Battery Balancer wire sizing for lithium

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leftcoastman35 asked Feb 26, '24

Battery Balancer wire sizing for lithium

I am considering purchasing a Victron Battery Balancer to add to my system.

My system currently has six 2018 vintage SmartLithium 12.8/150AH batteries in serial-parallel to create a 25.6v/450AH bank on my sailboat. Charging is done via Quattro, Skylla-i 80 and Smartsolar MPPTs.

We persistently have an issue where the series pair battery that is connected to the "positive" is at slightly lower voltage than the other battery. I am thinking a Battery Balancer may help with that since the current SmartLithium manual indicates that we can add it.

4. Installation (victronenergy.com)

However, I'm a little confused after reading the Battery Balancer instructions, which state:

"In case of series-parallel connection, the midpoint interconnecting cables must be sized to at least carry the current that arises when one battery becomes open-circuited. - In case of 2 parallel strings: cross section 50% of the series interconnecting cables. - In case of 3 parallel strings: cross section 33% of the series"

Currently, each series pair (pos to neg for each series pair) is connected using a copper bar. I expect that we may draw up to 500amps (24v) from the bank.

What size should I use between the midpoints and to the Battery Balancer? It seems the maximum size the Battery Balancer will accept is 6mm2 (10AWG).

Thank you!

Battery Balancer

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Answer 1 · 2024-02-26T10:02:31Z

Alex Pescaru answered · Feb 26 at 10:02 AM

That info refers to the interconnecting cables between string midpoints. See picture below.

Those interlinks, which sometimes are missing, now will be needed in order for the balancer to "reach" and balance all batteries from all strings.

The battery balancer balances with a certain maximum current, mentioned in the specifications. Also see the picture below.

The balancing wires must be sized in order to accommodate that current, no more.

Use 1mm² for 0.7A.

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leftcoastman35 commented · Feb 26 at 10:54 PM

Great, thanks! So to make sure I understand correctly, the wires between the midpoints should be 1mm2?

What about the wire between the Battery Balancer and the positive bus bar?

And the blue line going between the battery balancer and the first midpoint?

Are those all 1mm2?

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Alex Pescaru leftcoastman35 commented · Feb 27 at 07:38 AM

No.

Like shown in the picture, the wire section between string midpoints (1) must be a certain percent from the wire between batteries midpoints (2).

For example, if you used between each series pair a 50mm² copper bar (2), between those middle points copper bars you must have at least 50/3=16mm²wires (1).

As for the balancer, you must use 1mm² wires for those 3 wires (red, blue, black) from the balancer to the the batteries.

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