This is a Q&A Question from Home Power #48 which deals with battery equalization and touches on battery water use as a measure of charge performance. Posted with permission. http://www.homepower.com

*In discussions of battery charge/discharge rates the term C/x refers to the battery capacity in ampere-hours divided by some factor (as in C/10-C/20 below) DCK

Battery Equalization

A Reader Question on Battery Equalization:

Every time I think I understand Equalization I read something that gives me a problem. In HP #42, your article Build Your Own 12 VDC Engine/Generator (available in the Home Power Website download area at http://www.homepower.com as MARK8.PDF) I conclude that to equalize charge you should recharge between C/10-C/20* for at least five hours with battery voltage at or above 16 Volts. If this is so, then it appears I can not equalize with my current equipment. I have a set of Trojan L-16s (350ah@12vdc), three Kyocera LA51s, a Trace 812SB and a C30A controller. If I understand your article I should charge my battery pack for five hours at 16 Volts with at least 17.4 Amps. My PVs are rated at 16.9 Volts but the maximum Amps is only 9 Amps-OK on the Volts but the recharge rate is C/38. The Trace charger is 25 Amps but it has a maximum 14.7 Volts before it cuts off-OK on the amps but cant get to 16 Volts.

I live in SE Arizona and even here it would be difficult to get five hours at 16 Volts with a C/20 rate even if I had three additional panels. I have had this system for two years and the method I have been using is to boil the battery for 3-4 hours, one day a month. This brings the battery up to 15-15.5 Volts. I have my controller set to shut off at 14.7 Volts. Very seldom (four or five times a month) does the controller not shut off every day, so I dont think I am deep cycling the battery. I am using about 40 Amps a day, on average. Also I have used about 1 1/2 gallons of water in two years (regular caps-not Hydrocaps).

Two questions: 1) Am I taking proper care of my battery? 2) Is there any way to get the 16 Volts-C/20-5 hours charge without building an engine/generator like the one in the article? I already have a Honda ES6500 generator to run my well pump and Trace 812SB. Would appreciate any help you can give me. Thanks!! Tom, Wilcox, AZ

The answer from Richard Perez: Hi, Tom. Dont worry you are doing OK by your batteries, but you could be doing even better. First off, you dont have to get the battery up to 16 VDC to equalize. If you do equalize at a C/20 rate or faster, then expect the battery voltage to go over 16 VDC. In fact, the voltage is allowed to float during an equalizing charge  it really doesnt matter what the voltage is. What counts is giving the already recharged cells an overcharge. This overcharge can be at rates far slower than C/20, but the overcharge must continue for a longer period. For example, a 12 Volt lead-acid battery equalized at C/40 or so will only attain voltage in the region of 15.5 VDC. The actual voltage depends on temperature, colder cells will show higher voltages during equalization.

In your case I would recommend setting your Trace C30A regulator at a higher voltage such as 15.0 to 15.2 VDC. Set the return voltage at above 14.3 VDC. This will give the battery micro-equalizing charges on a daily basis. If your controller is shutting off the PV power daily, then you are wasting valuable equalization power.

From the description of your water usage, you are undercharging your cells. You should use a gallon of water in those two Trojan L-16 batteries every six months. Water consumption is a very good indicator of cell overcharging. When I flew a pack of two L-16s, they would use a gallon of water every two months. I flew the battery hard, with daily deep cycles and regular equalization. I would equalize about once a week at C/20 for five to seven hours. That set of L-16s endured twelve years of brutal service. A more modern approach is to set the voltage regulators in the system to provide daily micro-equalizing charges. The key is watching water consumption. Gallons per month mean too much overcharging. A gallon per year means too little overcharging.

In your case, if you want to equalize from your generator, then get a new battery charger. Statpower makes a series of very smart, three-stage chargers, that will run great on your generator. And it will equalize. Almost all of the battery chargers built into inverters are anemic (not necessarily true anymore -DCK 8/23/99) , but they are inexpensive since the inverter already has a big transformer anyway.

(Note that the L-16's discussed form a 350ah battery and factor your battery water consumption by this).

-- Don Kulha (dkulha@vom.com), August 23, 1999