A battery charges when an external electrical current is applied to it. This current causes the battery's chemical reaction to reverse, storing energy in the battery's electrodes.
The time it takes to charge a battery depends on the type and size of the battery, as well as the charging method being used. Typically, it can take anywhere from a few hours to several days to fully charge a battery.
Yes, overcharging a battery can damage it by causing the battery to overheat and potentially explode. It can also shorten the overall lifespan of the battery.
Most batteries have a built-in indicator that will show when the battery is fully charged. This can be a light on the charger or a change in the color of the battery's indicator. Some batteries may also stop accepting a charge when they are full.
The number of times a battery can be charged before needing replacement depends on the type and quality of the battery. Generally, rechargeable batteries can be charged hundreds or even thousands of times before they need to be replaced.