seespotrun avatar image
seespotrun asked

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.


Orion DC-DC Converters not smartfusesalternatormarine
24-12-top-up.png (92.1 KiB)
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ingo21 avatar image ingo21 commented ·
you could use a solarpanel + charger to keep your starter/house charged?
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seespotrun avatar image seespotrun ingo21 commented ·
Thanks, good idea. I might go that way some time down the track. But for now, the trolling battery has more capacity than I need, so a 24/12 converter is a simple way to use its excess as a virtual house battery. If I start doing multi-day trips then will probably add solar.
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1 Answer
kevgermany avatar image
kevgermany answered ·

1 fuses on the Orion dc:dc are too large. If you do add fuses, fuse as close to the source/batteries as practical. You need a fuse at the starter battery. .

2 No

3 If set correctly, yes

4 If not a charger, float is probably safest. 12.7V.

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seespotrun avatar image seespotrun commented ·

Thanks for the thorough reply.24-12-plus-12-24.png

Updated sketch with revised fuses and also adding a 12/24 charger so that (1) the alternator can top up the 24 array when motoring and (2) I can use my existing 12V shore charger to recharge the whole boat, saves me buying a new Mains-to-24V charge. Added DPDT switch to select charging direction; not sure if it can be done with single pole.

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24-12-plus-12-24.png (145.4 KiB)
cdgatti avatar image cdgatti seespotrun commented ·
Seespotrun, I would like to gain the same functionality on my boat. Did you ever do this project, how were the results? Thank You!
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Victron DC-DC converters product page

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