries. The are usually sealed and contain valve regulators that release pressure if too much internal pressure is created. These AGM batteries are also VRLA (valve-regulated lead-acid) batteries. You can charge them like any lead acid battery, but, since they are sealed and valve regulated, you do not want to over-charge them and produce gassing.

--a lead-acid battery with a gelled electrolyte is still a lead-acid battery. As far as I can tell, the only difference in operation of a gelled electrolyte battery is a preference to keep the float voltage lower so that there is very little risk of the electrolyte bubbling out any gas. In the thickened or gelled electrolyte, gas bubbles tend to remain in the place where they are created, at the plates themselves, and reduce plate surface area. The bubbles cannot easily move to the top and become gas. So with a gelled electrolyte there is usually instruction to not let the charging voltage get too high. You do not want to over-charge them and produce gassing, as this will lead to reduced capacity and eventual failure.

As for charging a deep-cycle battery, I do not know of any special regime that is needed.

A battery with no-discharge, that is, at its full charge, will be about 12.9 to 12.8-Volts at its terminals. It varies with temperature. This is the open-circuit, no load, terminal voltage after the battery recovers from charging. It may take a few hours for the battery voltage to stabilize after charging.

I like charging batteries at low current rates.

The self-discharge rate depends mostly on the composition of the lead in the lead-acid battery. If the battery plates are made of high purity lead their self-discharge rate will be very low. Some AGM batteries are made with very high purity lead. Not all AGM batteries are made with very high purity lead. Accordingly, the self-discharge rate of AGM batteries varies as a function of the lead purity. There is nothing intrinsic in the AGM construction that imbues the battery with a very low self-discharge rate. You only get that in an AGM battery if it is made with very high purity lead.

Batteries contain sulfuric acid, usually in a solution with water of about 30-percent by mass or about 4-molar when at full charge. Battery electrolyte is usually replenished with distilled water, which then decreases the concentration slightly. Re-charging of the battery restores the acid concentration through the chemistry of the lead-acid battery.

The electrolyte in sealed batteries cannot be replenished with water. For this reason, there is proper concern about over-charging sealed batteries and producing gassing. Sealed batteries, or VRLA batteries, contain recombinant material that tries to return expelled gases to a liquid form and return them to the electrolyte. In this way a VRLA battery can tolerate some over-charging. If too much over-charging occurs, the amount of gas produced may be too much for the recombinant materials to handle, and the gas may produce pressure in the battery case. This pressure is vented to the atmosphere if it becomes too high, resulting in a loss of electrolyte.

jimh posted 08-07-2013 01:57 PM ET (US)     Profile for jimh  Send Email to jimh     
As far as I can tell, the feature of a lead-acid battery which tends to cause it to be described as a deep-cycle battery--meaning a battery that can tolerate many cycles of deep discharge and recharging without losing its capacity--comes mainly from the structure of the lead plates. Batteries which are deep-cycle batteries tend to have lead plates of greater thickness, while batteries described as engine-starting or cranking batteries tend to use thinner plates. The use of thinner plates allows a cranking battery to be able to deliver more current from a battery of a particular size and volume compared to one with thicker plates, as the surface area of the plates is what determines the current capacity. Deep-cycle batteries with thicker lead plates will have have as much surface area as a cranking battery of the same physical dimensions.

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