A rechargeable battery is recharged by reversing the chemical reaction that occurs within the battery. But a rechargeable battery can only be recharged a given number of times (recharge life). Even built-in batteries, cannot be recharged forever. Moreover, each time a battery is recharged, its ability to hold a charge goes down a bit. Non-rechargeable batteries should not be charged as various harmful substances can leak out, such as potassium hydroxide.