Trouble shooting uneven cells

[bruce.nali]

Thanks in advance for any help. I have a simple beach cruiser conversion I did about 18 years ago with a 400 watt front hub motor. In 2016 I put in my second Ping battery (36v 15Ah LifePO4 battery pack with a 36V High Rate BMS). About once per month I charge for 24 or more hours but otherwise charge for about 4 hrs per night.
Battery has worked great but now cuts out after a short ride or hills.
Following advice here on some great threads, I charged it for 48 hours. At end, charger was green but BMS lights were blinking, not steady. Took off the charger waited for the lights to go out and found the following cell voltages:
1. 3.91 (B- to first pin)
2. 3.31
3. 3.57
4. 3.48
5. 3.57
6. 3.57
7. 3.57
8.3.57
9. 3.56
10. 3.57
11. 3.31
12. 3.57

Same voltages when measuring unplugged from the BMS.
Are the cells likely to balance out if I leave it longer on the charger (weeks?) or have I just gotten my monies worth from the this old Ping stack?
Could the BMS be bad - if so not sure how to test.
If a new battery - recommendations for a Ping Li substitute for this low tech machine.
Thank you in advance for any help - this forum has been wonderful and great to see the evolution of the ebike.
Bruce

 

[amberwolf]

The 3.91v cell could be a failed balance connection--does it read the same voltage at the BMS pin as it does at the cell? if it does, it's not the connection, but more likely the balancer itself has failed, and can't drain the cell down anymore.

If it's different at the cell, then reconnecting the cell to the BMS will probably fix it.

To rebalance all the cells, you'd need to leave it on the charger until all the lights are steady. That will at least give you back a bit of your capacity, but the real problem is that all the cells that are different in voltage from each other are actually different from each other in capacity and resistance.



To fix that, you'd have to replace the cells; since it's aged a fair bit it'd be easier to replace them all (and that way they'd all match and need less or no balancing until they age), but at that point it's far easier and more certain to replace the whole pack.

Other than EM3EV I dont' have a specific seller recommendation unless you want to build your own (in which case used EV modules from Batteryhookup, etc. are a good way to get (probably) matched cells.

I *can* recommend to stay away from Ebay, Aliexpress, Alibaba, and Amazon, etc., unless it's a well-known respected seller and the deal does not appear "to good to be true". Pretty much all the cheap packs are cheap for a reason, and will likely start out brand new in the condition yours is in after years of use...and will only get worse from there. At best. :/

Start out by looking for a pack that says it can handle *more* current than you need, and has more capacity than you need. Then it will still work for your needs even as it ages. I'd recommend at least 25% more. (if you get one that *just* matches your needs, then in a year or two, it won't quite meet your needs, etc).

 

[bruce.nali]

The 3.91v cell could be a failed balance connection--does it read the same voltage at the BMS pin as it does at the cell? if it does, it's not the connection, but more likely the balancer itself has failed, and can't drain the cell down anymore.

If it's different at the cell, then reconnecting the cell to the BMS will probably fix it.

To rebalance all the cells, you'd need to leave it on the charger until all the lights are steady. That will at least give you back a bit of your capacity, but the real problem is that all the cells that are different in voltage from each other are actually different from each other in capacity and resistance.



To fix that, you'd have to replace the cells; since it's aged a fair bit it'd be easier to replace them all (and that way they'd all match and need less or no balancing until they age), but at that point it's far easier and more certain to replace the whole pack.

Other than EM3EV I dont' have a specific seller recommendation unless you want to build your own (in which case used EV modules from Batteryhookup, etc. are a good way to get (probably) matched cells.

I *can* recommend to stay away from Ebay, Aliexpress, Alibaba, and Amazon, etc., unless it's a well-known respected seller and the deal does not appear "to good to be true". Pretty much all the cheap packs are cheap for a reason, and will likely start out brand new in the condition yours is in after years of use...and will only get worse from there. At best. :/

Start out by looking for a pack that says it can handle *more* current than you need, and has more capacity than you need. Then it will still work for your needs even as it ages. I'd recommend at least 25% more. (if you get one that *just* matches your needs, then in a year or two, it won't quite meet your needs, etc).

Thanks so much Amberwolf. I'll try leaving the battery on the charger for an extended time to see if the lights stop blinking and all come on. But sounds like a new battery is going to be the real solution. I've looked at some older threads regarding my set up - EZee 400 W (at 36V) and 550 (at 48V) motor with the older controller (60V max, 20A max) and wondering what at '25%' increase in capacity might look like. Would that mean going to 48V and 17Ah? Just a bit worried to not overload my controller.
Thanks again,
Bruce

 

[amberwolf]

Voltage wouldn't change. Only capacity (which, if you use a pack with the same or better cells, would then be capable of proportionally more current).

Cells have a "C rate" which is a wierd sounding term that simply means how much current the cells can supply. It's a number that is a multiplier for the capacity that gives you amps, which is confusing. For example, if you have a 5C cell (or battery pack) and it's 10Ah capacity, then it can supply 50A. So any pack or cell that isn't marked or specified with an actual Amp (A) limit (not Ah, or Amp-hours) but does have a C-rate, you can calculate the max A from the Ah and C-rate.


If your present pack worked great for you up to now, you can simply get another one with the same capacity (Ah) and current (A) capabilities.



The controller controls the current (that's what it's for), based on the load the motor puts on it, which is based on the riding conditions/etc. So a higher capacity or higher current-capability battery won't hurt the controller, though a higher voltage one might if the controller wasnt' designed for it.

 

[bruce.nali]

Voltage wouldn't change. Only capacity (which, if you use a pack with the same or better cells, would then be capable of proportionally more current).

Cells have a "C rate" which is a wierd sounding term that simply means how much current the cells can supply. It's a number that is a multiplier for the capacity that gives you amps, which is confusing. For example, if you have a 5C cell (or battery pack) and it's 10Ah capacity, then it can supply 50A. So any pack or cell that isn't marked or specified with an actual Amp (A) limit (not Ah, or Amp-hours) but does have a C-rate, you can calculate the max A from the Ah and C-rate.


If your present pack worked great for you up to now, you can simply get another one with the same capacity (Ah) and current (A) capabilities.



The controller controls the current (that's what it's for), based on the load the motor puts on it, which is based on the riding conditions/etc. So a higher capacity or higher current-capability battery won't hurt the controller, though a higher voltage one might if the controller wasnt' designed for it.

Thanks so much Amberwolf, really helpful and educational. Just a follow up on this thread in case it is of interest to someone in a similar situation. I left the battery on the charger for 14 days, the lights continued to blink, never reaching a full on status so I am assuming one or more cells are bad or the BMS. Given the old age of the battery I have ordered a new one from EM3ev (36v, 17ah), will put this one out to pasture, and get on the road again...
Bruce

 

[whatever]

This guy seems to have a good wide selection of batteries: Jehu Garcia
youtube: https://www.youtube.com/@jehugarcia/videos
website:

Lithium-ion Batteries

Lithium ion batteries tend to have higher energy density compared to LiFePO4 chemistry, making it a great choice for space-constrained or weight-sensitive DIY energy projects.

jag35.com

Seems he might be in Rancho Cucamonga near LA? but not sure if still there.