3 Outboards, DC-DC chargers to House Lithium - unwanted paralleling?

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colindavis asked Jun 30, '24

3 Outboards, DC-DC chargers to House Lithium - unwanted paralleling?

Just installed a system on a boat with three large (300HP each) outboards. Each engine has its own AGM start battery. These three batteries can be connected in parallel when desired, via dedicated emergency parallel switches. Each AGM battery also serves other needs (bow thruster, windlass, etc.)

We installed three non-isolated DC-DC chargers (one for each start battery) with charger outputs connected to a bus bar that then connects to new house lithium. Chargers are configured to sense engine start. Everything seems to work and charge as desired.

Question:

Are the DC-DC chargers inadvertently connecting the three AGMs in parallel (through their connected ‘outputs’?). I didn’t think they would, but when checking the ‘emergency parallel’ switches for function, I see continuity (via multimeter) from AGM battery to AGM battery ‘through’ the chargers. Do I need to install diodes, or perhaps Battery Protects for each charger to prevent current flowing from one AGM to the other through the chargers? I thought the chargers would already do this (so that current would not flow from lithium house bank back to start batteries), but the fact that I see continuity surprised me. I would like to keep the AGMs truly independent ‘normally’, for redundancy (i.e., so if one battery goes bad, it doesn’t drag down the others).

starter battery dc-dc charger

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Answer 1 · 2024-07-01T06:19:24Z

JohnC answered · Jul 01 at 06:19 AM

Hi @ColinDavis

I don't see a need for concern here. 'Continuity' in an electronics sense doesn't necessarily mean Amps, and you'd need to see A to have an issue. If these Orions(?) allowed current in reverse then a 12/24 would be useless when turned off.

Redundancy is good, but I eventually graduated/downgraded? to a (well monitored) all-in 450Ah 12V battery bank for 2 engines and house lights, etc. The main house is 48V (solar/genset powered), and I dose the 12V with a 48/12 20A Orion when necessary. Auto with manual override. Goes through a Smartshunt too, and there's no current flow visible when the 48/12 is off.

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colindavis commented · Jul 01 at 04:37 PM

Thanks @JohnC. Yes, Orion 12|12-30’s. We thought about combining the AGM’s to one bank, but given the existing wiring/connections/loads, it was actually easier to leave them separated.

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Answer 2 · 2024-07-01T21:56:00Z

Kevin Windrem answered · Jul 01 at 09:56 PM

Because you are using non-isolated DC-DC chargers, the negative connection of all three starter batteries and the house battery will be connected together. Because the positive connections are isolated, there is no way for energy to transfer from house battery to any starter battery or from on starter battery to another. You should be fine.

If you are looking at resistance between positives of the starter batteries, then you will measure some resistance but it's really just showing you the voltage difference between the batteries. In fact, you may see a negative resistance! You really can't measure resistance (or continuity) with anything powered.

There will also be resistance between positive and negative of each starter battery and because the negatives are common, you'll see resistance between the positives also (with the batteries disconnected).

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