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Author Topic:   Mixed 12-volt and 24-volt Primary Power with Three Batteries
Jefecinco posted 05-19-2010 10:35 AM ET (US)   Profile for Jefecinco   Send Email to Jefecinco  
The new 190 Montauk is now at home and I need to get the battery situation squared away. Currently a single low maintenance lead acid starting battery is installed. The boat has the optional trolling motor wiring installed. I plan to eventually install a trolling motor. My goal is to correctly wire the boat for two trolling motor batteries, for which the boxes have been installed. I will replace the existing battery with a Sears AGM. I will install two more AGM batteries and connect them to the two trolling motor battery cables. I will install a 24-volt trolling motor.

I want to be able to use the trolling motor [24-volt series connected] batteries for starting in case the starting battery fails. I'm sure this is not extremely complicated, but I have never done it. I don't want to begin without some guidance. Is the Blue Sea Systems Add-a-Battery, PN 7650, the device to use [to allow a connection between a 24-volt two-battery series connected string and a single 12-volt battery so that these batteries can be shared for 12-volt engine starting]?

Recommendations and comments are welcome.



jimh posted 05-19-2010 09:05 PM ET (US)     Profile for jimh  Send Email to jimh     
I don't think there is a simple or uncomplicated way to accomplish what you have in mind. The Blue Seas Add-A-Battery switch is intended to connect two 12-volt batteries together, not to connect a 12-volt battery to a 24-volt series string of two 12-volt batteries.
bluewaterpirate posted 05-19-2010 10:55 PM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
I agree with Jim ...... I'm doing some research on this.


jimh posted 05-19-2010 11:26 PM ET (US)     Profile for jimh  Send Email to jimh     
Here is one possible way to use three batteries for the purposes you mentioned.

Schematic diagram

The drawback with this approach is only two of the batteries will get charged by the motor. The third battery is never charged.

jimh posted 05-20-2010 08:31 AM ET (US)     Profile for jimh  Send Email to jimh     
In a prior discussion I suggested two ways in which a both a 12-volt battery and a 24-volt battery could be provided with charging current from a single 12-volt charging current source such as an outboard motor. See

Charging Three Batteries In Separated 12 and 24-volt Systems

for the links to the only two products I could find that accomplish this.

Jefecinco posted 05-20-2010 10:55 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     

Thank you. Great information. It appears the Dual Pro may be what's required. Now I have to figure out how to incorporate it into a total boat charging/switching system. I will first see what I can learn from their web site and customer service/sales telephone number. Also thank you for the subject name change. It makes much more sense now.


I appreciate your response and look forward to any advice you may offer.


If I'm able to get this all sorted out I'll report back with the result.


jimh posted 05-20-2010 10:50 PM ET (US)     Profile for jimh  Send Email to jimh     
Another method of connecting three batteries so that they can be used in a mixed 12-volt and 24-volt system, yet all be charged by the main engine:

Schematic diagram of three batteries connected in a mixed 12-volt and 24-volt system

This design requires a special double-pole, double-throw, high-current switch to handle the connection of Battery-A into the circuit. In the "24" position of the switch Battery-A and Battery-B are connected in series to run the 24-volt trolling motor. In the "12" position of the switch, Battery-A and Battery-B are connected in parallel. They can be charged by the main engine if the primary power distribution switch is put into the "1" position.

The engine starting battery is Battery-C. The normal position of the OFF-1-BOTH-2 switch will be "2." This lets Battery-C start the engine. The engine charges Battery-C only.

The Trolling Motor runs of Battery-A and Battery-B only when the special switch is in the "24" position. When the special switch is moved to the "12" position, the trolling motor should be shut off. The OFF-1-BOTH-2 switch can be moved to the "1" position, and the outboard motor will charge Battery-A and Battery-B in parallel.

Battery-A and Battery-B should be of the same type, same age, and same condition.

One problem implementing the circuit I have sketch above will be finding the special DPDT high-current switch. You might try using an electrical relay. A Potter-Brumfield PRD-11-DGO-12 series relay can handle 50-amperes, but it is an open relay and might not be suitable for a marine environment.

I am not aware of an off-the-shelf marine product that would be suitable for the special switch function.

jimh posted 05-20-2010 11:13 PM ET (US)     Profile for jimh  Send Email to jimh     
Improved circuit:

Schematic diagram of three batteries connected in a mixed 12-volt and 24-volt system

jimh posted 05-20-2010 11:27 PM ET (US)     Profile for jimh  Send Email to jimh     
You could probably use two SPDT relays in combination. This vendor site has a lot of choices that are under $10:

Jefecinco posted 05-21-2010 09:13 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     

Terrific information! I'll be studying your drawings very carefully. It is looking very possible. Thank you.


jimh posted 05-22-2010 01:30 PM ET (US)     Profile for jimh  Send Email to jimh     
In the circuits I have sketched so far that include a DPDT switch, an important concern is the DPDT switch must be a break-before-make contact arrangement. Many marine battery switches are the opposite configuration, make-before-break, and cannot be used, as they would short out the battery terminals and create a huge problem. If relays are used, there is also a risk of short circuit of the battery if one of the relays were to fail to operate. For these reasons, I am not so fond of the circuits as sketched above with the DPDT switch that manages the third battery (or the "A" battery).

A simpler arrangement would be as shown below:

Wiring diagram: three batteries in 12 and 24 volt system.

In this arrangement, batteries A and B are connected to form a 24-volt string for the trolling motor. They receive charging current from a 24-volt charging device that runs off the 12-volt main distribution system and charging current from the outboard motor. Battery C is the engine starting battery and runs the 12-volt system on the boat. A simple main ON-OFF switch controls the 12-volt system. In an emergency, the boost ON-OFF switch is closed, and battery B is put in parallel with battery C for engine starting.

To use this arrangement you need to find a device that converts 12-volts into 24-volts and is suitable for battery charging. See the other article I linked for some leads to a device like that.

jimh posted 05-22-2010 11:40 PM ET (US)     Profile for jimh  Send Email to jimh     
Look at the Stealth-1 Charger:

as a possible solution to how to get 24-volts from a 12-volt charging source.

Jefecinco posted 05-23-2010 05:52 PM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     

Thank you again. You've provided a wealth of possibilities for what I assumed would be a simple system because of the high number of fishermen using 24 volt trolling motors.

Due to the complexity of the systems required for a three battery 12 volt and 24 volt system I am going to reexamine the option of a 24 volt vice a 12 volt trolling motor.

At least I now have a clear picture of the requirements of both.


Jefecinco posted 05-24-2010 10:04 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
I've been doing my homework on the issue of what size, type, thrust and voltage trolling motor is best suited for my boat.

Trolling motors are basically rated by the pounds of thrust they produce. Shaft length is the second major issue under consideration.

Twelve volt trolling motors appear to have maximum thrust ratings of 55 pounds. That is the minimum recommended for my boat. More thrust is recommended if operation is to be in rough or windy conditions or if operation is to be in grassy waters. I'm a fair weather fisherman and don't fish in shallow waters with grassy bottoms very often. It follows that I can get away with a 12 volt trolling motor for my boat even though it is a small compromise from the ideal. I'm thinking the trade off is worth it for a far less complex electrical system that conforms to more normal practice for Boston Whaler Boats.

I now have to do some measurements to determine the required length of trolling motor shaft for the boat. If I can find the required shaft length in a 12 volt motor I will certainly install a 12 volt trolling motor and will not require a 12 - 24 volt combination electrical system.

Again, thanks to all.


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