DC to DC Charging Trolling Batteries from Outboard

DIY
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1

Hi All,

I’ve been trying my best to read up and understand how best to use my outboard motor to charge my trolling motor batteries. Here are the highlights of my setup:

  • Mercury Four-Stroke 75HP, with a 50 amp/630 Watt alternator
  • 12v lead acid house/starting battery (which also runs my Helix 10, stereo, lights, etc.)
  • 2 -12v AGM (Fullriver DC105-12) wired in series
  • ProSport 20, dual bank 20 amp charger for shore power (wired in parallel)
  • Terrova 80 trolling motor

I think the Orion Tr Smart 12/24 - 10 (isolated) is the unit I need. I’ve read the installation manual, as well as a few other useful links, and I think I have everything, but I’m not sure if I need the battery balancer (and I don’t understand how it would be wired in my situation).

Here is a pdf showing how I think things should be connected. I’ve shown the fuses and wire sizes based on the table I found in the Orion installation manual. I don’t think I need the 6AWG, but seems to me it won’t hurt to upsize. Am I missing anything, or have anything bass acwards?

JCG_Wiring Diagram - 12-24v Charging_2026.JUN.24.pdf (2.6 MB)

Seems like a great community! Thanks for your time!

JG

2

Your shorepower charger, does it have 2 completely independent outputs. Often these chargers have a common negative and can only charge 2 x 12V batteries with the same negative, not 2 x 12V batteries in series as one of the negatives is connected to +12V. You need to check this out. You may be better with a single 25V charger and balancer. The balancer has 3 connections, 0V, Common and 24V. 24V goes to the 30A fuse, Common goes to the negative on the same battery, 0V goes to the negative on the other battery.

3

The charger has two sets of cables, each with a positive and negative lead, as shown in the diagram. It seems to work, since this is my third season with this boat, but are you saying it shouldn’t work because the batteries are hooked in series? Sorry if I don’t follow exactly, but I am just learning the finer points of the DC world. Thanks for you help!

4

What I am saying is some dual output chargers have 2 completely independent outputs and can do 2 x 12V batteries in series, some have 2 outputs that are not independent and would cause a short with 2 x 12V batteries in series. If you have successfully used it with 2 x 12V batteries in series so far then it is OK. If you have not used it with them in series then you need to check.

5

Gotcha! Yes, my charger has two independent outputs - each attached to one battery. I got confused thinking that the connection between the batteries in series could be a problem. Thanks for your explanation!

6

If I introduce the balancer into the equation, do I need to be concerned about the AC charging system as it is currently installed? Are there any other considerations I’m missing?

I have to admit, the alarm stuff is going over my head a bit as well.

7

The balanver works independent of the charge source so will be OK for both ac and dc to dc. The alarm functions are optional.

8

Thanks for your help kind sir! There is so much to learn that it can be overwhelming, but with your guidance from this post as well as the others I have read I feel less intimidated by it all. Much appreciated!