Variety of Types

The industrial batteries like those used for fork lifts will last the longest (after submarine batteries - not available). These come in 2volt. You buy as many as you need to get the correct voltage and the number of amp-hours (staying power) you need. These are rather too pricey for most people. The most popular batteries after golf cart and marine batteries are the Trojan L-16, 350amp-hour(ah) at 6Volts which run around $200 a piece. Most home systems are 12V to 24Volt configurations. 12 volt has fallen out of favor because of thicker wire needs but it is simpler. 24volt systems are more common especially for needs over 1KW, though 48volts are beginning to gain more favor.

The difference between a car battery and a deep discharge battery is that an auto battery is designed to supply a large amount of electricity in a short burst. Deep discharge batteries used in renewable energy systems are designed to give a moderate amount of amps for a long period of time, usually rated at a 20hr discharge time frame. Nicad batteries can be used but are 5-6 times more expensive and have trouble copying the voltage range and charging characteristics of a lead acid battery. Also, most inverters are designed for lead acid batteries and it is hard to truly gauge their state of charge.

An average 6 volt deep discharge battery has three 2.1v cells. Each made up of nine negative plates and 8 positive plates. In car batteries, there might be more plates that are thinner to provide more surface area needed for discharging. Since deep discharge batteries have thicker plates, they can withstand more discharge/charge cycles than auto batteries, hence making them more effective for renewable energy systems. The plates are made with a porous paste that allows a higher surface than just a flat plate. When fully charged, each cell will be about 2.1 volts. When discharged, it is about 1.75 volts. This makes it easier to check the charge state of lead acid batteries. This assumes that the battery has been at rest for at least 30 minutes.

Offered by Steve.