This will help when you have a battery that will completely charge but doesn't seem like it holds a charge when your out on the lake, trolling batteries.
I picked this up at Harbor Freight for a little over $20.00.
You charge a battery (make sure the cells have water in them). Hook it the device up to the battery and test it with 100 amp load for 10 seconds to see if it maintains the cold cranking amps printed on the battery. If it drops off the battery needs to be replaced.
There are a number of videos on YouTube demonstrating how to use it!