Amazon offers its Prime members Jensen's Schumacher 1.5-amp Slow Charge Battery Companion, model no. SEM-1562A, for $19 with free shipping. (Non-members can spend $25 or more to get free shipping; otherwise, shipping adds $6.49. Also, Walmart charges the same via Site-to-Store.) That's a buck under last month's mention and the lowest total price we could find by $2. (Most stores charge $30 or more.) This automatic battery charger maintains a battery's charge for vehicles that aren't used every day. It features reverse-polarity protection for both 6- and 12-volt batteries.

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. (Note that currently there's a waitlist for Amazon Mom.)