A deep-cycle battery is a battery designed to be regularly deeply discharged using most of its capacity. The term is traditionally mainly used for lead–acid batteries in the same form factor as automotive batteries; and contrasted with starter or 'cranking' automotive batteries designed to deliver only a small part of their capacity in a short, high-current burst for cranking the engine.
For lead-acid deep-cycle batteries there is an inverse correlation between the depth of discharge (DOD) of the battery and the number of charge and discharge cycles it can perform; with an average "depth of discharge" of around 50% suggested as the best for storage vs cost.
Newer technologies than the traditional lead-acid (such as lithium-ion batteries) are becoming commonplace in smaller sizes in uses such as smartphones and laptops. The new technologies are also beginning to become common in the same form factor as the automotive lead-acid batteries, although at a large price premium.