Battery Tender Charger. A battery tender® is a trademarked name but is also a trickle charger and a maintainer as far as how it functions. All battery tender® smart battery chargers automatically fully charge and maintain your battery safely for months at a time.
The best way to test a tender (or charger for that matter), is to use a known good battery, and stress it using a load checker then read the output of the battery, then connect the charger and read the output of the battery. Battery tender 6/12 volt 3 amp selectable battery charger. Battery tender junior 12v, 750ma battery charger and maintainer: