Charging starter battery and trolling motor battery

[Guest]

Can I use the motor to charge both my starter battery and my trolling motor battery?

 

[jtexas]

Re: Charging starter battery and trolling motor battery

what model/year/hp?

 

[Silvertip]

Re: Charging starter battery and trolling motor battery

The answer is yes -- but how much of a benefit it be for the troller battery is questionable. Troller batteries tend to get deeply discharged so with most midrange and smaller outboards, you would need to run for miles at wide open throttle to add any appreciable charge since those motors have pretty small alternators (low output). Even some older big engines have rather small alternators so the same applies to them.

 

[Guest]

Re: Charging starter battery and trolling motor battery

My motor is a 2003 Yamaha 50 HP, 4-stroke.

 

[Silvertip]

Re: Charging starter battery and trolling motor battery

You have a 17A alternator. That's big enough to at least allow you a little longer time on the water but it will not keep the troller battery fully charged. You need to add an on-board charger so you can plug in at the dock. Or else use a portable charger. Failure to keep that battery topped off will shorten it's life. Just to give you an idea how electricity works, if you ran a trolling motor for just three hours at 10A rate (slow setting) you would take 30A out of the battery. Your alternator has a maximum 17A output so you would need to run nearly two hours to fully charge the troller battery.

 

[Boatist]

Re: Charging starter battery and trolling motor battery

I will disagree with Silver some here. Even if your alternator can supply 17 amps it will not. Yes when it is deeply discharged it will come close to 15 amps or so but as it charges it will not. The alternator will supply voltage of somewhere between 13.8 volts and 14.8 volts and in the begining with the battery deeply discharged it may take 15 amps. The voltage will remain constant but as the battery charges the amps it takes will drop. When it is half charged it will likely only charge at 5 or 6 amps and will take many hours to finish the charge. The bass boat I fish on with a good friend we charge a group 27 deep cycle with a 20 amp automatic charger and after a days fishing it still takes 10 hours or more to fully charge.

I do agree with Silver if you do not recharge it right away it will shorten the battery life. Really a deep cycle battery should not be discharged more than 50 percent but we all do it. The longer it is before it is fully charged the shorter the battery life will be.

 

[Silvertip]

Re: Charging starter battery and trolling motor battery

Boatist -- I agree. The numbers I used were simply to illustrate the futility of trying to keep a deep cycle fully charged using the engine alternator.

 

[Boatist]

Re: Charging starter battery and trolling motor battery

SilverTip
We agree completely then. Charing on the water can give you a little more time and help getting it out of the deep discharge damage zone but not practicle to fully charge or charge with a small 10 or so alternator.

 

[jtexas]

Re: Charging starter battery and trolling motor battery

out of curiosity...what effect if any would it have on the alternator, in terms of wear & tear or longevity?

any thoughts?