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Battery Equalization

I've been a reader since HP1, and have upgraded my off-grid solar-electric system four times, from one module to thirteen; from a 12-volt, 600-watt inverter to a 24 V, 4,000 W unit; from 6

V golf cart batteries to 2 V submarine batteries to three sets of 6

V deep-cycle L16s.

The main problem over the last 20 years has been battery life. I've talked with a few installers who advised me to equalize the batteries twice or even four times a year. Even with the sunny climate here in Hawaii, equalizing at 30 V for three to four hours or until the specific gravity reaches the right place on the hydrometer on all the batteries without using a generator is impossible.

When wired into the 4,000-watt Trace charger/inverter, a generator will charge the batteries at 30 V when it is idling. Is that sufficient, or should I run the generator at the maximum of 13.8 amps? How many amps are needed to equalize my eight L16 batteries? How often should they be equalized, and for how long? What specific gravity should I strive for?

Karim Wingedheart • Maui, Hawaii

I remember you well—you've been off grid about as long as I have! For battery equalization—a controlled overcharge of the battery—I use the battery ampere-hour meter to determine overcharge and try to overcharge the batteries each day by at least 2% of their capacity. Weekly,Isetthe PV regilatorsto equalize and overchige the batteries by at least 5% of their ampere-hour capacity. Monthly, I use the generator to overcharge the batteries by 10% of their capacity. I don't pay attention to the battery voltage during equalization, but to the amount of overcharge (in ampere-hours) that the battery receives. I don't routinely do hydrometer measurements.

This regimen requires having an array large enough to not only meet electrical consumption demands, but also to provide needed overcharge. If your array cannot do this, consider adding more modules. If the array is not up to the job, then an engine-generator must be used.

I try not to use the batteries too heavily. My average depth of discharge nightly is about 12% of the battery capacity. On most sunny days, the battery is fully recharged before noon (and then the daily overcharge begins). If the battery drops below 30% depth of discharge, I start the generator. I use the generator an average of 160 hours per year—mostly during the depth of Oregon's cloudy winters.

Battery makers and distributors are just now becoming aware of the type of cycling that a battery sees in PV service. Their usual recommendations are for batteries that are deeply discharged, then refilled rapidly (a far tougher service than in a well-designed PV system).

If you are using the generator to equalize, do it at least monthly. Voltage doesn't matter—it's the amount of overcharge ampere-hours that matter. Recharge until the ampere-hour meter shows a 10% overcharge, and don't forget to add water to the electrolyte as needed.

Richard Perez • Home Power

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