Amps Divided when pulled through parallel battery strings

Netsua

Solar Enthusiast
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Jul 22, 2021
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Up until this point, I've basically just been following instructions and "over-building" whenever in question.

But I'm curious, if a system has 2 parallel strings and you're pulling 100 amps on the total battery, would the cable between each battery string and the bus bar read as 50 amps? I'd never do this, but hypothetically if you wanted to fuse each string as low as possible (forgetting that the cable itself could handle much more), how would one determine the fuse size you could get away with?
 

schmism

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Jul 13, 2020
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Current through parallel path is determined by the resistance in each path. V=IR This is why instructions on parallel path setups stress ensureing that everything is identical. Wire length, wire size, connectors etc.
 

shadowsteve

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Feb 11, 2021
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For two equivalent batteries with same wire gauges and pretty much the same connection cable lengths then yes you'll get 1/2 the current coming from each battery. If you measured accurately enough you'd see a difference due to slight diff in connection resistance, lug connections etc. but we're building solar systems not grand pianos so I don't measure that close
 

740GLE

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Apr 30, 2021
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I highly recommend looking up KCL and KVL calculations for further explanations.

Kirchhoff laws are pretty powerful stuff.

For fusing, that’s set by your conductor limitation (with BMS factored in as well). If the BMS is 100amp, 125a fuse works, if two batteries in parallel, that’s be 250a fuse.
 

Hedges

I See Electromagnetic Fields!
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Mar 28, 2020
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If possible, design the system so no failure occurs even if 100% of current comes from one battery string.
You could do that if the 2nd string is for added capacity, not for higher current draw.

I think lead-acid batteries, which have higher resistance and more linear V/Soc curve (until discharged) will balance reasonably well.
Lithium has a knee in the curve near full, which could result in significant imbalance in current.

I had a similar issue with my inverters, which are wired 2s2p and pass through AC from the grid.
With 60' of 6 awg, I expected wire resistance to balance current through their relays fairly well.
Instead, 3:1 current imbalance occurred. Nothing tripped because total current was within the limits set for one relay.
The problem turned out to be Square-D QO270 circuit breakers - they different enough in resistance to cause that imbalance.
Substituting Schneider 63A DIN rail breakers resulted in perfectly balanced current.

After you've built your battery system, use a clamp DC ammeter to measure current in each string.
That should identify any resistance differences (which could change later if a screw loosens due to cables wiggling).
But you could still see an issue when discharge crosses knee of curve.
While drawing no more than current limit for one battery, watch for current imbalance as it discharges past knee.

A clamp ammeter measures net current through the clamp. If you ran one wire through clamp in one direction and one in opposite direction, meter will read the difference in current. Then you can just watch that reading as battery discharges. Balanced, it will read zero. You may see current imbalance reading rise to a peak and drop off again.
 

time2roll

Photon Sorcerer
Joined
Mar 20, 2021
Messages
3,449
Up until this point, I've basically just been following instructions and "over-building" whenever in question.

But I'm curious, if a system has 2 parallel strings and you're pulling 100 amps on the total battery, would the cable between each battery string and the bus bar read as 50 amps? I'd never do this, but hypothetically if you wanted to fuse each string as low as possible (forgetting that the cable itself could handle much more), how would one determine the fuse size you could get away with?
I have pulled 150 amps out of my parallel batteries and they each contribute within about 5 to 8 amps of each other. To minimize the fuse rating and keep it tight I suggest 20% over the expected maximum. For your example 60 amp fuse would be as small as I would go. Could make your system prone to nuisance shut down. If your imbalance is greater it might be time to check connections etc. Easier to use a clamp-on ammeter to verify the situation.
 

Netsua

Solar Enthusiast
Joined
Jul 22, 2021
Messages
89
Thanks everyone - all super helpful.

I'm also seeing more and more info that differentiates "chassis" wiring from "distribution" wiring also. Sounds like a given wire gauge can handle more amperage when used in the chassis space - or connecting the batteries etc. as opposed to a full 5-10ft distribution. Appreciate all the input and reading up on Kirchhoff now.
 
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