The ammeter indicates whether or not the alternator or generator is producing sufficient current to power the electrical components and recharge the battery. The instrument has the zero point in the middle. When the needle is shifted toward the positive side on the right, it indicates that the battery is being charged. When it is shifted toward the left, it indicates that more current is being drawn from the battery than is being replaced. A full-scale negative deflection means that the alternator or generator has failed. A full-scale positive deflection means that the voltage regulator has failed, allowing the battery to receive too much charge and potentially overheat. In this case the circuit breaker for the alternator / generator should be pulled to prevent the possibility of the battery catching on fire.