Hello,

First off…..Let me say, that I have enjoyed reading through a ton of posts on your site. You perform a great service, and knowledge is power.

I am going to be adding a trolling motor soon to my 191 CC Mako. The trolling motor is a 24 volt model. I would like to add two Group 31 deep cycle batteries for the trolling motor, and have one dual purpose battery to crank with. Three batteries total.

I would also like to pull 12 volt power leg out of my 24 volt series to power accessories and act as a house battery when I am not using the trolling motor, and an emergency crank battery.

I have included a wiring diagram that I drew up. Please take a look at it for me.

What concerns me most is the 12 volt ground coming off one of the batteries in series. Have I done this right? I thought this problem might be a good one to discuss on Easy Ac/Dc.

I have found no answer to this on the internet yet. I would be happy to update this diagram per your recommendations and give you the corrected one in the end, if you would like it.

Thanks,

David

Hi David,

Very nice schematic.

I would suggest some minor changes.

I would use battery A instead of battery B as your paralleling battery. Basically you would be paralleling to your 0 to 12 volt battery instead of your 12 to 24 volt battery. This will create a common ground in your boat wiring and help reduce stray current corrosion and accidental mis-connected grounds.

After you switch your paralleling switch to be connected to battery A, pay close attention to 12 volt devices that you may connect only to Battery B. If their ground wire comes in contact with any other ground wire in the system, it will create a dead shore across Battery A. You may want to consider installing circuit protection in the ground wire of any 12 volt item that you connect only to battery B.

Your bilge pump “manual on” switch needs to draw power from the same fuse as your bilge pump “auto” switch. You should only have a single source of power for any device. I would keep the switch at the helm, but run a wire from the auto bilge fuse (load side) to the line side of your bilge pump switch.

Make sure your wire sizes are appropriate. When your cranking battery switch is in the parallel or trolling motor battery position, the cranking circuit will include the wire from the switch to Battery A. Battery A to your Ground buss. Ground buss to Battery C. These wires should be at least the same size as the main engine battery cables.

Let me know if you have any other questions,

Kevin

Dear Kevin,

I am currently beginning my first boat wiring project, a total rewire of a small Owens cruiser from the late ’60s.

Here’s my problem:

The old wiring runs behind the original paneling and headliner. I could try dismantling everything to remove and replace the wires, but am concerned about damaging any of the woodwork. It’s old. It’s gorgeous. It would be a bitch to replace.

I don’t mind leaving the old wiring in place and just running a new harness, but that still leaves the problem of installing the new wires without causing any damage.

Any ideas?

RP

Dear Ron,

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Kevin.

Kevin,

I want to put on a 36 volt trolling motor, replacing the 12/24 version that I currently have.

I have a two bank charger. After I add the third battery, can I still use the two banks on two of the batteries and then hook up a portable charger for the third?

Chris

Chris,

Yes. Your plan will work. BUT…remember that your portable charger is probably not ignition protected. It should not be used in a compartment where gasoline is stored.

Kevin

Hi Kevin,

Just purchased the 2501 battery isolator for my boat. Going to use a two battery system, one for starting, and the other for trolling motor and accessories. I have an outboard mercury 70 hp motor.

Do I need to come directly from the alternator to the isolator on terminal A? If so, how is that done? And do I use the existing alternator wire, or add a wire from the alternator?

Thanks!

Paul

Paul,

You need to connect the isolator directly to the alternator output. The alternator sense wire needs to be connected to the positive post of one of the batteries. This can be difficult with an outboard motor and impossible on some models.

I would strongly recommend returning the 2501 battery isolator and purchasing a VSR based automatic battery switch. This unit will allow you to charge two batteries with your motor, but does not require you to connect to your alternator output. The VSR connects between the positive battery posts and monitors the voltage on each battery. Your outboard motor connects to one battery and your boat accessories connect to the other. The grounds are tied together.

Hope this helps,

Kevin