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 tried 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? Or maybe my trolling motor having an issue?


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

If you have a fully automatic onboard charger you should be able to leave it plug in until your next use.

The motor will only charge the cranking battery, but not fully. You still need to charge it after each use.

All accessories can and should be ran off the cranking battery. 14v is common when motor is running and battery is charging. I wouldn't worry about too many volts unless it is 18v or so when charging.

As for your trolling battery it sounds like it is bad. I would take it back while it is still under warrenty and try to get a new one.

I leave mine plugged in all the time it is in storage. Make sure your charger is automatic, most are. Your battery running down fast is normal for a worn out battery. Deep cycle batteries run at full power until the charge is gone. When this happens, they go downhill fast. Regular lead acid batteries lose their power gradually until they are dead. Deep cycles maintain full power until they are dead and then they just fall off quickly. If your battery is not lasting long it probably needs to be replaced. Nothing should be run off the cranking battery other than the starter. Trolling motor batteries should be used for everything else. Your engine driven alternator is designed to charge one battery. It does not have the amp output to charge more than one battery at a time and could cause it to trip off line if you try to do so. Cranking battery for starter only. Alternator to charge only the cranking battery. Trolling batteries to power everything else, and charge them with onboard or external charger. Leave automatic charger plugged in all the time.

Gary, I disagree with you on the matter of the trolling motor batts running everything but the starter. If you charge your cranking battery correctly (and often), then everything but the trolling motor can be ran from the cranking battery. I have all my electronics (two GPS units, pumps, lights, etc.), running everything--the only item I have running on my trolling batts is the trolling motor. Haven't had an issue with anything...yet :-). Bottom line here is charge your cranking battery--whether you use an onboard charger or manually, before you head out to the lake--it is an integral part of your arsenal!

My experience, the cranking battery can handle the accessories just fine as long as your motor is charging the way it should - leaves more power for trolling. Might make an exception for a bait tank running full time, or anchored for hours with a high-power stereo amp.

Always charge your trolling battery soon as you get home - too much time at less than 100% charge shortens its useful life. When you discharge a battery (thru a trolling motor for example), sulfur builds up on the plates. Recharging converts the sulfur back to sulphuric acid. Too many hours without recharging, the sulfur hardens & can't be converted ("sulfation"). That portion of the plate (and battery capacity) is lost.

Note that when these guys say "automatic" charger, they mean one with "maintain mode", or "float charge", where it turns itself off & on as needed; that's the best way to maintain batteries. It's *not* the same as "trickle charge" (constant 2-amps), which is ok for overnight charging but don't leave it on 24/7. If you don't have float charge, just top 'em off every couple weeks if you don't use 'em.

Main causes of premature battery deaths: heat and lack of maintenance. Check the water, more often in summer, add only distilled water, enough to cover the top of the plates.

Avoid discharging a cranking battery below 80% (about 12.5 volts) or a deepcycle below 20% (12.03 volts).