A break through in battery technology would be really good about now...more capacity for cost and longer life times would have a major impact for those of us going off the grid. A cheap hydrogen cell, or similar is not yet on the horizon!

Maintenance free car batteries are very nice, and for a grid connected system acid gel batteries work well with a good life expectancy. For off-grid systems though, the old traditional Flooded Lead Acid battery is still the way to go.

At night and in times of low energy generation the batteries have to support the houses system. Of course they will be backed up by a generator. The number of batteries used determines the 'Period of Autonomy' , the amount of time the house can survive at normal loads without any power generation. Systems are typically designed for 4 days of autonomy.

Batteries for off grid systems must be able to deep cycle many times. A car battery releases a short burst of high power to start the car and then is recharged very quickly so it spends most of its life fully charged. An off-grid system will partially discharge the batteries every night and recharge during the day. When power is available we want to get the batteries recharged as fast as possible with the energy available.

Sealed batteries have to be charged within limitations or the battery will overheat and be damaged. FLA batteries can be charged faster at higher current rates. When this happens the batteries will generate hydrogen gas, so need to be in a well ventilated space.  The gassing also reduces fluid levels and so regular maintenance and topping up of batteries is required. Life expectancy of batteries depend on many factors, such as the number and depth of recharge cycles they must go through and good maintenance.  With the present batteries we can expect 8 to 14 years. Good news is that with these kind of batteries the recycling is done by the manufacturer and return of old batteries gets a 'core' discount on the replacements.

Lead prices have been climbing over the last few years and so have battery prices. We can expect to pay $1,000 for a 4V 1000 AmpHr deep cycle battery.  The batteries will be the major coast factor in the total system. Later in the power usage section we will be calculating the number required for the house. (More info: Batteries - very technical)