5S2P - Strange Problem - Two Low V Pairs

markz

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What interests me the most is the first module.
New Battery in DMM and solid contact with leads.
These two modules Z and Y were paralleled, that is why they are so similar.

MODULE Z - Can save four 18650's maybe six.

I measure entire 5S2P group and I get 5.40V and moves down slowly, never stable.

I measure 2 parallel groups at a time and it becomes interesting.
First two on negative side (call them A + B) 7.92V which is good!
Now row B + C is 0.98V not good
C + D same voltage 0.98V
D + E same 0.98V

Individual Pairs 1S2P
E = 2.75V Hopefully GOOD
D = 4.0V BAD
C = 2.96V BAD
B = 4.0V GOOD
A = 3.9V GOOD

The measurement of 1S2P look normal but out of balance but within cell range, but the 0.98V 2S2P I get from measuring is not good. I am wondering why I get those measurements. I kept measuring full pack voltage, and was getting lower then expected voltage after measuring individual voltages of each 1S. So you see my dilemma. I knew something was up so I broke the pack down even further and came across these two 5S2P modules that were bringing the entire pack down. I find it odd that the voltage reading starts at a given voltage, then slowly and always move lower and lower. Obvious sign of weakness for those pairs.



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MODULE Y - Can save four 18650's maybe six.

Individual Pairs - parallel groups (2P)
E = 3.9 negative side of module GOOD
D = 4.0 GOOD
C = 0.9 never stable
B = 4.0 Maybe GOOD
A = 1V never stable

Entire module 2.5V never stable

A and B - Nothing stable
BC - Nothing stable
CD - Nothing stable
DE - 7.90V GOOD
 
markz said:
E = 2.75V Hopefully GOOD
D = 4.0V BAD
C = 2.96V BAD
B = 4.0V GOOD
A = 3.9V GOOD

I don't understand your criteria for "good" and "bad". :?

if you call C=2.96v bad, then E=2.75v is worse, so it can't be good.

if you call B=4.0V good, then D=4.0v can't be bad, because its' the same. Or else, if D is bad, tehn B can't be good, because it's the same--and then A has to also be bad because it's worse, at 3.9v.

I find it odd that the voltage reading starts at a given voltage, then slowly and always move lower and lower.
This usually indicates a high impedance or capacitance that has a residual voltage on it, and the meter's impedance when connnected is draining that residual voltage. For instance, this happens with a BMS that's shut off it's output, so it appears to have a voltage on it, but will drop slowly...or a controller with big caps in it, etc.


Are you measuring at the actual cells, or are you measuring at the BMS or balance leads?

If the former, they should be accurate readings. If the latter, it's possible a broken wire or bad crimp at the contact could give the high impedance causing falling voltage or wierd readings.
 
There is no BMS, its strictly 18650 in a 5S2P config, so I measured a few things.
A) Each paralleled paired group, or 1S, at Vmax is 4.20V
B) Entire 5S2P at Vmax is 21.00V
C) Then I measured every 2S, which at Vmax is 8.40
*But realize this pack was at some voltage not Vmax, I'd assume each can was at roughly 3.89V when I see most of the others at that 3.90V.

I mentioned criteria bad because when I measured 2 in series, the resulting voltage came up a dud when compared to them singularly as in when I measured one 1 in series, it came up good, but 2 in series was bad. It confused me too.

That makes sense AW about the IR and DMM.

Time to take the weak cells out.




amberwolf said:
 
If you measure a cell good, but two in series including that cell as a voltage lower than the good cell, then either you have:

a connection problem between the cells

or

a connection problem from meter or probe to cell

or

the bad cell is actually reverse voltage (very bad), so that the two cells added together make a voltage lower than the higher voltage cell.
 
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