Home Depot offers the Sunforce 1.8-watt Solar Battery Maintainer, model no. 50012, for $16.97. Opt for in-store pickup via ship-to-store to avoid the $5.99 shipping charge. That's the lowest price we could find by $8. It's designed for use in cars, boats, ATVs, and personal watercraft and features a 12-volt DC plug for cigarette lighters.