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Deep cycle batteries are designed with the intention of having a high percentage of their stored electricity removed on a regular basis. On the other hand, shallow cycle batteries are engineered to provide infrequent stand-by service, with only 15-20% of their stored power intended for use. Examples of deep cycle service are golf cars and fork lifts, shallow cycle examples would include hospital and telephone back-up batteries. Deep cycle batteries are the best and indeed the only suitable choice for solar electric storage.
Battery capacity is expressed in amp-hours at a given discharge rate (X A/hr @ Y hour rate) to full discharge. For example a Surrette KS 13 battery has a capacity of 788 amp hours at the 72 hour rate. Another way to express this would be to say that this battery would be fully depleted if an 11 amp load was continuously connected for 72 hours. (Rated battery capacity divided by the hour rate determines the size of load). The number of times a battery can be cycled before it is worn out increases significantly as the percentage depth of discharge declines. A battery which is required to deliver 50% of its power will provide 30% to 50% more cycles than when it is drained to 80% depth of discharge. In other words the battery will last approximately twice as long if it is continually being charged.
Another way to look at how batteries work: there is sulfur in the acid that has a tendency to settle, and become a solid, as the battery is discharged. To keep the sulfur mixed with acid the battery has to be recharged. A battery that is being charged continuously has a boiling effect to keep the sulfur from separating from the acid and becoming a solid. This produces a longer battery life.
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