72V lithium battery only charge to 76V

S

sonoaatta

10 mW
Joined
Aug 18, 2023
Messages
22
Location
canada
Recently bought a 72v ebike. I took the battery out, and I noticed that when I charge it the charger cuts off (go green) early, when I measured the battery voltage from charger terminal or discharge terminal it measures 84V, but once I take the charger off it measures 76V. I tried other chargers too, they all stop charging at 76V.

Also while charging when the charger light is red, I kept measuring the voltage from the battery it was measuring and increasing normally until 76V once reached it jumps to 84V suddenly, then when I remove the charger it goes to 76V.

It feels like the BMS limits the charging to 76V, is that possible? or does this mean there is a bad cell?

Unfortunately the battery has no labels at all, but it came from a 72V bike and with a 72V charger. I opened the casing and saw the BMS there are also no labels.
 
harrisonpatm said:
If you've already opened the casing, then I can suggest you try measuring each parallel group with a multimeter to report voltage.
Click to expand...
Thank you yes I measured each of the red wires to black and each group was increasing for example from (7V, 14v,.. to 26V) I don't have exact measurement now, I already put the battery back, but I think the voltage measurement made sense as it was increasing for each group.
 
sonoaatta said:
Thank you yes I measured each of the red wires to black and each group was increasing for example from (7V, 14v,.. to 26V) I don't have exact measurement now, I already put the battery back, but I think the voltage measurement made sense as it was increasing for each group.
Click to expand...
Is this a lithium ion pack and not lifopo4?
 
E-HP said:
Is this a lithium ion pack and not lifopo4?
Click to expand...
I Believe they are lithium there is only one label on the casing showing lithium power. When I opened the case the batteries were wrapped with blue heat shrink but BMS was outside.
 
sonoaatta said:
Thank you yes I measured each of the red wires to black and each group was increasing for example from (7V, 14v,.. to 26V) I don't have exact measurement now, I already put the battery back, but I think the voltage measurement made sense as it was increasing for each group.
Click to expand...
These would be the important numbers to write down. That they are increasing only means none of the parallel groups are at zero Volt. But one could be at 4.2V, whereas all others are at 3.5V, for example. If something like that were the case, then you might need to give the BMS some time to rebalance the groups. The charger might already indicate green while the rebalancing is going on.
 
Gruesome said:
These would be the important numbers to write down. That they are increasing only means none of the parallel groups are at zero Volt. But one could be at 4.2V, whereas all others are at 3.5V, for example. If something like that were the case, then you might need to give the BMS some time to rebalance the groups. The charger might already indicate green while the rebalancing is going on.
Click to expand...
ok i will have to take the battery out open again to check. I just found it strange that when voltage reach 76V it jumps to 84V suddenly to stop the charger.
 
sonoaatta said:
I measured each of the red wires to black and each group was increasing for example from (7V, 14v,.. to 26V) I don't have exact measurement now, I already put the battery back, but I think the voltage measurement made sense as it was increasing for each group.
Click to expand...
Were each of the twenty parallel groups (72V = 20s) the same increase in voltage? Just because there was an increase proceeding along each parallel group (B1-B20) doesn't mean the battery is balanced. The advantage of a so-called SMART BMS is a voltage readout of each parallel group (with active balancing) instead of very slow passive resistance balancing of a so-called dumb BMS.

Did you mean to say 76V instead of 26V? What was the voltage at each parallel group? What was the voltage at B20(76V?)

Being you took the time to measure and record the increase in voltage proceeding from B1 to B20 you could have at least listed the voltage increase from B1 to B20 for us to see ... without doing so how could we get some idea if there was any imbalance and amount of imbalance in a parallel group?

(7V, 14v,.. to 26V) tells us nothing as far as which of the twenty parallel groups may be underperforming (assuming 18650 or 21700 cells).

"but I think the voltage measurement made sense as it was increasing for each group" ... again that alone is no indication that the parallel groups in your 20s battery are balanced (having equal voltage in all twenty parallel groups). Was the voltage increasing the same amount for all twenty parallel groups as you proceeded from B1 to B20?
 
eMark said:
Were each of the twenty parallel groups (72V = 20s) the same increase in voltage? Just because there was an increase proceeding along each parallel group (B1-B20) doesn't mean the battery is balanced. The advantage of a so-called SMART BMS is a voltage readout of each parallel group (with active balancing) instead of very slow passive resistance balancing of a so-called dumb BMS.

Did you mean to say 76V instead of 26V? What was the voltage at each parallel group? What was the voltage at B20(76V?)

Being you took the time to measure and record the increase in voltage proceeding from B1 to B20 you could have at least listed the voltage increase from B1 to B20 for us to see ... without doing so how could we get some idea if there was any imbalance and amount of imbalance in a parallel group?

(7V, 14v,.. to 26V) tells us nothing as far as which of the twenty parallel groups may be underperforming (assuming 18650 or 21700 cells).

"but I think the voltage measurement made sense as it was increasing for each group" ... again that alone is no indication that the parallel groups in your 20s battery are balanced (having equal voltage in all twenty parallel groups). Was the voltage increasing the same amount for all twenty parallel groups as you proceeded from B1 to B20?
Click to expand...
Thank you for your reply, yes it is 76V voltage from the charging port and the discharge port. I will take the battery out on the weekend and report the measurement of each parallel group, it is one of those motorcycle ebike, it is not very easy to access the battery with all the covers.
 
sonoaatta said:
I Believe they are lithium there is only one label on the casing showing lithium power. When I opened the case the batteries were wrapped with blue heat shrink but BMS was outside.
Click to expand...
Let's see the label.
sonoaatta said:
Thank you yes I measured each of the red wires to black and each group was increasing for example from (7V, 14v,.. to 26V) I don't have exact measurement now,
Click to expand...
These are obviously just made up numbers, so not useful, but seeing what is stamped on the cells in the pack would be. IF the pack is actually 20S and charging to 76V, then it would seem to be a lifepo4 pack, since a fully charged cell would be 3.8V maximum safe voltage. 3.8x20 = 76V. Any higher and you would be overcharged, so if you are using a charger made for lithium ion cells, you could be damaging your pack. It could be that the BMS is shutting down the charge, not the charger, which results in the spike to 84V when the BMS cuts out to protect the pack at 76V.
 
E-HP said:
Let's see the label.

These are obviously just made up numbers, so not useful, but seeing what is stamped on the cells in the pack would be. IF the pack is actually 20S and charging to 76V, then it would seem to be a lifepo4 pack, since a fully charged cell would be 3.8V maximum safe voltage. 3.8x20 = 76V. Any higher and you would be overcharged, so if you are using a charger made for lithium ion cells, you could be damaging your pack. It could be that the BMS is shutting down the charge, not the charger, which results in the spike to 84V when the BMS cuts out to protect the pack.
Click to expand...
That makes lots of sense that it could be a lifepo4. Is there a way to verify without removing the heat shrink wrapping from the battery pack? Can we know from just the voltage of each of the parallel connection? Yes the voltages mention they were just an example how it was increasing not the real measurements. I will get actual measurement asap and photos.
 
sonoaatta said:
That makes lots of sense that it could be a lifepo4. Is there a way to verify without removing the heat shrink wrapping from the battery pack? Can we know from just the voltage of each of the parallel connection? Yes the voltages mention they were just an example how it was increasing not the real measurements. I will get actual measurement asap and photos.
Click to expand...
Assuming the BMS is shutting it down, then the individual voltage should tell the story.
 
E-HP said:
Assuming the BMS is shutting it down, then the individual voltage should tell the story.
Click to expand...
Thank you again for the help. I was able to take the battery out and measure again.

Measurements are as follow.
B1=3.709v
B2=7.46
B3=11.15
B4=14.9
...
B10=37.11
B11=40.8
...
B17=63.5
B18=67.6
B19=71.3
B20=75.1V
Only 20.

So it looks like they are increasing by 3.7v.
Does this mean it is lifepo4? Are the voltages normal?
 
sonoaatta said:
Thank you again for the help. I was able to take the battery out and measure again.

Measurements are as follow.
B1=3.709v
B2=7.46
B3=11.15
B4=14.9
...
B10=37.11
B11=40.8
...
B17=63.5
B18=67.6
B19=71.3
B20=75.1V
Only 20.
So it looks like they are increasing by 3.7v.
Does this mean it is lifepo4? Are the voltages normal?
Click to expand...
This is after the battery is fully charged?
3.7V is high if it's a resting voltage for lifepo4; 3.65V fully charged would be normal. Lithium ion fully charged would be 4.2V. Are you able to see the label/numbers on the side of one of the cells? Can you post a pic of the BMS?
 
sonoaatta said:
Thank you again for the help. I was able to take the battery out and measure again.

Measurements are as follow.
B1=3.709v
B2=7.46
B3=11.15
B4=14.9
...
B10=37.11
B11=40.8
...
B17=63.5
B18=67.6
B19=71.3
B20=75.1V
Only 20.

So it looks like they are increasing by 3.7v.
Does this mean it is lifepo4? Are the voltages normal?
Click to expand...
No, they are not all increasing by 3.7V. B18 - B17 is 4.1V. There might be other outliers that we don't see because you are too quick to jump to conclusions and are not giving us all the numbers. It's only 20 numbers, not a hundred. But even if there were a hundred, there still wouldn't be any shortcuts if you want to understand what is going on.

If one of the groups is at or near the maximum voltage (as judged by the BMS), then the BMS might very well stop charging and start balancing. What happens to the voltages if you leave the battery plugged into the charger for another 12 or 24 hours?
 
Gruesome said:
No, they are not all increasing by 3.7V. B18 - B17 is 4.1V. There might be other outliers that we don't see because you are too quick to jump to conclusions and are not giving us all the numbers. It's only 20 numbers, not a hundred. But even if there were a hundred, there still wouldn't be any shortcuts if you want to understand what is going on.

If one of the groups is at or near the maximum voltage (as judged by the BMS), then the BMS might very well stop charging and start balancing. What happens to the voltages if you leave the battery plugged into the charger for another 12 or 24 hours?
Click to expand...
Thank you again for your reply and support. This the full list. I have not tried leaving the charger for this long, It takes about 2-3 hours to charge to 76V then when the charger turns green I take it off. I can try to leave after it helps.
ConnectionVoltageDifference
B13.709
B27.463.751
B311.153.69
B414.93.75
B518.63.7
B622.243.64
B725.973.73
B829.683.71
B933.413.73
B1037.113.7
B1140.83.69
B1244.593.79
B1348.333.74
B1452.043.71
B1555.963.92
B1659.873.91
B1763.53.63
B1867.64.1
B1971.33.7
B2075.13.8
This is a photo of the battery with the BMS outside the case.

1721362943631.png
 
Somebody who knows more about how BMSes work should reply, but in my limited understanding the BMS would very slowly try to reduce the high group in this situation. So, I would leave the battery connected to the charger so the BMS can do its job.
 
Please do not leave this battery, or any battery, plugged in and charging for extended periods of time.

Your cells aren't a little bit out of balance; they are drastically mismatched. A low of 3.69v and a high of 4.1v. What's happening when you charge is that the charger is attempting to reach 84v (4.2v per cell x 20 cells). It stops at 76v or so because group 18 reaches 4.2v, and the bms cuts power. This will continue to happen every time you charge it; you are now relying on your BMS overvoltage protection to function every single cycle. If it fails once, you'll have a battery fire.

Some BMS'S have balancing function, to bring cell voltages closer together. But this is too much different for even a good BMS to deal with. you said you recently acquired this battery, so I would go to the seller with your chart of cell voltages and (politely) ask him what's up. If the cells weren't balanced prior to assembly, then it's the seller's fault, and who knows what other shortcuts were taken? Alternativelty, if the seller can prove that the cells were balanced, that means they became severely unbalanced in a short period of time, meaning poor quality cells.

Tldr; bad battery. I would not feel comfortable continuing to use it myself.
 
For reference, take a look at my newly made 28s battery:
Screenshot_20240717-122500.png

Group 15 is high, group 11 is low. If you were to ignore those two groups, you would see that total cell deviation is only 0.002v (4.141-4.143). This is what you want to see. And even you factor in my two problem groups, the deviation is still only 0.028v. Whereas your pack has a deviation of 4.1-3.69 = 0.41v.

Can you elaborate on where and how you got this battery?
 
sonoaatta said:
Thank you again for your reply and support. This the full list. I have not tried leaving the charger for this long, It takes about 2-3 hours to charge to 76V then when the charger turns green I take it off. I can try to leave after it helps.
ConnectionVoltageDifference
B13.709
B27.463.751
B311.153.69
B414.93.75
B518.63.7
B622.243.64
B725.973.73
B829.683.71
B933.413.73
B1037.113.7
B1140.83.69
B1244.593.79
B1348.333.74
B1452.043.71
B1555.963.92
B1659.873.91
B1763.53.63
B1867.64.1
B1971.33.7
B2075.13.8
Click to expand...
Well that’s bad. Probably would have saved some time getting the accurate measurements up front. The charger probably stops when B18 hits 4.2, and if the BMS doesn’t have a balancing function, it can only shut itself down so you don’t burn down your house. You could try charging it for a day outside, even if the light goes green, and see if any of the cells start getting closer, or manually discharge the highest cell(s) then try charging again.
 
harrisonpatm said:
For reference, take a look at my newly made 28s battery:
View attachment 356689

Group 15 is high, group 11 is low. If you were to ignore those two groups, you would see that total cell deviation is only 0.002v (4.141-4.143). This is what you want to see. And even you factor in my two problem groups, the deviation is still only 0.028v. Whereas your pack has a deviation of 4.1-3.69 = 0.41v.

Can you elaborate on where and how you got this battery?
Click to expand...
Thank you, it is a used bike I got from someone, he bought locally from a store that is also permanently closed. Just as project I would like to try to fix this, would it help if I drain the whole battery and charge again?
 
E-HP said:
Well that’s bad. Probably would have saved some time getting the accurate measurements up front. The charger probably stops when B18 hits 4.2, and if the BMS doesn’t have a balancing function, it can only shut itself down so you don’t burn down your house. You could try charging it for a day outside, even if the light goes green, and see if any of the cells start getting closer, or manually discharge the highest cell(s) then try charging again.
Click to expand...
thanks, I always charge those unknown batteries/projects outdoors, and even charge it inside an old bbq with some fans inside :).
Any suggestion how can I drain just those high voltage cells? do I need to take the heat shrink from the battery and access each of the cells? or can I just drain from the B18 cable?
 
It's not just B18. All the groups are completely out of whack, and since you got it used, you don't know how or why they got that way. The way I see it, there are two possibilities:
1. The pack was made properly with balanced cells, but has aged to a point where the cells are seriously out of balance, which means it's a significantly old pack and you shouldn't use it.
2. The pack is new, but was made improperly with out-of-balance cells, meaning who knows what other steps were skipped in its construction, which means it's a bad/unsafe pack and you shouldn't use it.

It's entirely possible that you may be able to bring them back into balance and get a bit more use out of it, but it would require adding a strong active balancer, accessing all the groups individually, and monitoring them closely after you bring them back into balance to make sure it doesn't happen again. It's your project, but if it were mine, I would start over with a new pack and scrap the current one down to individual cells.

sonoaatta said:
would it help if I drain the whole battery and charge again
Click to expand...
Nope, because attempting to drain it, the BMS should prevent discharging after the lowest cell group (B6?) reaches 3.0v or so, at which point B18 would still be .5v higher than that, and you'd still have an out of balance battery
 
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