Why does your car battery self discharge?

Your car battery can self-discharge on the basis of chemical reaction, or due to temperature changes (when it gets colder, it has less amperage available). It can also be caused by parasitic draw due to a improper connection or a device left on. Typically if the electrical system has been evaluated and there is no parasitic power-off draw, then the battery's chemical composition may have been compromised or a cell may have been crossed. This means your battery is bad and typically needs to be replaced.