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Graydon avatar image

multi-battery banks?

I was unable to find an answer, but please point me to it, if I missed it.

I currently have a 4 PV LG array of 1600W and a 2 PV array of 340W, the former on a Victron MPPT 150/70 and the later MPPT 150/35. They are both connected to copper busbars, then to six 100Ahx12 LFP batteries (2s3p) for 7200W, also running through a 500A Victron SmartShunt; as well as a Victron Multiplus 24/3000/70 inverter.

The batteries are all the same make, chemistry and capacity. Im going to add 2 more, and want to split the single bank into two banks, 2s2p2p, 4800W each @ 24v. I was going to add a smarthsunt or BMV to each bank to monitor its individual SOC, as well as a third 'main' smartshunt connecting both banks to the NEG busbar. Each bank is fused and has a dedicated cut off switch. The individual battery bank shunts will have a 24v200A capacity set, and the main shunt a 24v400A capacity setting.

So, essentially, treating each bank (of 4 batteries) as a single 24v200Ah battery running in parallel with a duplicate. Can I simply connect both banks to the busbar, and expect good performance for the system?

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2 Answers
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wkirby answered ·

What is the reason for splitting the banks up that way?
You are effectively adding more batteries to the same bank, but with two shunts between each bank.
There can be issues with current sharing between two separate battery banks connected in parallel,. There is no easy way to ensure that the current will be equal from each 2s2p bank.
Only one SmartShunt can be assigned to be the system shunt, this would be the "400Ah" one. The other two would show up on the devices page, but apart from casual viewing, they will be wasted resources.

Why not just add the new batteries to the existing battery bank as 2s4p and treat the whole thing as one bank with one shunt?

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Actually, there is no reason I couldn't do that. My long term plans are to add 16 to 20 batteries. So I was thinking of splitting them into banks of 4. Would your suggestion hold for continuing to add more batteries to the one bank, in parallel?

There's really no limit to the number of strings you put in parallel. Just fuse each string for safety.

Keep all positive leads from battery, through the fuse to the bus bat the same length.

Same for the negative leads, then one from the battery bus bar to the shunt, then to a system negative bus bar.

Equal length leads is essential to balance the current between batteries.

One shunt for the entire bank. Multiple disconnects is not necessary and complicates keeping all leads between batteries and bus bar the same length.

Is there any need for using battery balancers with individual LFP cells in series? If so, I think there is a limit of 3p per balancer.We've solved this before by splitting into banks of 3p per balancer and putting those banks onto busbars with equal length cabling and then connecting the loads to busbars.

Balancers for series connected batteries (not just individual cells) depends on the battery. Up to 4 Battle Born batteries can be connected in series without balancers.

Ya, I used "cells" incorrectly there. I meant a 12V "cell" in 2s 24V "battery".


OK, so balancers not necessary for lithium.

Graydon avatar image
Graydon answered ·

Kevin, I just want to clarify your suggestion "There's really no limit to the number of strings you put in parallel. Just fuse each string for safety." do you mean putting each 4 battery (2s2p) bank in parallel with another 4 battery bank (2s2p2p)? Or putting ALL the batteries in parallel, so 2s4p now, then eventually 2s8p?

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No each 2s is a "string". So you end up with 2s4p with each 2s fused