OptiMate TM-500 DC to DC charger draws power from your vehicle’s or an external 12 V battery to save, charge and maintain another 12 V battery. The OptiMate TM-500 is has a highly efficient long term maintenance mode protects the source battery, drawing only when the battery under charge needs it. The OptiMate maintenance mode keeps the battery at full charge, never overcharging, prolonging the life and improving performance of the battery. The OptiMate TM-500 can save a deep discharged, sulfated battery from as low as 1 Volt using a larger source battery (ideal 1.5x capacity) or DC Supply without fear of discharging the source thanks to OptiMate's Source Battery Protection.*Flat Rate shipping only vaild within the 48 United States.