Our slick new Voltmeter replaces the original Ammeter for safety and more reliable charging rates. Voltmeters take the guess work out of an Ammeter. Also, with a Voltmeter, full current does not have to run through it to tell you what's going on in the charging department like an Ammeter does. Voltmeters splice into the system and monitor what is going on with the charging system. Also has optional internal lighting bulb and socket.