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Repaired my Grin pack, I think...

OCD Garage

10 mW
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Mar 12, 2024
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Canada
A while back I posted looking for info on large format cell packs after my 72v 23ah pack from Grin started only charging to 82.4v and shutting down at 71v.

I have since opened the pack up and found a very nicely build battery pack with a tiny BMS in it. I decided to change it out for a much larger BMS I had bought for another project some time ago. Originally the pack would only charge up to 82.4v when the charger would stop, even when left to balance charge for hours.

After changing the BMS and letting the pack balance charge for a day, then cycling it through use and charging/balancing again, the pack seems to have reached a maximum voltage of 83.9v, I have yet to see how low it will go while in use as I don't like pushing home. I have read that a cheap BMS will only charge up to the level of the worst cell, whereas better quality ones will drain all the cells down to the same level and then charge them all up together. Should it actually come all the way up to 84v or do you think this is it 'back to normal'?
 
Did you inspect the cell groups while you had it open? The voltage you measure at end of charge depends on the charger and BMS specs, but 83.9V is very close to 4.2V per cell which is pretty much as high as you would want to charge that pack.
 
I have since opened the pack up and found a very nicely build battery pack with a tiny BMS in it. I decided to change it out for a much larger BMS I had bought for another project some time ago. Originally the pack would only charge up to 82.4v when the charger would stop, even when left to balance charge for hours.

After changing the BMS and letting the pack balance charge for a day, then cycling it through use and charging/balancing again, the pack seems to have reached a maximum voltage of 83.9v, I have yet to see how low it will go while in use as I don't like pushing home.
Most likely either the BMS included doesn't have a balance function (seems unlikely from Grin) or has a very small balancing current capability (which can take days to weeks to balance a pack with serious cell problems).

The new BMS probably has a higher-current balancing function, so it can drain the higher capacity cells faster to allow the low-capacity cells to catch up in voltage.

Note that neither one fixes anything, it just charges the pack up to a higher voltage, and may allow you to use more of what capacity remains.

To fix it requires replacing the low-capacity cells.


I have read that a cheap BMS will only charge up to the level of the worst cell, whereas better quality ones will drain all the cells down to the same level and then charge them all up together.
Neither is strictly true.

For the first, some BMS include a balancing function, and some don't. Technically the ones without will probably cost less than those with, but that's not usually what people refer to when stating "cheap BMS". ;)

Cells with a balancing function vary a bit in detail, but they all have some cell voltage near full at which balancers turn on and begin shunting current around the near-full cells (or shuffling charge to less-full cells for active-balancer types) so the less-full cells can catch up. When the fuller cells reach HVC the BMS turns the input off, and continues draining any cell that is over "full" until it is safe to turn the input back on and continue charging to let the less-full cells continue to catch up, repeating this cycle until they are all within whatever range the BMS is designed or programmed to allow.

This can take hours, days, or weeks depending on the amount of shunting / draining current the BMS has (or shuffling charge amount for active balancer types), vs the difference in capacity between cells.


There are more details about this sort of thing in the many BMS discussions around the forum, often in pack repair and build threads.





Should it actually come all the way up to 84v or do you think this is it 'back to normal'?
Depends on what is wrong with the cells. You'd have to have actually measured the cell voltages while you were in there before changing the BMS and recharging to find out.


If you didn't do any testing then you'd have to run the pack down to empty and then measure the cell voltages to get a guess as to what might be wrong / how bad the problem is.

To find out what's really wrong with the cells you'd have to test them individually, but it's doesn't really matter--knowing which cells are not staying identical in voltage with the rest when it's drained tells you which ones to replace. The lower their voltage, the less capacity they have.

Note that the opposite problem happens during charge as it reaches full--the cells with less capacity are at higher voltage (because they fill up first) and the ones with more capacity are lower voltage as they take longer to fill.
 
Thanks for the above responses/opinions. Prior to changing the BMS I needed to charge at work to make it home, now I don't.

Yes, I did measure the group voltages and one was below the others, the other 19 were all virtually the same. Clearly the new BMS (which is 3x the size of the original) must be providing better balancing as I can see the LEDs switching on/off as they work. I can now ride to work with 80v left in the pack instead of 73v so no need to charge before returning home.
 
Someone should mention that regular DMM's typically have a voltage accuracy of +/- 0.5% i.e.at 80V nearly half a volt either way.
 
Yes I have a BT BMS and after two years. I still finding out things.
I would change the low cells manually before I put a BMS on. As it makes it ezer on the new BMS.
After you put the new BMS on did you wait before you sealed up the battery again to make sure all cells were where you want it ? 4.18v or so Or ?
 
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