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Mistake busbars ?
#1
Hi all,

Did i made a (big) mistake in the design of the busbars in my setup ? Huh
A few members made a correct observation in my build thread, so just a bit unsure
I know the general idea of the busbars on the packs is to do the terminals of the positive side oposit to the negative side. This to distribute the load more even over all the cells in the pack... I do understand this, and do believe it is the best way... no discussion there.
I had to do it otherwise due to design reasons.

I want to know how bad it is in our type of systems to do it my way, with both terminals on the same side ?

Are the cells charged/discharged uneven? are they getting  (temporarily) out of balance in one and the same pack? hard to believe...
Are the cells in the front of the pack more stressed?
Is a setup with a (even a small) difference in lenghts of fuse wire not worse?
Internal resistance beween cells in a pack, same problems? 

In my setup busbar resistance is negligible ( 5 x 32cm copper wires of 6mm2 in "parrallel" is 0,0002 Ohm, even less with the "middle" busbar connection )
Currents more than 150A (75A / string) are rare, there are 90 cells in a pack.:
More specs here: http://secondlifestorage.com/t-Instant-powerwall

Is there a way to do some tests?

My way:


Other (right) way (thanks Peter)
___________________________________
18x 300Wp solar off grid and 10x 180Wp solar tracker grid-tie
10KW 3phase hybrid inverter. 40Kwh 18650 storage (for now)
My setup: https://secondlifestorage.com/t-Instant-powerwall
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#2
It's not a "big" mistake, and possibly not even a "mistake", per se.
But I would venture into the area of possibly not recommended.

You are correct as you will have a little bit of imbalance. However, this isn't as much of a big deal as imbalance in serial cells. Parallel connections can get imbalanced for a little while with little to no issues. I doubt you'd have more than 100mA/mAh difference between cells close to the busbar, and those at the other end. Especially after resting for a few minutes.

If you think it's a big issue, you could always just move the one bar to the other end. Your design makes doing that a big easy to do. Albeit, the bar moved to the other end just won't have solid copper to the bar from the pack (unless you resoldered those runners, which, might not be feasible as you have a bunch of fuses)

I personally don't believe you'd have a lot of issues with it.
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#3
Surely it is a total non-issue with *adequate* bus bars? 

What matters is the difference in resistance between the pathways to the different cells. I can't see there being a significant difference with plenty copper in the bars along the pack. Sure more is better, but even the manufacturing differences between your (all new) cells are going to be more significant. 
Nickel has a much higher resistance than copper, and some of the nickel strips being used are quite thin. There probably is the potential for such an effect with nickel strips, but not with a few square mm cross-section copper, and you have 30mm2...

As a counsel of perfection, certainly opposite ends, equalising the path through each cell is ideal. But not needed if your bus bar resistance is insignificant, 

The opposite ends design is less convenient for racking, but does have a safety advantage in keeping the terminals as far apart as possible!
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#4
The Variance would only be evident is Really high Amp draw conditions. We are talking about max discharge so around 1A per cell and in an 80p packs that's 80 Amps. Not sure what your max load is going to be but even with 80Amp draw the difference we are talking about is .01V from end to end. and if you are leaving .1v wiggle room at the top and bottom you still have a lot of wiggle room. For example i run my Wall from 4.1 to 3.6V.

I did do a test when i was first looking at pack design (and I'm still making changes as I go lol) and took 8 cells hooked them together in parallel with the power leads off cell 1 only. Cycled the pack a few times then pulled a high drain (8Amps ) from the pack and then removed the batteries from the holder right after it was done and I only saw 2 batteries that were different and they were in the middle so I think that was just a happenstance of the cell being not exactly the same as the others in capacity or max draw

So personally I would not sweat it at all.
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#5
You variance due to difference in individual cells are easily 10mOhm or more ofr used cells.

Therefor your design is fine, since you have bus-bars with resistance which is a lot less than IR-variance.

Secondly, is only relevant at extreme currentloads.

And even with small busbars, the problem is stressing of front cells compared to back cells, and some extra charge-discharge cycles locally in the packs whilst it eaqualizes  - so some coulomb-losses (<5% of the imbalance-charge), and some wear on the cells - not optimal for teh pack, but possibly optimal in other regards such as access to maintenance. You get some and you loose some.

Your current* design is fine, dont sweat it.
 
* pun intended
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#6
As said its not a big deal as long as you keep your max current low and you have thick busbars. I have packs like that too and still no issues 1 year later... Not ideal no but it works.
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#7
A little test, with 2 V-meters, one in the front of the pack and one in the back, both on the same bus-bar in the middle, this is te one with 2 rows of cells.



In the cupboard,left top corner, without load:
 

With a 10min load of 130A, still the same voltage :
 

And after 45min "medium" load of +/- 60A, no difference between the voltages.


I also measured the voltage between the common-pack negative and the positive on a random cell in the pack (as Daromer suggested ) with a handheld V meter, and there is no difference between the cells ( only a 0,01V precision meter )
So, maybe some unbalance in the 0,001V range on a heavy load, but i think we can put this to rest and carry on happy Smile

I will leave the Volt meters in there for a while...

suggestions are still welcome.

My build: http://secondlifestorage.com/t-Instant-powerwall
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___________________________________
18x 300Wp solar off grid and 10x 180Wp solar tracker grid-tie
10KW 3phase hybrid inverter. 40Kwh 18650 storage (for now)
My setup: https://secondlifestorage.com/t-Instant-powerwall
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#8
The volt meter wont show discrepancies while they are hooked up. you would need to disconnect the cells to be tested immediately after drain, if not while dain so you can see it before and during bounceback
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#9
(09-20-2017, 03:21 PM)wim Wrote: A little test, with 2 V-meters, one in the front of the pack and one in the back, both on the same bus-bar in the middle, this is te one with 2 rows of cells.
...

My build: http://secondlifestorage.com/t-Instant-powerwall

I think your test demonstrates that your mini-meters are consistently calibrated!
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#10
(09-20-2017, 04:13 PM)Korishan Wrote: The volt meter wont show discrepancies while they are hooked up. you would need to disconnect the cells to be tested immediately after drain, if not while dain so you can see it before and  during bounceback

Very much true, but disconnect the cells immediately after drain is a bridge to far for me, before they "self-balance" i aim not fast enoug for that... sorry Wink

I believe the un-balance is going to be too small to messure with my voltmeters, maybe when discharged very fast ( way beyond normal use ).

This kind of test only shows, as hoped, there is no significant voltage drop in the lengt of the bus-bars under load, no voltage drop means the cells in front of the pack will not ge (much) more stressed then those in the back.

If there is a way to test without having to dismantle the pack, i would do it... i can not think of one....
At the end, for sure time will tell i guess....

(09-20-2017, 04:14 PM)dougal Wrote:
(09-20-2017, 03:21 PM)wim Wrote: A little test, with 2 V-meters, one in the front of the pack and one in the back, both on the same bus-bar in the middle, this is te one with 2 rows of cells.
...

My build: http://secondlifestorage.com/t-Instant-powerwall

I think your test demonstrates that your mini-meters are consistently calibrated!
Yep, they even blink between the numbers at the same time... Big Grin  was Lucky ... at last some great chinese stuff Wink
___________________________________
18x 300Wp solar off grid and 10x 180Wp solar tracker grid-tie
10KW 3phase hybrid inverter. 40Kwh 18650 storage (for now)
My setup: https://secondlifestorage.com/t-Instant-powerwall
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