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can my merc 40 charge my trolling motor battery as well as the outboard battery?

E

ertauber

New member
I have a merc 40 about 15 years old. It does charge its own battery but can I hook that battery to my trolling motor battery in parallel?
 
you can but its not advisable to do so..first of all your charging system is probably only 6 amps or so..to try to charge one rundown battery with it causes too much stress on the system...to try to charge two will eventually cause a failure..those battery charging systems ,in that size motor in particular, are designed to maintain a charge...another problem will be that you are pulling current from both batteries when running the trolling motor...thats a good way to have no ignition when you need it...another problem is that sooner or later the weakest battery or a shorted battery will pull them both down...they build diode packs and switches to do what you want to do but unless you have the 35 amp charging system its not worth the trouble..just my opinion..
 
thank you for the info. Probably saved me from ruining my system. Guess I just have to remember to put the charger on the batteries every night before fishing next day. Florida may be chilling right now but fishing is still great.
 
Put the batteries in parallel as you wanted, but install a high power switch between them to enable you to isolate the trolling motor battery. So, when starting the outboard, isolate the switch (open position). After the motor is running, close the switch (trolling motor battery can charge). Make sure the batteries were charged the night before (no high demand from the charging system). When using the trolling motor, isolate the switch again. Make sure the trolling motor is connected across the correct battery (not the main outboard motor battery), so that it does not drain the outboard motor. Make sure to keep the switch isolated until the outboard is running again (if you used the trolling motor so much that it's battery is flat, keep the switch open so as not to kill the charging system). The high amp switches are normally available from auto stores.
 
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