I have a 90 merc with an onboard charger that supposedly charges the cranking battery while the motor is running. I also have a built in charger hooked up to both batteries,supposedly a very good one, where I can just plug it in when I get home from fishing and it charges both batteries.

What is the best way to charge batteries? How long do you keep them charging? Some say keep them plugged in, while others say just charge for a day.

What should be run on the cranking battery and what should NOT be run off it? If it's charging while the motor is running will it be putting off more amps and damage things like spotlights Etc? Why not have the motor charging or at least partially charging the trolling motor batter also?

I have a big deep cycle 30 month Interstate battery thats 2 years old. When I use the trolling motor it runs it strong for maybe an hour then suddenly loses power. It happens fast, like one second it's running great then Bam! no power. I thought it was my trolling motor but I just tries a buddies new battery and it ran it great for a long time.

Is this a common battery failure? Could it be caused by improper charging?


Thanks for helping me out. I have no idea how all this stuff works.

2 things here: the engine battery should be capable of running most of your accessories, your trolling motor battery is probably just about to bite the dust. Have you checked the water level?

Thats how I have it now. Everything on the engine battery and just the trolling motor on the other.

Water level looks ok. If it's a 30 month battery and it's 24 months old how do they pro-rate the warranty? Do they assume the battery is worth zero at 30 months so I would get 20% back(or off my next battery)?