A battery is a means of storing and releasing electrical energy. A battery releases the stored electrical energy at a certain voltage: 12v for most automobile batteries, 6v for some marine batteries, 1.5v for most batteries for consumer electronics (eg. the AA, AAA, C, D batteries.) As a battery becomes drained, this voltage decreases; slowly at first and then more rapidly as it is increasingly depleted. At a certain point, the battery continues to hold a charge but is no longer useful for its intended purpose. In the case of a 12v car battery, this point is 10.5 volts, which is used as a cutoff point for a lot of measurements.

The measurement that is rarely discussed for consumer-grade batteries but of constant interest for RV, solar power and houseboat enthusiasts is amp-hours (Ah). Ah is the measure of the battery's lifespan, or at least until the battery's 10.8v retirement.

A typical rating for your average car battery is 120 Ah, suggesting that you could drain the battery for 120 hours at 1 ampere, or for 2 hours at 60 amperes, or even 1 hour at 120 amperes.

But this assumption is wrong. Draining a battery at twice the current burns it out in less than half the duration, and that's because of the Peukert Effect:

I n T = C

where I is the discharge current, T is time, and C is the Capacity loss, and n is Peukert's Exponent, which varies with battery construction methods. (Units for I is usually amps, T in hours, and C in amp-hours). Usual values for n are 1.05 for a really flexible (read: expensive) battery to 2 for a battery intended for a very slow drain.

In English: The faster you drain a battery, the fewer total amp-hours you get out of it. If you drain a battery at twice the amps, it will last less than half as long.

The usual method to minimize this effect is to build banks of batteries, connected in parallel. The drain to each battery is reduced, so the total Capacity loss is reduced. Thus, two batteries will last slightly longer than twice the life of a single battery.