A battery's discharge rate vary depending on the battery's design and the internal and external conditions of the battery.
Conventional "flooded" or "wet" lead-acid batteries, fr example, have a higher natural discharge rate than Gel-Cell and Absorbed Glass Mat (AGM) batteries do.
All lead-acid (flooded, gel-cells, and AGM batteries) discharge faster at higher temperatures. BatteryStuff.com (which sells different types of batteries and chargers), for example, says that a battery left at 110-degrees (F) for 30 days probably won't start a car.
Low temperatures slow the battery's discharge rate, but also slow the chemical reactions that release energy from the battery down, so cold batteries can't produce as much power as warm ones, either.
As lead acid batteries get older or are abused by excessive discharge they become "sulfated" by the formation of hard lead-sulfate crystals on the surface of the lead plates in the battery. Where the sulfation collects on the plate the battery can no longer store or exchange electrical
charges, which also speeds the discharge process.