The chemistry that makes batteries rechargeable also makes it easier for other side reactions to occur. These side reactions are not always detrimental, but it does mean that instead of all the energy being stored indefinitely inside the battery cell, some can be lost in powering these reactions.
The result is a battery that loses some of its chemical energy over time without even being in use. Depending on the battery chemistry, this can reduce the future capacity of the battery if it isn't completely recharged before use. Some batteries can even be irreversibly damaged if they are over-discharged. The end result—manufacturers recommend charging the battery before use.
Not all batteries are the same, however. The lithium-ion batteries in mobile phones have a fairly good self-discharge rate of around 2–3 percent per month; lead-acid type batteries tend to lose 4–6 percent per month; nickel-based batteries lose around 10–15 percent of their charge per month; non-rechargeable alkaline batteries only lose around 2–3 percent of their charge per year.