The Schumacher Battery Monitor allows instant monitoring of both battery and charging system voltage, alerting you once the battery drops to 12 volts, giving you approximately one hour of run time, depending on the system"s wattage. With the mounted display on your car"s dashboard, you will have constant monitoring of your electrical system, with LED warning lights alerting you when your battery or charging system is not functioning properly. The digital display is easy to read, indicating the electrical voltage of your battery. The monitor detects charging input voltage within .1 volts, and detects problems with the alternator, diode and any variance of more than .8 volts. Conveniently plugs into a 12 volt automotive cigarette lighter receptacle.