Like other renewable energy systems, solar energy systems use a lead-acid deep cycle battery. This type of battery is different from a conventional car battery, as it is designed to be more tolerant of the kind of ongoing charging and discharging you would expect when you have variable sunshine from one day to the next.
Lead-acid deep cycle batteries last longer but they also cost more than a conventional battery. The major difference between lead acid batteries and other batteries is that they have solid lead plates; in conventional car batteries, the plate is made of a sponge-like material. You may not be able to tell this difference simply by looking at the battery, and some batteries are labeled "deep cycle" when they are not. If you are looking at a battery that is rated for cranking amps, this will likely mean that it is not a deep cycle battery.
You would use a battery in an independent solar energy system, and it is also used in some grid inter tied with battery backup systems.
Battery Size to your System
Be sure that you have a battery properly sized to your system. If you size your batteries too small, they may discharge too often (even for lead-acid deep cycle batteries) so you would have to replace them more often. When you size your battery to your system and maintain it well, it will last 10 to 15 years. This will make it well worth the investment in money and time. A solar installer will understand how to size your battery properly.
Depth of discharge
Depth of discharge is a measure of how much energy has been taken from a battery. With the lead-acid deep cycle battery used in a solar electric system, there is more tolerance for discharging. You can discharge the battery of a solar energy system 50% to 80% with no damage to the battery. This makes it very different from a car battery, (also called a start battery).
Want to know more? There are several kinds of deep cycle or deep discharge batteries. The most common kind is assumed here: flooded cell batteries. These can withstand overcharging, are low cost (compared to others) and built to have a long life.
Other kinds also have their advantages and features, including the following:
- Sealed flooded cells are maintenance free but can be damaged by overcharging.
- Recombinant flooded cells are more expensive but will not spill acid.
- Gelled electrolyte cells are also expensive and are more versatile in mounting.
A word of caution
Like all large batteries, the batteries for your solar system can be dangerous. According to Prevent Blindness in America there are over 6,000 serious accidents every year involving car batteries. Be very aware of the dangers of any battery you use: lead-acid batteries contain a diluted sulfuric acid electrolyte. It is highly corrosive in both liquid and gas form. Use protective glasses any time you are working with your battery.