ASK THE EXPERTS: Battery Watering

Looking down into the cell at the fill level of a Trojan L-16 flooded lead-acid battery.

At which state of charge/depth of discharge (SOC/DOD) is it best to top off flooded lead-acid (FLA) batteries with distilled water?

Ben Aeschbach • via

I recommend the following quarterly battery maintenance procedure. Before you start, put on your protective gear—safety glasses and nitrile gloves, at a minimum. And have an acid spill-control kit nearby—this can be as simple as a box of baking soda.

1. Get the batteries to float stage, either by PV input and load shifting or with a fossil-fuel generator.

2. Take and record voltage and specific gravity readings on each cell.

3. Top off each battery cell to the fill line with distilled water.

4. Although you can start the equalization cycle with your PV charge controller or inverter/charger, I recommend using a generator. Equalize cycles are timed, and a cloudy afternoon could make your PV equalization attempt abort until the next day, and the day after that. It’s best to get it done immediately—and completely—with the generator.

5. The equalization cycle mixes the electrolyte thoroughly to prevent stratification. After the cycle is over, check each cell’s voltage and, if needed, top off with distilled water. Once each cap is back in place, gently clean the tops of the batteries with a damp (not wet!) rag.

6. Check for loose connections and corrosion each time, addressing any problems found.

Dan Fink • Buckville Energy Consulting

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