Battery Tender

Battery Tender, an automatic battery charger, is smart enough to continuously monitor your battery and adjust from full-charging power to low-maintenance power, as needed. Your battery won't overcharge or discharge. This handy charger can actually pay for itself over time, because a battery kept at full charge lasts longer. The Battery Tender can even charge a totally dead battery in 1-2 days. Reverse-polarity safety guard protects you from injury if it's connected backward.
Comes with two types of connectors: a small harness you permanently attach to the vehicle, and a set of alligator clamps. Plugs into standard wall outlet. Available in 12-volt and 6-volt versions.