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Calculate amps through each bus bar?

ohthetrees

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I'm planning on assembling a 4P4S battery bank. Hypothetically, if I used the layout in the attached diagram, and was pulling, say, 100 amps, how many amps would be flowing through each link? My instinct is that the serial links would have more amps flowing through them than the parallel links. Would each serial link be the same as every other serial link? Is there a way each and every link could have a calculated number? Let's assume perfectly matched cells.

Screen Shot 2021-08-26 at 1.48.29 AM.png
 
100 / 4 = ~25

1st 100, 2nd 75, 3rd 50, 4th 25.....it looks like it will very up and down in 25 amp increments as it works its way around.
 
100 amps through any point measured over the bus bars. Voltage difference between the cell and the contact area of the bus bar determine the amps flowing from invidual cells to bus bar. It should be "close enough" to 25 amps on all cells unless you got unbalanced cells and bad contacts
 
The two previous replies seem to conflict a bit. Maybe can be explained if @newbostonconst is discussing the parallel links only, and @Scph9002 is discussing serial links. I've had a bit of a think about it, and made a diagram with my guess about what is going on. I've labeled the current through each bus bar. Anyone agree/disagree?

Screen Shot 2021-08-26 at 9.39.00 AM.png
 
Did a little test because this is a bit unclear to me also lol. See for yourself.

This is 2p4s btw. Not 4p

First video is discharging

Second video is charging



Im not sure what to make of this information.

Add aditional bus bars each step the amp rises? Measure voltage with balance leads between lowest amp points?

Does it matter enough as long as the bus bars dont get hot? ??
 
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The current flowing through each busbar on parallel side depends on internal resistance of each battery/battery connection and resistance of each bus bar and the way how/where the "overall" battery lead(s) will be located. Only if all those values are known, the current on every bus bar can be calculated.

As already said: If things get hot, current is too high/bus bars too small. And check *each* busbar's voltage drop, they shall be each similar (and reasonably low).
 
Did a little test because this is a bit unclear to me also lol. See for yourself.

This is 2p4s btw. Not 4p

First video is discharging

Second video is charging



Im not sure what to make of this information.

Add aditional bus bars each step the amp rises? Measure voltage with balance leads between lowest amp points?

Does it matter enough as long as the bus bars dont get hot? ??

Good videos.
Perfectly shows that parallel connected cells share the load ... and the serial connections have x2 Amp of the cells.
 
I'm planning on assembling a 4P4S battery bank. Hypothetically, if I used the layout in the attached diagram, and was pulling, say, 100 amps, how many amps would be flowing through each link? My instinct is that the serial links would have more amps flowing through them than the parallel links. Would each serial link be the same as every other serial link? Is there a way each and every link could have a calculated number? Let's assume perfectly matched cells.

View attachment 61727I would suggest additional buss bars as noted.
 

Attachments

  • BATTERY LINKS.jpg
    BATTERY LINKS.jpg
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Not enough.
Left side 2x4 and right 2x4 cell connecting with 1 busbar (on the middle) ...

Also I do not suggest the 4p4s config. The 4s4p config is more maintainable.
 
Not enough.
Left side 2x4 and right 2x4 cell connecting with 1 busbar (on the middle) ...

Also I do not suggest the 4p4s config. The 4s4p config is more maintainable.
I’m open to other configurations of 4P4S, but I’m not doing 4S4P. Both have pros and cons, and I’ve read a lot about it, and I’m comfortable that I prefer to parallel cells before I put them in series to form one large battery.
 
Last battery I built was a 3P4S in the same layout as you have; just 3P versus the 4P.

It works well, and been deployed for about 2 months now without issues.

I did add all 3 bus bars crossing between cells middle top - where you only have one.
 
I dont see a problem with 4P4S, but I personally think that the parallel connections shouldn't exceed 500-600AH. After that becomes a challenge to keep them balanced. Even well matched cells.
 
I’m open to other configurations of 4P4S, but I’m not doing 4S4P. Both have pros and cons, and I’ve read a lot about it, and I’m comfortable that I prefer to parallel cells before I put them in series to form one large battery.

If you want 4p4s then linear
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I dont see a problem with 4P4S, but I personally think that the parallel connections shouldn't exceed 500-600AH. After that becomes a challenge to keep them balanced. Even well matched cells.
My BMS (REC Active BMS) has a 2 amp active balancer, and I will usually be charging and discharging at low C, so I'm not too concerned about keeping them balanced.
 
Last battery I built was a 3P4S in the same layout as you have; just 3P versus the 4P.

It works well, and been deployed for about 2 months now without issues.

I did add all 3 bus bars crossing between cells middle top - where you only have one.
Thanks for your report, @jwelter99 glad to hear the 3P is working well for you. Like you, I'm not too worried about paralleling 4 (in your case 3) cells. Lots of high-knowledge builders (e.g. Battleborn, electric car companies, etc) do parallel many more cells, and let the BMS monitor groups of cells, rather than individual cells.

Regarding when you said you "did add all 3 bus bars crossing between cells middle top" (which was also suggested by @grizzzman), I think I'm missing something. Please help me understand. OK, so I have 4 groups of 4 cells. Each group of 4 will be parallel to each other. Then I have to serial them up. Group 1 to group2, then group 2 to group 3, then group 3 to group 4. If I understand you correctly, both of you and @grizzzman are suggesting extra bus bars for the serial connection between group 2 and group 3 (what you refer to "middle top". But with this layout, there will only be a single connection between group 1 to group 2 (bottom left to top left) and group 3 to group 4 (top right to bottom right). To sum up, with a battery that has 3 serial connections, you suggest one of the serial connections have much higher capacity than the other two. Can you please explain why I'd want this?
 
I'm planning on assembling a 4P4S battery bank. Hypothetically, if I used the layout in the attached diagram, and was pulling, say, 100 amps, how many amps would be flowing through each link? My instinct is that the serial links would have more amps flowing through them than the parallel links. Would each serial link be the same as every other serial link? Is there a way each and every link could have a calculated number? Let's assume perfectly matched cells.

View attachment 61727

Looks to me reasonably close to ideal.
Similar arrangement for 2p4s would be perfectly matched. This one for 4p4s will have some imbalance between cells. And, of course the links carry about 25A, 50A, 75A, 100A as another poster said. So making their cross-sections proportional to what they carry could help.

That still isn't a perfect balance. You could draw a resistor ladder representing busbars and cell IR in LTSpice and observe the outcome.

There are schemes to perfectly balance wires to a 4p bank of batteries. Not sure if worth trying to do for these sorts of packs. For 4p4s, would be done at the two ends, and between groups of 4p they would stitch together in X and Y.


We know that 4p4s instead of 4s4p carries the risk that bad contact leads to a bank of 3 cells in series with banks of 4 cells.
 
Looks to me reasonably close to ideal.
Similar arrangement for 2p4s would be perfectly matched. This one for 4p4s will have some imbalance between cells. And, of course the links carry about 25A, 50A, 75A, 100A as another poster said. So making their cross-sections proportional to what they carry could help.

That still isn't a perfect balance. You could draw a resistor ladder representing busbars and cell IR in LTSpice and observe the outcome.

There are schemes to perfectly balance wires to a 4p bank of batteries. Not sure if worth trying to do for these sorts of packs. For 4p4s, would be done at the two ends, and between groups of 4p they would stitch together in X and Y.


We know that 4p4s instead of 4s4p carries the risk that bad contact leads to a bank of 3 cells in series with banks of 4 cells.
Thanks for the comments. I'm not worried about minor imbalances, as my BMS has 2amp active balancing, so I think that should be able to keep up with any minor imbalance issues.
 
Did a little test because this is a bit unclear to me also lol. See for yourself.

This is 2p4s btw. Not 4p

First video is discharging

Second video is charging



Im not sure what to make of this information.

Add aditional bus bars each step the amp rises? Measure voltage with balance leads between lowest amp points?

Does it matter enough as long as the bus bars dont get hot? ??
Great stuff, thank you.
 
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