Any standalone off-grid solar system needs a Rechargeable Battery. Nearly everyone I can think of, is used to the idea of emergency lights and cell phones. In fact, battery life and charging time are two points of great interest in cell phones. Whether the source of power is the electricity grid or sunlight, we use it to charge up our batteries for use ‘on the move’ or in darkness. Some popular rechargeable batteries are lead acid, NiCd, NiMH, Li-ion and Li-ion polymer. Lead Acid batteries are a popular choice for  standalone off-grid solar system because they can store a lot more charge than the other kinds of batteries. One of the major causes for the fall of the Roman Empire is the lead that leached from their water pipes into their drinking water and food causing gout. (Reference.) Of course there are the socio-political and economic reasons as well. But it is very important for our society to take care of properly recycling our lead acid batteries. It is normal to use 6V and 12V batteries in solar power systems. Because of the internal resistance of the batteries, the actual terminal voltage reduces with the current load on the battery. Also the battery gets discharged as it is used to power the various circuits and appliances. Once a battery reaches it’s cut-off voltage it should be disconnected from the load. Every battery has a Storage Capacity based on it’s internal chemistry. A practical measure of this is Ampere Hours. A current load of 1 A for 1 h  or a load of 2 A for 0.5h require a battery storage capacity of 1 Ah. A battery with 150 Ah storage capacity can deliver a 15 A current for 10 hours. Further References:

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