I use the Griots Garage Battery Manager IV.

Battery Manager IV - Battery Care - Car Maintenance - Griot's Garage

The advanced, microprocessor controlled, quadphase charging process ensures you don't have to worry about a thing. It even has an automatic "desulphation mode" that sends an electrical pulse to help revive sulphated batteries that are dying! Performs its magic on any 12-volt or 6-volt battery, including standard lead-acid and AGM batteries used in motorcycles and boats. There are three ways you can connect the manager to your battery, all included: simple alligator clips, permanent rings, or cigarette lighter adapter, all with a quick disconnect plug. We also include an adaptor for use with the previous barrel-plug style connector so you can upgrade without having to change your current setup if you already own one of our battery managers! This is not the typical "trickle charger" you've seen elsewhere. For safety, a built in electrical detector prevents sparking and the error light glows if the polarity is crossed. This device is designed with modern integrated circuitry to keep batteries at full charge without overcharging or boiling out battery fluids