Vicar,

As bugseigel said, there are two devices. The first is the charger. It's like any charger except it is magic and won't mess up your batteries, yet it keeps them charged. Also you need a 12v battery to run it.

The second unit is a tester. You simply plug the battery in, the unit discharges the battery and gives you a readout of it's capacity in ma (Mili amps). On an older battery, the two units used, one after the other, will recondition a battery back to it's new or better condition. Of course that is assuming the battery is not shorted out.

Bish
K7QCP