BMS discharging/balancing issue

ebike11

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Hi all!
I have a question regarding my ant bms.
Im trying to slowly let the bms discharge/balance them to the same level as the lowest cell. You can see one of my cells is quite low

When the low cell reaches the cutoff voltage limit thats programmed in the app, the balancing stops totally and the rest of the cells are still much higher.

How can I bring all of the higher cells down to the same level as the lowest cell in order to make them all even?


Thanks
 

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You can use a resistor (or light bulb, other resistive load) to drain each cell group down, but it's going to take time and care not to short anything in the process.

Less chance of doing that if you instead manually charge up the low cells.


However, with that big a difference in cells, you have a fairly serious problem with the cell(s) in the groups that is only going to get worse with time, and I would recommend replacing the cells with non-defective ones, or repair faulty group interconnects, etc., that are causing the problems.

The problem could be with the low group (it could be self-discharging so losing more of what goes into it), or it could be with all the other groups (they could be lower capacity so filling up faster). so some testing would be needed to find out where the problem lies. There are numerous battery repair threads with variations on the testing process you can use to find a process you can perform with your available test equipment and abilities.


The BMS is probably refusing to balance or continue because it knows a difference that big indicates a serious problem and doesn't want to do anything that could potentially cause a fire now or in the future--it's job is to prevent that sort of thing.
 
amberwolf said:
You can use a resistor (or light bulb, other resistive load) to drain each cell group down, but it's going to take time and care not to short anything in the process.

Less chance of doing that if you instead manually charge up the low cells.


However, with that big a difference in cells, you have a fairly serious problem with the cell(s) in the groups that is only going to get worse with time, and I would recommend replacing the cells with non-defective ones, or repair faulty group interconnects, etc., that are causing the problems.

The problem could be with the low group (it could be self-discharging so losing more of what goes into it), or it could be with all the other groups (they could be lower capacity so filling up faster). so some testing would be needed to find out where the problem lies. There are numerous battery repair threads with variations on the testing process you can use to find a process you can perform with your available test equipment and abilities.


The BMS is probably refusing to balance or continue because it knows a difference that big indicates a serious problem and doesn't want to do anything that could potentially cause a fire now or in the future--it's job is to prevent that sort of thing.

Thx for the reply
But in the App setting i put cell max. at 4.2 volts. But when the highest cell reaches 4.2V it shuts off the charger. But how can I get all the other 26 healthy cells to also reach 4.2V if the BMS cuts off the charger due to the single higher cell??
 
Not all BMS support balancing. Some BMS that do have a button in the app you can press to trigger manual balancing. E.g. here's a screenshot of hitting it from an ANT listing:
Screenshot_20221128-011204.png

Without a manual trigger, for BMS that do support balancing, the criteria for auto-balancing is often very strict. E.g. it may balance only at the very end of the CV charge phase when the charger has reduced charging current to be very low. This is because many BMS only have some small resistors for bleeding off current from the highest cells to implement balancing and these can only apply 50mA or less of balancing current. That's why you often see people say to leave the battery on the charger overnight to balance.
 
It will take a really long time to pull down all the cells to that level. It would be faster to individually charge the low cell.
I've done that in the past by disconnecting the BMS and using a pair of SOLID strand copper wires inserted into the balancing plug and connect that to a single cell charger.

Single Cell Charging Hookup.jpg
 
ebike11 said:
But in the App setting i put cell max. at 4.2 volts. But when the highest cell reaches 4.2V it shuts off the charger. But how can I get all the other 26 healthy cells to also reach 4.2V if the BMS cuts off the charger due to the single higher cell??

Manually, as described in my reply, or in the many various battery repair threads around the forum.

But you don't know that the other cells are the healthy ones. They could also be ones that are all low in capacity compared to the one that is lowest in voltage (which could simply be taking longer to fill because it's "bigger" than the others).

This is why you would have to test them all to find out which ones are not meeting the specs you need them to. Otherwise you're guessing and don't know the state of the pack.

Remember that cells that are equal in voltage (balanced) aren't necessarily equal in capability or capacity (well-matched), and will not then remain balanced, nor will the pack perform as well as if they were all well-matched to begin with.
 
lnanek said:
Not all BMS support balancing. Some BMS that do have a button in the app you can press to trigger manual balancing. E.g. here's a screenshot of hitting it from an ANT listing:
Screenshot_20221128-011204.png

Without a manual trigger, for BMS that do support balancing, the criteria for auto-balancing is often very strict. E.g. it may balance only at the very end of the CV charge phase when the charger has reduced charging current to be very low. This is because many BMS only have some small resistors for bleeding off current from the highest cells to implement balancing and these can only apply 50mA or less of balancing current. That's why you often see people say to leave the battery on the charger overnight to balance.

The Ant bms i have has a auto balance button..but not sure if it still auto balances on its own
 
amberwolf said:
ebike11 said:
But in the App setting i put cell max. at 4.2 volts. But when the highest cell reaches 4.2V it shuts off the charger. But how can I get all the other 26 healthy cells to also reach 4.2V if the BMS cuts off the charger due to the single higher cell??

Manually, as described in my reply, or in the many various battery repair threads around the forum.

But you don't know that the other cells are the healthy ones. They could also be ones that are all low in capacity compared to the one that is lowest in voltage (which could simply be taking longer to fill because it's "bigger" than the others).

This is why you would have to test them all to find out which ones are not meeting the specs you need them to. Otherwise you're guessing and don't know the state of the pack.

Remember that cells that are equal in voltage (balanced) aren't necessarily equal in capability or capacity (well-matched), and will not then remain balanced, nor will the pack perform as well as if they were all well-matched to begin with.

The packs were all recently purchased new..so for starters I think I will assume that its just 1 low cell...maybe ill try to charge the single cell up first
 
ebike11 said:
The packs were all recently purchased new..so for starters I think I will assume that its just 1 low cell...
Then they are not very good packs, and are not made of well-matched cells. If they were all equal-capability cells then they would all start out equal, and stay that way.

ATM your best bet (short of replacing the packs with well-matched ones) is to rebalance them by draining the one high cell with a resistive load, and charging up the one very low cell with a single-cell charger or USB power supply, etc. (being careful to monitor voltage continuously during the process to ensure you don't drain the one down below the others, or charge the other up above them).

Once all the cells are reasonably close (within a few hundredths of a volt) the BMS (if it is a balancing type) should be able to take over and finish the job.

Then you should monitor the cells during usage, especially at higher loads, to be sure none are dropping in voltage more than any others--if they are not all equal regardless of load and state of charge, then they are not all equal capability and whatever problems you see will only get worse as they age.
 
amberwolf said:
ebike11 said:
The packs were all recently purchased new..so for starters I think I will assume that its just 1 low cell...
Then they are not very good packs, and are not made of well-matched cells. If they were all equal-capability cells then they would all start out equal, and stay that way.

ATM your best bet (short of replacing the packs with well-matched ones) is to rebalance them by draining the one high cell with a resistive load, and charging up the one very low cell with a single-cell charger or USB power supply, etc. (being careful to monitor voltage continuously during the process to ensure you don't drain the one down below the others, or charge the other up above them).

Once all the cells are reasonably close (within a few hundredths of a volt) the BMS (if it is a balancing type) should be able to take over and finish the job.

Then you should monitor the cells during usage, especially at higher loads, to be sure none are dropping in voltage more than any others--if they are not all equal regardless of load and state of charge, then they are not all equal capability and whatever problems you see will only get worse as they age.

Thanks!!
I think Ill start off by trying to get that cell higher with a wall charger this weekend. Ill keep you updated
 
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