Revived battery pack

[eCruiser3k]

Yesterday I noticed that my bike battery seems to be drained almost empty when I switch it on, there was little light in ca screen but no numbers. Bike was sitting about three months and all that time tranformer of the headlight was on. My mistake.

Battery pack is 20s9p 72v li ion with LG hg2 cells and I measured it only 8 volts! I connected the charger and it started pretty soon charge it. I tried couple of times but finally charger stayed on.
Now, after about 11 hours (normally it takes maybe 7 hours from 60v to 84v) pack was full 84v and also balanced normally. I drove little test drive and it worked normally, now because of cold weather, it is hard to tell is there any noticeable lost of battery capacity. Also during the charging there was no heated cells at all, pack feeled cold all the time. Tomorrow I must check that is there any self discharge happened but today everything seems to be fine. When I unpluged the charger, there was no voltage drop.

My only concern is now that is there any fire risk to live with that battery?

 

[ZeroEm]

The BMS should have tripped the LVC to protect the battery. You should have checked the P groups for 2v or less to know how bad it was. You may have not done to much damage. Best keep an eye on it while charging the pack.

 

[Bbbbrass]

I’ve had fine experiences with high-power cells being brought back and working normally. That said, technically there is an elevated risk now.

Is the headlight wired directly to the battery, bypassing the BMS?

 

[eCruiser3k]

It seems that battery keep its voltages, now after 24h there was exatly same voltage than yesterday.
There is bms only for charging so nothing is limiting discharging all out.

Have I understood right that fire risk is only when using battery like charging or driving but if it sitting in the garage without any operating it should be happy and not burning everything?

 

[ZeroEm]

by eCruiser3k » Jan 15 2023 6:54pm

It seems that battery keep its voltages, now after 24h there was exatly same voltage than yesterday.
There is bms only for charging so nothing is limiting discharging all out.

Have I understood right that fire risk is only when using battery like charging or driving but if it sitting in the garage without any operating it should be happy and not burning everything?

Sounds ok just charge before riding, not before storing in garage. BMS should monitor parallel groups and and if one gets to low then shut off, if not then the BMS is not very good.

 

[eCruiser3k]

Yes, thats how I normally do, charging before riding and longer breaks are somewhere about 50%.
Battery is not self made so all I know is that bms is only for balancing when charging. Time will tell is there any problems in the future... Thank you guys at this point :thumb:

 

[docw009]

If you only had the BMS connected for charge protection, I could see how a headlamp took it down to nothing. How did you recharge it? The BMS should have stopped you. If you managed to recharge thru the normal charge port, maybe the BMS was also connected for discharge. If so, all should be well.

 

[amberwolf]

Even with a BMS that protects both charge and discharge, it's not uncommon for the discharge (or charge) FETs in a typical BMS to fail stuck on, so that the pack works apparently normally for discharge, but cannot turn it's connection off in the event of overcurrent, overdischarge, etc.

Same thing for the charge port, which wil be unable to prevent charging an overdischarged pack (for instance), or overcharging unbalanced cells, etc.


See Methods' recent Daly BMS thread(s) for some analysis of that model/brand in this kind of failure.

 

[eCruiser3k]

The battery was charged via the normal charging port just like I normally do and first when I plugged the charger wire into the port charger started and stopped immediately so I unplugged the wire and put it back in, I have to repeat this a couple of times until the charger stayed on. According to the CA, voltage goes from 8 volts to about 50 volts pretty quickly, maybe in one hour and then it slowed down to near normal rate wich was maybe 10 hours from 50v to 84v using 300w charger. Battery capacity is 27ah.

Is there anything abnormal that the charger accepted began charging from eight volts?

 

[docw009]

I think your BMS must be connected normally for discharge, and when the headlight drained the cells down to 3 volts each, the BMS triggered and isolated the cells from the output. You measured 8 volts, but that was just leakage around the BMS.

The clue is you saying it charged normally. A good BMS would never let a 20S battery recharge from 8 volts. That's like .40 volt per cell. The science says cells that sit around at .50 volts a while may form dendrites that create internal short circuits later.

.

 

[eCruiser3k]

Okay, how about that Cycle Analyst internal battery resistance function, does that tell anything if it shows same values like it always do about 0.050-0.065 ohms depending on battery stress and temperature?

It takes longer time to charge from 60v to 84v than normally but otherwice charging process was like normally.