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Author Topic:   Three-battery Electrical System
wally910 posted 05-28-2014 12:11 PM ET (US)   Profile for wally910   Send Email to wally910  
A supposedly novel-for-marine applications system [shown in pictures linked below] employs three 12-Volt batteries. Two of the batteries are normally connected in series and power a 24-Volt trolling motor. The third battery is the starting battery. Through a [carefully coordinated] throw of [three switches], however, all three batteries can be placed in parallel, giving more power to the 12 volt system of the boat. Also, the system allows all three batteries to be charged while underway.

Any thoughts on any problems with this setup? At first blush, I would think there may be a problem after heavy trolling motor use.

jimh posted 05-28-2014 12:32 PM ET (US)     Profile for jimh  Send Email to jimh     
The suggested arrangement of three batteries looks complicated. The suggested wiring provides for a very dangerous short circuiting of two large batteries if by some chance the switches (marked A, B, C) are operated in an improper manner. The existence of that risk seem to be to so serious a defect in the design that it outweighs any possible benefit the design might offer.

I don't have any first-hand experience in charging batteries in a 24-Volt system, but my first thought would be to use some sort of charging system which employed a 12-Volt-to-24-Volt voltage convertor to create a 24-Volt charging current source from the 12-Volt outboard engine alternator.

As mentioned by Wally, if the two batteries in the 24-Volt system are deeply discharged from running the trolling motor, when they are in parallel they will represent a very deeply discharged 12-Volt load for the charging system. When this load is suddenly placed in parallel with the engine starting battery, the current flow into the two discharged and paralleled batteries could be very high.

jimh posted 05-28-2014 01:01 PM ET (US)     Profile for jimh  Send Email to jimh     
On further study of the proposed design, I see an additional flaw:

One of the two batteries arranged for the trolling motor is also to power all the house electrical loads. This means that battery will have higher discharge current than the other battery of the 24-Volt pair. The result will be that two batteries of the 24-Volt pair won't be equally discharged.

jimh posted 05-29-2014 12:26 PM ET (US)     Profile for jimh  Send Email to jimh     
I seem to have been a bit negative in my comments about the potential of this proposed system. I do find one aspect of the proposed 24-Volt system to be interesting and perhaps positive. The 24-Volt potential for the trolling motor is made by creating a (+)12-Volt and a (-)12-Volt source relative to the boat's normal 12-Volt system's ground reference, the chassis of the outboard engine. This seems like a good method to me.

Also, I sketched out a wiring for the two batteries in which they would be controlled by a double-pole, pole-throw switch. In my implemenation (see below), the operator could never create a big short circuit hazard by inproper sequencing of the three switches of the original design. With the third battery's wiring controlled by one switch--which of course would have to be a break-before-make switch--the possibilily for an accidental short circuit is much less.

wally910 posted 05-29-2014 01:48 PM ET (US)     Profile for wally910  Send Email to wally910     
I appreciate the critique Jim. I personally do not have enough electronics running in my Montauk to justify further complicating my DC system, which consists of a starting/house battery and 2 12 volt agms in series and mounted in my old igloo cooler for running my 24 volt trolling motor. My starting battery powers a GPS, sounder, VHF, bilge pump, backlit compass, running lights, and baitwell pump. Rarely, if ever are all of these items running at once, and I haven't experienced any problems yet.
jimh posted 05-31-2014 10:00 AM ET (US)     Profile for jimh  Send Email to jimh     
As a rule for designing any system, simpler is better. Unless there is some special benefit to be gained from adding complexity, simplicity should be preferred.
jimh posted 06-01-2014 10:22 AM ET (US)     Profile for jimh  Send Email to jimh     
A simpler approach to charging a second battery that is part of a 12-Volt and 24-Volt circuit can be found in a product called TROLLBRIDGE24 from YANDINA LTD. See

YANDINA is a small manufacturing company that seems to specialize in marine electrical products. Their solutions seem to be well designed and tend to be very simple.

The TROLLBRIDGE24 will charge the second battery in a series connection of two 12-Volt batteries by creating the charging current from the 12-Volt circuit. It looks like a simple solution.

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