Deep-cycle lead-acid battery is designed to be regularly deeply discharged using most of its capacity. In contrast, starter batteries (e.g. most automotive batteries) are designed to deliver short, high current burst for cranking the engine, and to be frequently discharged of only a very small part of their capacity. While a deep-cycle battery can be used as a starting battery, the lower “cranking amps” imply that an over-sized battery may need to be used.

The structural difference between deep cycle batteries and cranking batteries resides in the lead battery plates. Deep cycle battery plates are thicker, active plates, with higher-density active paste material, and thicker separators. Alloys used for the plates in a deep cycle battery may contain more antimony than for starting batteries. The thicker battery plates resist corrosion through extended charge and discharge cycles.

Major modes of failure of deep-cycle batteries are loss of the active material due to shedding of the plates, and corrosion of the internal grid that supports active material. The capacity of a deep cycle battery is usually limited by electorlyte capacity and not by the plate mass, to improve life expectancy.

A deep-cycle battery is designed to discharge between 50% and 80% depending on the manufacturer and construction of the battery. Although these batteries can be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 50% discharge, as there is a direct correlation between depth of discharge on your battery and the number of charge and discharge cycles it can perform.