Battery voltage drop overnight

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utsi

10 W
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Oct 11, 2013
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Norway
Hi!

I recently had a problem with my 72v 21ah triangle pack when a charging wire had fallen off one of the cell groups. After soldering it back on, it seems degraded. All the other groups can hold their voltage (or drops ~0.01V after charging), while the said cell group drops by a whopping 0.1V over a couple of days.

I know it's bad, but how bad is it - Should I continue to use the battery as is or is it worth taking it to a repair shop? They will charge me at least 300 USD and that's half of what I paid for it in 2018. And if I continue to use it, will I have to continue to charge the bad group separately?

Note: The BMS stops at 4.1V for all groups, so the cells are never fully charged.
 
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E-HP said:
Does the BMS balance the pack when it reaches 4.1V as well?
Click to expand...

I looks like it stops charging when a couple of the groups reach 4.1V, the rest stays at 4.09. The lagging group remained at 3.65V before I charged it manually with a single cell charger. Having said that, I never left it charging for more than 9-10 hours after the regular charger had "finished".
 
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utsi said:
E-HP said:
Does the BMS balance the pack when it reaches 4.1V as well?
Click to expand...

I looks like it stops charging when a couple of the groups reach 4.1V, the rest stays at 4.09. The lagging group remained at 3.65V before I charged it manually with a single cell charger. Having said that, I never left it charging for more than 9-10 hours after the regular charger had "finished".
Click to expand...
I believe the BMS with bulk charge the pack until the pack reaches the cutoff voltage, then after that the individual balance leads will attempt to balance out the pack groups. If your original issue was that one of the balance leads wasn't attached, then the pack is still charging, but the balancing won't occur, at least for that group. That group could have been under or over charged over time, either of which could contribute to it's degradation. Not sure, but you may be able to somewhat nurse it back if you charge separately, but maybe not.
 
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E-HP said:
utsi said:
E-HP said:
Does the BMS balance the pack when it reaches 4.1V as well?
Click to expand...

I looks like it stops charging when a couple of the groups reach 4.1V, the rest stays at 4.09. The lagging group remained at 3.65V before I charged it manually with a single cell charger. Having said that, I never left it charging for more than 9-10 hours after the regular charger had "finished".
Click to expand...
I believe the BMS with bulk charge the pack until the pack reaches the cutoff voltage, then after that the individual balance leads will attempt to balance out the pack groups. If your original issue was that one of the balance leads wasn't attached, then the pack is still charging, but the balancing won't occur, at least for that group. That group could have been under or over charged over time, either of which could contribute to it's degradation. Not sure, but you may be able to somewhat nurse it back if you charge separately, but maybe not.
Click to expand...

So you're suggestion would be to use the battery and see what happens and charge the low group separately if it is considerably lower than the others after a ride? I'm just a little worried that using the pack could somehow make matters worse :confused:
 
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You don't want to overcharge the good groups. Are you able to to fully charge the bad group separately? If so, then I think if you can charge the bad group to be in line with the good groups first, then charge the pack normally. Hopefully you should be able to avoid overcharging. If you can't get the bad group to full charge, then you'll risk overcharging the good groups when charging normally.
 
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If that group is self discharging by .1v over a couple of days it indicates an issue with one or more of the cells in that group. Continuing to use it as-is or even keeping it in your home presents dangers at this point.

If it's already a few years old then repairing it by replacing the bad cells with new ones will only create a mismatch between that group and the rest of the pack. It would work but it would be less than ideal. Under the circumstances your best bet would likely be to bite the bullet and replace the pack.
 
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The charger seems to stop (or go into extremely slow mide) when one group reaches 4.1V. The rest of the groups were at 4.09V, except the bad one which I later charged separately until it was 4.12V. But it didn’t stay there, it kept falling. Was nearly at 4.0V when I started charging it again today.

Doesn’t seem like a good idea to seal up the pack with shrink wrap just yet🤔
 
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HK12K said:
If that group is self discharging by .1v over a couple of days it indicates an issue with one or more of the cells in that group. Continuing to use it as-is or even keeping it in your home presents dangers at this point.

If it's already a few years old then repairing it by replacing the bad cells with new ones will only create a mismatch between that group and the rest of the pack. It would work but it would be less than ideal. Under the circumstances your best bet would likely be to bite the bullet and replace the pack.
Click to expand...

Oh crap!😱 Will store it outside tonight!
 
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Often the BMS is powered off one cell group, which can drain it over time.

Try charging it up, disconnect the BMS, leave a few hours to settle then measure the voltages. Then leave for 48 hours and measure again.

If one group drops significantly more than the rest then you may have an issue
 
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electric_nz said:
Often the BMS is powered off one cell group, which can drain it over time.
Click to expand...
That makes sense, the problematic group is the first/last one. But the charging cable had also fallen off that same group before(?)
winter storage so it was probably way off the first couple of times I used the pack before I discovered the problem.

electric_nz said:
Try charging it up, disconnect the BMS, leave a few hours to settle then measure the voltages. Then leave for 48 hours and measure again.
Click to expand...
I did, at least 48-ish.

electric_nz said:
If one group drops significantly more than the rest then you may have an issue
Click to expand...
It does, ~0.1 vs 0.01, so I believe I have. Would searching for a repair shop with people willing to find cells with similar capacity be a viable option? I'm still hoping this pack is my last current gen before we can get our hands on Tesla-batteries or similar :lowbatt:
 
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If cells are actually bad, I wouldn't worry about replacing them with new cells that are slightly higher capacity. I've had to go that route with my pack that I abused temperature wise, and watching via the smart BMS it's been fine.
 
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thundercamel said:
If cells are actually bad, I wouldn't worry about replacing them with new cells that are slightly higher capacity. I've had to go that route with my pack that I abused temperature wise, and watching via the smart BMS it's been fine.
Click to expand...

Besides capacity, probably want to go with cells with the same (or greater) discharge rating as well.
 
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Another possibility is the BMS is bad and constantly discharging the cell group. You could try disconnecting the BMS and see if that group still loses voltage over time. If the voltage holds steady without the BMS then you could replace the BMS.
 
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The voltage on the bad group is dropping steadily, now it's at 3.93V while the others are at 4.09V. The BMS has been disconnected the whole time so it's not the problem.

I found a repair shop in my country who can fix batteries. The seller gave me a couple of alternatives:

1) I can give him the pack and he could replace all the existing Panasonic NCR18650PF 2900mAh cells with refurbished LG MH1 21Ah with 12month warranty. 600 USD.
2) He can exchange the bad group with some compatible cells. 250 USD.
3) He can build a new pack with LG MH1 21Ah with 12month warranty for 650 USD. (I get to keep my old pack)

I'm leaning towards the second alternative because I feel it's a shame to throw the pack away. What are your feelings about refurbished packs? 12 months warranty feels a little short.
 
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If he'll exchange the bad group and warrant that it won't be the first to hit lvc and won't sag more than the others under load, $250 is a cheap way to get back on the road. That said it's another $250 into an already 3 year old pack, and who knows how close to end of life the rest of the cells are. Repairing it may be penny wise and pound foolish, but your project, your budget, your level of risk tolerance
 
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i recycled some good panasonic 18650pd into a new pack, used them last year but this year one p-group has gone south
 
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utsi said:
I'm leaning towards the second alternative because I feel it's a shame to throw the pack away. What are your feelings about refurbished packs? 12 months warranty feels a little short.
Click to expand...

Might be a good time for you to describe the configuration of your current pack. 20S?P ? Math doesn't work assuming your original cells are 2900mAh, and the pack is rated at 21Ah. If the pack were 7P, that would be 20.3Ah; 8P would be 23.2Ah.

Assuming it's a 7P pack, then I'd do the $250. Reasoning: The LG's, when new, have higher capacity then the Panasonics (3200mAh vs 2900mAh). They both have the same discharge rates.

If he's rating his 7P pack made of used cells at 21Ah, then that means he's rating his cells at 3000mAh. If he actually tested them, then that's 94% of the capacity of new cells. That would seem to indicate the cells are barely used. All of this, of course, is based on whether you trust the guy. But $250 isn't too much to gamble.
 
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HK12K said:
If he'll exchange the bad group and warrant that it won't be the first to hit lvc and won't sag more than the others under load, $250 is a cheap way to get back on the road.
Click to expand...

That's an excellent suggestion, I'll ask him if he's willing to do that!

E-HP said:
Might be a good time for you to describe the configuration of your current pack. 20S?P ? Math doesn't work assuming your original cells are 2900mAh, and the pack is rated at 21Ah. If the pack were 7P, that would be 20.3Ah; 8P would be 23.2Ah.
Click to expand...
You are correct, thanks for pointing that out! I had forgotten to mention that upon opening the pack it became clear that the aliexpress seller had pulled one on me and given me cheaper cells than what the listing said - the pack is in fact 20.3Ah, 20S7P.

E-HP said:
Assuming it's a 7P pack, then I'd do the $250. Reasoning: The LG's, when new, have higher capacity then the Panasonics (3200mAh vs 2900mAh). They both have the same discharge rates.

If he's rating his 7P pack made of used cells at 21Ah, then that means he's rating his cells at 3000mAh. If he actually tested them, then that's 94% of the capacity of new cells. That would seem to indicate the cells are barely used. All of this, of course, is based on whether you trust the guy. But $250 isn't too much to gamble.
Click to expand...
Turns out the $250 will be closer to $400 with the current exchange rate and included delivery service but at least I won't have to wait more than a week or two.
 
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