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2 pack faling behind with charging and get unbalanced (update 9/07/2020)
#1
i hope you guys can help me.. i have 2 pack in my 14s powerwall faling behind with charging i had them with 0.007 balanced but after a few cycles the differece has gone up to 0.030 what could it be? maybe you guys had an experience with it?.
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#2
Are you using a bms?

How many cycles does it take to drift that much/little?
Generally speaking, drift can happen for lots of reasons. A cell that's very slow discharging, a connection is loose, missing/broken fuse wire to a cell(s), buss wire isn't large enough to handle the current, balance leads from bms not hooked up securely, etc
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#3
(08-10-2020, 10:53 AM)Korishan Wrote: Are you using a bms?

How many cycles does it take to drift that much/little?
Generally speaking, drift can happen for lots of reasons. A cell that's very slow discharging, a connection is loose, missing/broken fuse wire to a cell(s), buss wire isn't large enough to handle the current, balance leads from bms not hooked up securely, etc


Yes i am using a bms (ant 8s to 20s 300 amp) all the voltages displays the same as what the mulimeter says.
The drift happens after 5 cycles i will look for broken glass  fuses 
Also all  the cells are measured for IR and the were all very low (25 to 28 mohm) when i buils the packs.

you said "mutch/little" is the drif oke? will it be a problem?
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#4
(08-10-2020, 11:10 AM)barry Wrote: you said "mutch/little" is the drif oke? will it be a problem?

Some view <0.005V drift acceptable, others <0.01V acceptable.

It also depends on how many cells are in parallel. The more that are connected, the more drift that could occur. If it keeps happening, then you have an issue somewhere in your packs.
Another thing that could cause it to "seem" like there is drift, is if you did a test voltage while the charger/discharge was happening, or checking too soon after charge/discharge was disconnected. Don't do a measurement within seconds of disconnect. Wait about a minute to let the cells settle. Under charge/discharge, you could actually have slight voltage different from one end of the pack to the other (this is measured with one probe static position).

It's good that you are watching the voltages that closely, though. 3 digit readings can be pretty helpful in diagnosing issues.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
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#5
(08-10-2020, 11:18 AM)Korishan Wrote:
(08-10-2020, 11:10 AM)barry Wrote: you said "mutch/little" is the drif oke? will it be a problem?

Some view <0.005V drift acceptable, others <0.01V acceptable.

It also depends on how many cells are in parallel. The more that are connected, the more drift that could occur. If it keeps happening, then you have an issue somewhere in your packs.
Another thing that could cause it to "seem" like there is drift, is if you did a test voltage while the charger/discharge was happening, or checking too soon after charge/discharge was disconnected. Don't do a measurement within seconds of disconnect. Wait about a minute to let the cells settle. Under charge/discharge, you could actually have slight voltage different from one end of the pack to the other (this is measured with one probe static position).

It's good that you are watching the voltages that closely, though. 3 digit readings can be pretty helpful in diagnosing issues.


Thanks for your advise Smile i will look in to it i will report back when i find something

Well i think i found something holy crap... Sick Sick

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#6
it stil hold voltage at 4.1 volts and 20 mili ohm with a hole in it Huh
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#7
We need more info. How long (time not cycles) did it take for that balance drift? What type of balancer do you use? Did you do any type of matching when constructing the packs, e.g. by capacity, IR, or self-discharge?
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#8
(08-10-2020, 02:45 PM)gauss163 Wrote: We need more info. How long (time not cycles) did it take for that balance drift?  What type of balancer do you use? Did you do any type of matching when constructing the packs, e.g. by capacity, IR, or self-discharge?


all the cells are matched in capacity and IR so that cant be the problem SD cells goes right to the recyle bin...
The BMS i use ia a ANT BMS.
All the other12 packs stay perfecly in balance but i found the problem in one pack.... look at photo above Big Grin
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#9
^^ Ah, that photo clearly shows the culprit. How in the world did that hole occur? Was there some prior damage to that spot, or was something hot resting on it during cycling? Did you solder directly to the cell?
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#10
Woah you caught it in time! Glad you asked the question!

Drift is never ok. There should never be a time when any of your packs should self discharge ahead of your other packs. If you do, then pull that pack out.

Deviation between the packs may change during the day, and that's OK. You will never create an exactly balanced pack. So at certain battery state or SoC during the day, especially when reaching full charge or low charge, one battery may reach that goal faster so if you look at the charge curve the voltage will shoot up creating this deviation from the rest of the pack. For example my battery for the last 30 days may deviate by 20mV depending on the charge level. You see a repeating pattern. Also my balancer does not activate until it reaches 30mV. This is how I know my packs would have an issue if suddenly my balancer starts to activate daily and I see the graph hit 30mV for no reason.

 

Now your numbers and my numbers could be different, so maybe if your packs are more closely matched you will have a tighter deviation. Or if your packs are more loose then it might deviate more. For example it's ok to have a 100Ah and a 150Ah and 300Ah battery in series, but only as long as you take out of each pack the lowest common denominator, which is the 100Ah. So the pack even though has higher rated amps is only as strong as the weakest one. Take out any more than you risk ruining the smallest pack. Actually you would only want to take out 80% of the 100Ah pack, which means 80Ah is what you have out of each pack. But with that said, the deviation between these packs would be quite large depending on the SoC of each cell. When the 100Ah pack is depleted, the 300Ah pack would be at 60% charge and with a higher voltage. So not ideal and harder to manage but still a doable scenario.

But when you see a pack start to deviate from the normal, then you know you have an issue. So glad you noticed it. 

On your cell that burst, I can only guess that since it was at the corner that you may have bumped it against something hard, creating a dent in the cell. This dent pushes the layers inside the cell into a pinch point that probably over time weakens and creates a short between the layers. That's why we commonly tell everyone to discard any dented cells.  Now this is one possibility.
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