Amazon right now has this Schumacher SEM-1562A Speed Charge Maintainer
(1.5 Amp) price cut for only $19.54 (down from $59.80) FREE Shipping via Amazon Prime or on orders over $25.
* 6-volt and 12-volt battery charger; automatically changes from full-charge to float-mode monitoring to prevent overcharging
* Ideal for keeping batteries charged while in storage
* Rugged plastic housing
* Includes charger, jump cables, ring connectors
* 7.1 by 10.5 by 5.9 inches; 5 pounds; 1-year limited warranty