Why is a battery bank necessary? Because an off-grid system does not allow you to store electricity easily and economically. There are other ways to store electricity but they are more expensive.

Battery Selection

With thousands of batteries available, the selection process is very complex. There are many applications of battery use (cars, boats, trucks and telephones, for example) and there are "special" batteries such as NiCad, lithium ion and nickel metal hydride. Which is the best choice for a renewable energy system?

First, let's clear up some of the myths of quick fixes:

• Used batteries of any kind are not suitable. A deal that looks good or cheap usually comes with problems – batteries become useless after their life span expires or if they are incorrectly serviced. They may work for a couple of years but offer lower capacity.
• Quality batteries are expensive. A golf-cart battery that has a similar capacity to a high-quality, "deep-cycle" battery costs less, but has a shorter life span and is only good for small electrical loads or when there are short grid interruptions. It is dangerous and time-consuming to change batteries - it should be done once or at most twice in your life - so it's worth getting the best.
• NiCad or lithium ion batteries may function well in your cell phone and camcorder but they're not designed for a renewable energy system: you'd need so many that they'd cost you the same as another house!
• The best for a complete renewable energy system is the industrial lead-acid, deep-cycle battery with a liquid or gelled electrolyte which is "maintenance-free". Such batteries are solidly established with re-engineering research to offer the best value for your money. These batteries last around 15 or 20 years if they are taken care of properly. They are also recyclable, with many companies offering trade-ins or a refund allowance for old units which is a plus point for the environment and your green energy concept. Conversely, NiCad batteries and other fancy blends are a hazardous waste.

How Batteries Work

The conventional deep-cycle battery, such as a car battery, has a lead-acid composition. It has a plastic case for the single-cell battery to hold a group of lead plates made up of varying compositions. The suspended plates are placed in a case filled with weak sulfuric acid solution known as the electrolyte. This can come in gelled form to avoid spillage. Such batteries are "maintenance free." The lead plates are connected to a positive and negative terminal as with C and AA cellls. Such single-cell batteries have a nominal 2V rating.

An electrical load connected to the battery causes sulfur molecules to form from the electrolyte which bond with the lead plates and cause electrons to be released. These electrons flow through the conductors from the negative terminal to the load before flowing back to the positive terminal. This is repeated until all sulfur molecules are attached to the lead plate, which leads to a discharged or dead cell state.

When that happens, the acid becomes weaker. The battery electrolyte's