Three battery design idea - does this work?

capncaveman

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Jun 23, 2013
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I've looked in the forum and haven't seen a solution to the wiring problem I am trying to solve. I submit the below diagram and invite comments on whether it will work and achieve my goals plus any suggestions to improve it. Please note that I also have a three bank on-board charger, but have not included it in the diagram for simplicity.

My requirements:

With a three battery setup, provide for engine starting (single engine), backup start power source, 12 V house circuit and a 24V trolling motor circuit. I have a Perko switch available for the solution.

My proposed solution:

Battery 1 is the primary starting battery and is wired through the Perko as "Batt 1". Battery 2 provides the 12 V house circuit, but is also wired through the Perko as battery 2 for the occasional "jump start". This battery provides the -12V leg for the 24V trolling motor. Battery 3 provides the +12V leg for the trolling motor.

Note that the diagram includes a SPDT to switch the trolling motor from 12V to 24V. The diagram shows the trolling motor switched for 24V mode.

Thanks in advance for your comments and suggestions.


three battery circuit design.jpg
 

UncleWillie

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Oct 18, 2011
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3,995
Re: Three battery design idea - does this work?

That will work in principal. But it would not be highly recommended.

The Trolling batteries are going to be discharged at different rates because of the house loads.
You will need to monitor the House battery closely in operation as it will go dead first
and if you continue operating the trolling motor, its life will be short.

Charging the two batteries on the 24v side, will require a two bank charger, separate from the alternator.
 

capncaveman

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Joined
Jun 23, 2013
Messages
3
Re: Three battery design idea - does this work?

That will work in principal. But it would not be highly recommended.

The Trolling batteries are going to be discharged a different rate because of the house loads.
You will need to monitor the House battery closely in operation as it will go dead first
and if you continue operating the trolling motor, its life will be short.

Charging the two batteries on the 24v side, will require a two bank charger, separate from the alternator.

Thanks, UncleWillie. Great observations!

So, as a follow-up question, any design suggestions for achieving my design goals?

Thanks!
 

UncleWillie

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Oct 18, 2011
Messages
3,995
Re: Three battery design idea - does this work?

The trolling batteries should be used for trolling only.
Decide if you want 12 or 24volt power and stick with it.
Switching back and forth will unbalance the 2 batteries.

If you are trolling, I suspect we are not dealing with a 30 foot cabin boat with Refrigerators, TV, and a Mega-stereo on the house load.
If the house load is a fish finder, Small radio and NAV Lights; Your trolling batteries are going to give up before the starting battery gets low.

Either just move the house loads to the starting battery.
Or get a 4th battery for a dedicated House Battery.

Another Option .......
Parallel the two Trolling/House Batteries into a single large 12v bank.
The Batteries would remain balanced and charging from 12 volts is simpler.
The engine alternator would be able to Extend your trolling time if needed by switching to BOTH.

In The BOTH Position, The engine would be cranking from All Three batteries in parallel!
 

capncaveman

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Joined
Jun 23, 2013
Messages
3
Re: Three battery design idea - does this work?

The trolling batteries should be used for trolling only.
Decide if you want 12 or 24volt power and stick with it.
Switching back and forth will unbalance the 2 batteries.

If you are trolling, I suspect we are not dealing with a 30 foot cabin boat with Refrigerators, TV, and a Mega-stereo on the house load.
If the house load is a fish finder, Small radio and NAV Lights; Your trolling batteries are going to give up before the starting battery gets low.

Either just move the house loads to the starting battery.
Or get a 4th battery for a dedicated House Battery.

Right you are. It's a 21' fish/ski. I like your suggestion of moving the house circuit to the starting circuit -- simple and effective. I do plan to use the trolling motor in 24V mode only, the selector switch is already built into the control pedal so I'll leave it, but I don't intend to ever use the 12V mode.

Thanks again for the suggestions! I'm switching from design to implementation this weekend.
 
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