How do you know when your trolling battery gives up its ghost?

[fisherman]

The best way is to use a hydrometer to check the specific gravity of each cell. It only takes 1 bad cell to ruin the battery.
Autozone sells hydrometers for $5.00. I use the gauge on the charger, then check it with the hydrometer. You can see how well the battery is holding a charge by the specific gravity reading in each cell.
It looks like a giant turkey baster that you suck up the electrolyte and take a reading with.[cool]