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24/12 for starter battery top-up

Hi all,

I have a small fishing boat with the house load (12V at average load about 2-3A) supplied by the starter battery (conventional, not lithium). There is a separate 24V circuit (2 x 12V 100Ah lithium in series) to run the electric trolling motor. To reduce the risk of the starter battery charge getting run down by the house load, I'm planning to add a 24/12-9A Isolated Dc-Dc Converter to keep the starter battery topped up. In effect the house load will be supplied from the 24V system via the converter. I can protect the starter battery without the need for a dedicated 12V house battery.

Sketch of proposed setup below.


  1. Do I need the to fuse the input and output of the 24/12 converter? (am thinking only on output, since input is already fused)
  2. Does the converter need to be turned off before starting the outboard? (I assume not)
  3. Will the outboard alternator and the 24/12 converter play nicely with each other? (I assume yes)
  4. Optimum output voltage to set the converter output (I was thinking around "float" voltage i.e. 13-13.5V)

Thanks in advance.


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exact-electrics asked
matt1309 answered ·

1 Answer

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My wife and I took the plunge and bought a 1984 Roughwater Trawler for live-aboard (Semi-retirement). It needs a major upgrade when it comes to the electrical side of things that's for sure. So far I have made a list of the upgrades I'd like to make.

MultiPlus II 12/3000

Orion-Tr Smart 12/12-30A (360W) Isolated DC-DC charger

Victron SmartSolar MPPT 150|70

Lynx Distributor

Victron BMV-712 Battery Monitor

Smart Battery Protect 100A

Galvanic Isolator VDI-32

Cerbo GX

GX Touch 50

6x 200W Solar Panels

3x 200Ah AGM Batteries

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dirtdiver1977 asked
dirtdiver1977 answered ·

3 Answers

System Design (Marine)


I am designing an upgraded electrical system for my 32' sailboat to LiFePo4 batteries with built-in BMS. If anyone would be willing to review/provide feedback, I would really appreciate it. This design does not incorporate a multi-plus that is in many of the example designs. This is due to limited space, but if it's required please let me know, and maybe I can cram it in somewhere! Thanks!

My main question is how to best control the Orion so that I don't drain the starter battery. I have a Cerbo GX (not shown), maybe this can monitor the starter voltage to shut it off below a certain voltage? Or I could wire a manual switch to turn it on when the engine is running I guess too.


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I'm not sure how to ask my question.

I have this equipment, installed by the boat builder:

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  3. Whisper Power Belt Driven Generator W-BD 3.5.
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Item 3, the Belt Driven Generator, periodically displays a fault message that is not in the manual:


It happens when I run a 1600 watt clothes-washing machine. The boat builder told me the system would handle this.

I have asked two marine engineers, who are not sure what's wrong. I don't know the terminology nor the relative importance of things, so I don't know what questions to ask. So my question is very general: "Is this something that needs to be fixed? What do I need to know?"


Thanks in advance, and sorry if my question is incomplete. I'm doing the best I can. :)

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2 Yamaha 200's (50 amp Alternator Output each)

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Here is what I want to accomplish:

House battery setup - Either 1 Relion 12v 100 Amp lithium battery or 2 Relion 12v 60 Amp batteries in parallel

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Possibly a 400w REDARC inverter for phone charing and light loads like that.

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