brumbrum wrote: ↑Jan 02 2018 7:56am
Maybe ok for 30a draw, but you may find over time that the cell voltages begin to drift apart as some cells are taking the strain more than others.... In theory.
I really have to wonder about this "theory". If the cells in a parallel group started to "drift apart" -- by which I take it you & others) mean some cells will discharge more than others, thus reaching up at a lower voltage than those others, then the moment the demand eases off -- you shut the throttle -- the differential in voltages within the parallel group will cause a current flow from those with the higher voltage to those with the lower, effectively self-balancing the group.
I find it almost inconceivable that the tiny extra resistances between the cells at one end of a parallel group, and those at the other would be in any way sufficient to affect the long term health of the individual cells, regardless of whether the serial connection to that group is made at one end of the pack or the other.
And nothing I've read on the subject outside of this thread has even mentioned this as a possibility.
It seems to me that someone -- the OP? -- had the notion and posted it here, and it has gradually gained currency based purely upon that supposition. And the idea that people are inventing ever more elaborate connection schemas to try and counter a theoretical possibility that no one to my knowledge has provided the slightest piece of actual evidence for, just seems crazy.
Update: I just saw the link below where there are some calculations suggesting that in a 4P group, one cell is drawing 35.9% of a 100A draw, because of a 0.0015 Ohm resistance in the links between the 4 cells. Did anyone with good math/EE knowledge verify those figures, because they do not look right to me.
Update 2: I drew up a circuit equivalent of a 4P group connected to a load drawing 100A via 10mm x 0.15mm x 18mm long connections at
http://www.falstad.com/circuit/circuitjs.html which can be recreated by importing this text file:
Code: Select all
$ 1 0.000005 10.200277308269968 50 5 43
r 48 128 160 128 0 0.000008388
r 224 128 320 128 0 0.000008388
r 384 128 464 128 0 0.000008388
r 48 320 160 320 0 0.000008388
r 224 320 320 320 0 0.000008388
r 384 320 464 320 0 0.000008388
w 16 128 48 128 0
w 16 320 48 320 0
w 160 320 192 320 0
w 192 320 224 320 0
w 320 320 352 320 0
w 352 320 384 320 0
w 464 320 496 320 0
w 496 320 576 320 0
w 160 128 192 128 0
w 192 128 224 128 0
w 320 128 352 128 0
w 352 128 384 128 0
w 464 128 496 128 0
r 576 128 576 320 0 0.0295
v 16 320 16 208 0 0 40 4.2 0 0 0.5
r 16 208 16 128 0 0.05
v 192 320 192 208 0 0 40 4.2 0 0 0.5
r 192 208 192 128 0 0.05
v 352 320 352 208 0 0 40 4.2 0 0 0.5
r 352 208 352 128 0 0.05
v 496 320 496 208 0 0 40 4.2 0 0 0.5
r 496 208 496 128 0 0.05
w 496 128 576 128 0
It looks like this:
- PgroupEquivCircuit.jpg (69.3 KiB) Viewed 1338 times
And when I mouse over the 4 cells, the current draw from each of them, left to right is:
24.97A
24.98A
25.00A
25.02A
I find it hard to believe those differences would be significant?
It should be 838.8 uOhms.
FWIW a 200% difference in current sharing between cells would probably be a disaster on the first cycle!
