Amazon offers its Amazon Prime members the Schumacher SpeedCharge Automobile Battery Charger, model no. SC-600A, for $20 with free shipping. (Non-members can spend $25 or more to qualify for free shipping; otherwise, shipping costs $6.49.) That's $7 under our May mention and the lowest total price we could find by a buck. (Most merchants charge $31 or more.) It features 50-amp clamps for top and side-mounted battery posts, 2-, 4-, and 6-amp charging capability, and adjusts charge rate downward to prevent battery damage.

Amazon Prime membership is $79 per year; however, Prime is free for college students for six months, and free for parents and caregivers for three months.