Self-discharge decreases the shelf life of batteries and causes them to have less than a full charge when actually put to use.
[1] How fast self-discharge in a battery occurs is dependent on the type of battery, state of charge, charging current, ambient temperature and other factors.
[2] Primary batteries are not designed for recharging between manufacturing and use, and thus to be practical they must have much lower self-discharge rates than older types of secondary cells.
Self-discharge is a chemical reaction, just as closed-circuit discharge is, and tends to occur more quickly at higher temperatures.
Self-discharge is also thought to be reduced as a passivation layer develops on the electrodes over time.