12s2p used leaf cells pack BMS issue?

[evblazer]

I think my BMS tanked and May have brought a cell or two with it.
In January 2017 or so I installed a 12s2p Nissan Leaf pack in my mower. I used the included “Battery Management System SD Technology - SDT14S20A” which was with the kit.
The last couple time I charged it stopped at 48.7 volts or so and it was cold and I really didn’t think much about it. A couple weeks ago I checked and the cells were spread out as shown in the attached. The balancing seemed to flash on for all odd then all even cells which is similar to what I recall long ago except it used to do odd even not all at once. I nievily thought I could plug in the charger and it would do something but it ended up starting to flashing different voltages 1.# or 6.# randomly assigned to cells as the charger started and stopped.
The wiring looks good as it was and terminals are clear of debris.
Does the odd random voltages just mean the BMS is toast? With the large differences now should I individually or at least use a balance charge to bring everything back up?

 

[amberwolf]

Does the odd random voltages just mean the BMS is toast? With the large differences now should I individually or at least use a balance charge to bring everything back up?

First you need to use a separate voltmeter to verify the actual cell voltages, if you suspect the BMS is bad, because you may not be able to trust it's readings.

I'd also recommend disconnecting the BMS entirely, if it's bad, because if it has failed balancers (stuck on) they will drain cells down constantly. You can test for that by measuring directly across each of the balance resistors; if there is any voltage they are on and draining cells.

If you find no actual problem with the BMS, and the cell voltages it displays are accurate, then you should restart charging and see if it shows some cells suddenly jumping in voltage above others--if it does, and a voltmeter on those cells shows the same thing, then those cells have high internal resistance compared to the rest and are probably why the BMS shutdown charging; and it couldn't finish charging the rest because whenever it restarts charging they go too high again.

That can happen from the cell quality, or their tolerance for temepratures, or both.

 

[Volt_Ampere]

Since all of your voltages are within safe values, your battery is probably fine. I would carefully charge up the lower cells to get them all close to each other. Most BMS have trouble balancing these larger cells. I use Leaf modules in my golf cart and they work great. I built my own BMS / balancer that can balance with about 800 mA. I enable the balance once every 5 charges or so and the pack stays very nicely balanced. I have been running this pack for 3 years now.
After manually balancing, you can try your BMS again and see if it is reading the voltages correctly and see if any of the balancing circuits are stuck on. They will get hot if they are on. I added LED's for each balance circuit on my BMS so I can see what is going on with the balancing. My display also shows which cells are being balanced.

 

[evblazer]

Thanks for the input.
I had measured a few before and they agreed. I measured again after it was first disconnected and measured 3 volts in one cell on my meter when the bms was going nuts. A couple days later and much warmed up weather and it is at 4 again matching the image. So the imbalances are still there but maybe the cold snap messed with the bms connector and/or battery in my separate meter. Maybe even the bms.

 

[evblazer]

Rats
Literal rats.
I took off the cover to the controller section and it looks like they chewed through the wiring. The battery section looks clear of debris and no chewing on wires or broken wires there.
Maybe they are unrelated incidents but I know I turned on the controllers and ran the blades likely when the wires were exposed and tried today before I took it apart and found them eaten through.
A new bms with Bluetooth is on the way but I guess spring leaf pickup is delayed.