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dave.verry
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Paralleling Batteries

There is a major disagreement on paralleling batteries on this forum. As an engineer who parallels batteries for a living and designs & builds chargers for them, I’ll give you one guess where I line up.

This conflict has hijacked more then one thread, so probably deserves a posting of its own. Here goes!

I have been considering two states of the battery bank while answering these posts;

1. Batteries on a charger and charging or,
2. Batteries off a charger and discharging into a load.

It was pointed out that there is a third state where the batteries are off the charger and are not discharging, i.e. no load.

Examining each state with a simplified battery bank of two parrelled batteries, both rated at 100 AH (amp-hours), and one “good” at rated capacity and one “bad” with only 50% capacity or 50 AH. We will have to assume good connections, since everyone does monthly preventive maintenance every month on there battery bank, right? We will also have to assume the “bad’ battery does not contain shorted plates. The loss of capacity will be due to plate corrosion, sulfation and/or electrolyte loss (drying-out).

Under charge, the batteries behave independently, each charging to there maximum capacitance. The “bad” battery will charge faster, given the same energy input, but with two in parallel, the amount of current may be limited by the “good” battery, depending on the charger. Once the float voltage level of the batteries has been reached, both batteries will be at approximately 80% state of charge and the two batteries will continue to charge the remaining 20% independently. Using the 80/20 rule of thumb, 80% of the charge takes 20% of the time, the final 20% taking 80% of the charge cycle time. Because of the limiting effect of the good battery, both batteries will reach full charge state at about the same time. (Assuming full discharge was the initial state of both batteries. You can start without full discharge and the charge time of both batteries may differ.)

At the end of the charge cycle, both batteries will be at full charge, one at 50 AH and one at 100 AH.

During discharge, the “bad” battery will discharge 1/3 of the current level of the “good” battery. For a 10 amp load, 3.33 amps will be coming from the “bad” battery and 6.66 amps from the good battery. Both batteries will be at the same voltage level and because of this discharge the same percentage of capacity. Both batteries will be at the same state of charge during the discharge.

If the batteries are left off the charger they will drift down to their open circuit voltage after several hours. If the specific gravity of the electrolyte has not undergone change, i.e. the loss of capacity of our bad battery is due to plate corrosion or drying-out, the open cell voltage of both batteries should be the same for given state of charge. (close enough anyway). Since there will not be a potential difference between the batteries there should be minimum to no current flow between batteries. Any difference will be equalized once the batteries stabilize any specific gravity differences. This will be done by discharging the battery with the higher specific gravity and charging the battery with the lower specific gravity. There is no guarantee which battery will be discharge as the one with the lower capacity may have the higher specific gravity electrolyte. In any case the overall system capacity will remain constant. The only loss will be a small resistive loss due to current flow (negligible compared to system capacity).

If the “bad” battery has a low specific gravity electrolyte due to sulfation (this is the only cause I can think of in a battery system) the parrelled batteries will once again try and equalize the open cell voltage and the “good” battery will continue to “charge” the “bad” battery until the specific gravity of the electrolytes are the same. However, any charge taken from the “good” battery will be stored in the “bad” battery minus resistive loss again. Again the battery system capacity should remain constant.

If any work is done by the system, i.e. hydrolysis of water within the electrolyte in to hydrogen and oxygen, that energy may be lost from the system. This should occur, especially in a system off the charger, only when there is physical damage such as shorting of a cell of one of the batteries. In this case the “good” battery will OVERCHARGE the “bad” battery causing electrolysis and gassing and possibly venting. This energy will be lost to the system.

Basically the conservation of energy prevails. Energy from one battery may move to another (current flow between batteries) but the overall system energy will remain the same (discounting the negligible loss from resistive heating).

Does this mean that you should not replace all of your batteries if you find one bad? Maybe, or maybe not. Only a load test of the batteries can answer that question and that is beyond the means of most sailors. Good practice for reliability is to change all of the batteries at one time. This helps insure that all batteries are in good condition and that you have the battery system capacity you think you do.