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Old 10-05-2020, 16:00   #1
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Battery wiring quesions

Yet another battery wiring question…

I have had installed a Balmar SG-200 smartguage this weekend, which has prompted a few broader questions on my part. Current wiring diagram attached (simplified).

The prior owner appears to have added a third house battery at some point (Bat 3). I have just replaced all batteries. Ideally they’d be next to each other but there is no way we can achieve that for now due to space constraints.

Boat is on shorepower for 300 days per year. Boat has a generator. Most trips are (sadly!) only a night or two, with a 3-4x per year trips for 3-10 nights. We have what I would call moderate power demands – have gas hobs for cooking, use inverter for microwave (used infrequently), the toaster and the TV, but have a full sized DC fridge.

Grateful for any feedback. My main question is how are we best to wire in the port battery (no 3) given the distance from the other two. One thought was to run charger +ve to Bat 3 so at least that port house battery is not ‘hanging off’ the main circuit. Someone also suggested an AC post with equal length leads to all batteries. I ran a fairly crude test with what I had onboard in the weekend: All batteries fully charged and had inverter pulling 160amps - Bat 3 showed 12.61v at the terminal and Bat 1 and Bat 2 both showing 12.55v at the terminal (which to my basic mind suggests that, as you’d expect, Bat 1 and 2 were doing the heavier lifting).

NB: I’ve left the existing Mastervolt shunt in because the display in the salon provides some useful data.

Thanks in advance
Trevor
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Old 10-05-2020, 16:57   #2
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Re: Battery wiring quesions

Connect the three battery return's (negatives) on one end of the shunt and all loads and charging sources returns on the other end. Don't get your panties in a wad over getting all the battery connections balanced, nice but not a "world altering situation".


Frankly
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Old 10-05-2020, 19:14   #3
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Re: Battery wiring quesions

I agree with Frankly's advice.


The best way to do this, in theory, would be to feed the + and - from opposite ends, like this:


Fuse block + -----+-----+-----+
. . . . . . . . .bat1 .bat2 bat3
. . . . . . . . . +-----+-----+-------- Shunt -


That way, the voltage will stay balanced across all three batteries. You would add a disconnect and overvoltage protection at each battery.



I don't think the benefits are worth the cost of the copper to do that, and if it were my boat, I'd leave it the way it is.
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Old 10-05-2020, 19:34   #4
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Re: Battery wiring quesions

Unless you can move the battery I would just leave it like that.

When the inverter load is off they will just rebalance again.

I assume you have a 4th battery for starting?
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Old 10-05-2020, 19:55   #5
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Re: Battery wiring quesions

Quote:
Originally Posted by Frankly View Post
Connect the three battery return's (negatives) on one end of the shunt

I should have said electrically connect (as is shown) not to indicate an actual physical connection.


For 15 years I have been operating with two batteries in the middle of the yacht and a third in the bow (all electrically in parallel). If that is not bad enough the bow thruster and anchor windlass are tapped from the fwd set. If there have been any adverse effects I have not noticed.
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Old 10-05-2020, 20:18   #6
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Re: Battery wiring quesions

Quote:
Originally Posted by smac999 View Post
Unless you can move the battery I would just leave it like that.

When the inverter load is off they will just rebalance again.

I assume you have a 4th battery for starting?
Thanks for this. Yes - 2x12v in series for 24v start bank. Port engine alt feeds those, along with a 24v charger when at dock. (And just to bump up the complexity, there is also an interconnect to parallel the 12v house bank in an emergency.)
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