Most passenger vehicles use a 12-volt battery system. When your battery has a full charge, it should be around 12.5 volts on your voltmeter, which is the battery gauge in your vehicle. Once you turn on the engine, your voltmeter should show around 14-14.5 volts.
If your battery has a charge below 12.5 volts when the engine is running, there may be a problem with your alternator or charging system that will require maintenance.