Charging trolling motor battery with main engine electrical help needed

[madman2021]

I've been working on this for a while now and need to get some help. I am trying to charge my trolling motor battery with my mercruiser 140 engine alternator. I have a solenoid rigged up to connect both the main battery and the trolling motor battery only when the key is on and engine is running. When the key is turned off the trolling motor battery is disconnected from the alternator circuit and the main battery. I used a trim solenoid for the power trim thinking it was going to hold the current but it's getting really hot. What solenoid could I use instead. The trim solenoid I used was an old one I pulled off when I replaced my trim solenoid. Any help to improve my design is much appreciated. Thanks.

 

[Bondo]

Re: Charging trolling motor battery with main engine electrical help needed

Ayuh,.... Ditch the solenoid idea, 'n get 1 of these...

 

[madman2021]

Re: Charging trolling motor battery with main engine electrical help needed

If I used a perko switch then every time I start and turn off the motor I need to turn the switch on/off by hand.

 

[fucawi]

Re: Charging trolling motor battery with main engine electrical help needed

The best charge current you will get is 25 A rapidly dropping to 15 and below ...so even if you average 10 A you are looking at 14 hrs for a recharge ....

 

[Bondo]

Re: Charging trolling motor battery with main engine electrical help needed

If I used a perko switch then every time I start and turn off the motor I need to turn the switch on/off by hand.

Naw,... Just put it on All, 'n go boatin'....

The best charge current you will get is 25 A rapidly dropping to 15 and below ...so even if you average 10 A you are looking at 14 hrs for a recharge ....

It's a 140 I/O, probably with the 80 amp alternator...

 

[Silvertip]

Re: Charging trolling motor battery with main engine electrical help needed

Look at this way. Your trolling motor likely draws about 40 - 50 amps at the fastest speed. Granted, you probably run it at about 50% power most of the time so drop those current numbers to 20 - 25 amps for each hour of operation. Again, using Fucawi's charge rate of 25A you would need to run the engine one hour at something over 2000 rpm for each hour of motor operation and that's just to maintain the battery at its current level. Anything less than that you are losing ground. Using a solenoid to accomplish what you want to do is not an ideal setup because the solenoid itself requires power to remain engaged. The bigger the solenoid (which should be something like an outboard starter solenoid is required to handle the current. A dual battery switch or a combiner is a much better solution. With a switch you do not need to continuously fiddle with it. Leave it on BOTH unless you tend to stay in one spot for an extended period.

 

[fucawi]

Re: Charging trolling motor battery with main engine electrical help needed

You cannot use a starter solenoid to do the switching as the coil is not continuously rated and it will burn out........80A alternator does not mean an 80 A charge ..it means 80 A for the powering of resistive loads like lights fish finders etc and of course if you have two batteries the makers have allowed 25 A for each battery ( 50 A) and 30 A for accessories...

 

[1SaintsFan]

Re: Charging trolling motor battery with main engine electrical help needed

I put a battery isolator switch on my boat. charges both batteries when running and separates the starting battery from my deep cycle battery. Got stuck in the Louisiana marsh with that duel battery switch in both position.Had to call sea tow over $600.00 for a jump