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

[Gumbo]

Badfish, I have 3, sometimes 4 batts aboard my boat. Hooking them up to a bench charger and charging them all individually would take a week to bring all of them back to a full charge. And the last battery charged has sat a week in a depleted state, shortening its lifespan. With an onboard charger, you plug into the nearest outlet at home, hotel, campsite, restaurant, and they're all fully charged by morning.

I agree that I wouldn't leave an onboard charger plugged in indefinitely over winter, but I know guys who do. And if you listen closely, you might hear them panting.