Your batteries are the only part of your system that will wear out with use. Keeping them healthy is the most important part of your system maintenance. A well designed system will have enough PV panels to not only replace the power you use every day but to also provide enough extra charge to make up for sunless days. A small generator can also be used to boost your system.

Batteries that are allowed to go too long without being fully charged can begin to lose their storage capacity. Flooded cell batteries (the most common type in RE systems), which have a liquid electrolyte (battery acid), need what is called an equalizing charge. This is a deliberate overcharging of the batteries that causes the liquid electrolyte inside to bubble. Regular equalizing charges will maximize battery life.

Consistently undercharged batteries get sulfur deposits (called sulfation) on their lead plates. These deposits can reduce a battery's ability to hold a charge and severely shorten its life. Equalizing the batteries regularly forces those sulfur deposits off the plates and back into solution in the electrolyte.

You can tell if your batteries need an equalization charge by checking them with a digital voltmeter. When checking the voltage of your batteries you should always shut off the charging source and all power draws. The individual batteries in a system using 6-volt batteries should show no more than a three-tenths of a volt variation between batteries. Any more than that and your batteries are starting to sulfate.

There are two ways to overcome sulfation. One is to periodically give the batteries a strong charge, either with a generator, through a separate battery charger or an inverter with a built-in battery charger, or with your RE charging source. This requires a charge of at least 30 volts for 24-volt systems or 15 volts for a 12-volt system. How long it takes will depend on the amount of sulfation on the plates. A healthy set of batteries will read 25.2-25.5 volts (for 24-volt systems) when at rest, when no power is being used and no charging is occurring.

The second way is to keep a constantly high charge on the batteries from your PV panels. You can accomplish this with an adjustable charge controller that allows you to set the maximum voltage for your system at 28-29 volts (half of that for 12 volt systems). This works best when your charge capacity exceeds your average usage by at least 30%, for example 80 amphours per day of charge for systems that use 60 amphours per day.

Batteries will last longer through extended periods of minimum charging if they start out at peak capacity. Batteries will also recover faster from an extended period of current draw and it reduces the amount of time you need to run a generator when your batteries get low and the sun isn't shining.

Battery maintenance also means checking the electrolyte level in the batteries often and adding distilled water if it gets too low. With my system, since I use sealed batteries (called AGM, for absorbed glass matt) I neither need to add water or equalize my batteries. In fact, with sealed batteries equalizing will actually ruin the batteries. Sealed AGM batteries are becoming more common these days, but can be expensive when bought new. I got mine for free by scrounging them from a company that removed them from a power backup system. They had only been used for 2 or 3 years and kept at full charge for that whole time. They had considerable life left in them and the guy I dealt with was happy to see me put them to use. If I hadn't taken them he would have sent them off to a smelter to be recycled. A waste of good battery life, if you ask me.

Please remember these safety points when working with your batteries.

1. Always wear old clothes, battery electrolyte is sulfuric acid and it will destroy your favorite shirt. I prefer to wear old sweatsuits
2. Remove all rings, watches, jewelry, belt buckles or any other metal from your person (yes, even jeans with rivets.).
3. Always Wear Goggles. Believe me you don't want battery acid in your eyes and a pair of cheap yellow rubber kitchen gloves will protect your hands.
4. Make sure the tops of the batteries are clean and free of debris. Use a clean sponge and clean clear water to wipe the batteries down, if necessary, before you remove the caps. If the batteries were overfilled previously it is possible for some of the electrolyte to overflow during equalization. If this happens us a mild solution of baking soda and water on the battery tops to neutralize the acid, then wipe them down with clean water (don't get any of the baking soda in the battery, it will neutralize the electrolyte)
5. Remove the battery caps and check each cell with a flashlight. The surface of the electrolyte should always be covering the battery's plates and never higher than just below the bottom of the filler hole.
6. Use only distilled water to refill the battery. Be careful not to spill or splash when filling as this can cause the acid electrolyte to get on the one place on your body you failed to protect. I use a small plastic cup to refill each cell, it takes a bit longer but it is a lot safer than pouring the water out of the jug.
7. Make sure the area around the battery box is clear of debris and clutter at all times. When maintaining your batteries keep the area clear of dogs, cats, kids or anyone else who might get in your way.
8. Batteries need to be in a sturdy enclosure, vented to the outside. Make sure you can get to the tops of all the batteries easily and that the box is secure from children and pets.

Good battery maintenance takes very little time and the rewards of a few minutes a month will be healthy long-lived batteries that will serve you well for years.