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BMS heating up, how to diagnose BMS/Battery

mscir

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Oct 25, 2022
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I got a very old used, non-running ebike on CL, it had a 37V battery but no charger. I ended up removing the electrics because I couldn't find parts, the controller was damaged no joy finding one.

I bought a charger on BangGood and charged the battery about 3 wks ago, it's still holding 37.7V. I left the charger connected for maybe 4 hrs, after disconnecting the charger, with no load on the battery - nothing connected to it, the BMS has a hot spot on it 10 min's after charging. I couldn't find anything else warm but quickly unplugged the 8-pin white connector because I had read that the BMS can help prevent explsions due to too rapid charge/discharge and I didn't want to take any chances. Interestingly with the battery red and black leads attached to the BMS but the white 8-pin connector disconnected it stopped getting hot. That will help in the diagnosis if I try to repair it.

I don't know anything about Lithium Ion batteries or BMS tech, so I thought I'd ask here, how would I go about diagnosing this?

- The batteries have a batch code 20150803, can I assume that means Aug 2015?
- If the batteries are 2015 does that mean they're not worth keeping?
- How do I verify the battery cells are working normally before spending $ on it if it's worth repairing?
- If the BMS is faulty, is it worthwhile attempting a repair?
- will any 37V BMS with the same 8-pin white connector be suitable as a replacement?

I have a bunch of solar cells being given to me so I can charge these batteries if i get this sorted then use them for indoor LED lighting at night.

Thanks in advance, I'm a complete newbie, I was an electronics tech and am familiar with basic electronic concepts.
Mike
 

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The resistors get hot when they are balancing cells. If they are getting hot, the cells are not in balance, and those cells EXPLODE AND SELF IGNITE… so be very mindful when charging.
It sounds like the BMS is doing its job. I would measure each set of cells and determine which ones are getting charged fast, and which ones are lagging.

Or just get new cells, or an entire new battery.
 
The resistors get hot when they are balancing cells. If they are getting hot, the cells are not in balance, and those cells EXPLODE AND SELF IGNITE… so be very mindful when charging.
It sounds like the BMS is doing its job. I would measure each set of cells and determine which ones are getting charged fast, and which ones are lagging.

Or just get new cells, or an entire new battery.
RE: the resistor heat, it was not uncomfortable to keep a finger on it, so it wasn't scorching hot.
Base on your comment I went ahead and finished charging the battery, the charger light turned green, the voltage on the charge voltage wires of the BLM showed 42V. When I disconnect the charger the output voltage on the BLM is 38.2V. The battery says it's 37V. Should the fully charged battery voltage be higher?
 
The resistors get hot when they are balancing cells. If they are getting hot, the cells are not in balance, and those cells EXPLODE AND SELF IGNITE… so be very mindful when charging.
It sounds like the BMS is doing its job. I would measure each set of cells and determine which ones are getting charged fast, and which ones are lagging.

Or just get new cells, or an entire new battery.
If I remove all of the wrapping from the individual batteries, what are acceptable voltage ranges for them now that it is as charged as the BLM can manage? Do you have any idea how I would determine which cells are bad? Thanks.
 
If I remove all of the wrapping from the individual batteries, what are acceptable voltage ranges for them now that it is as charged as the BLM can manage? Do you have any idea how I would determine which cells are bad? Thanks.
There are 18650 cell testers out there. I would run one and test each one’s capacity.
 
I opened up the battery pack and found (pic below) 10 cells. I measured the voltages at the spots where the 9 white wires attach, with the negative voltmeter lead on the B- of the BMS board, and got the following Voltages(below).

Am I correct in thinking there's a problem with the first cell where the white wire measures 0V? The cells are labeled 3.7V, and all but the first one measure roughly 4.16V.

The battery output measures 37.3V.

My quesiton is, should my battery be charging closer to 41.5V total, and do I have a bad cell where the white wire measures 0V? My working 48V bike battery charges to over 52V if I remember correctly. so I'm thinking the fact that a 37V battery (labeled) only charges to 37.3V sounds low.

1 0V
2 4.16V
3 8.35V
4 12.53V
5 16.71V
6 20.91V
7 25.10V
8 29.30V
9 33.49V
 

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Ok, it looks like the bms is positive linked, so start at main positive, measure to the first white wire, then heck each cell from there down the line.
 
Lithium ion polymer? As in LiPo ?
I would run as fast as I can... It's a bomb waiting to explode!

Well, maybe I'm exaggerating, but that Lithium chemistry is one of the most dangerous ones. A friend of mine accidently made some sort of short while working on his hobby drone, and the battery simply exploded into a big ball of fire! It took 5 large fire extinguishers to put that fire out.
Battery was a LiPo...
Please be careful, especially if your not sure what you're doing!

Anyways, you didn't mention at what current you are charging this 10Ah battery with?
Maybe you are charging too fast? What Ah rating is the charger?

For a 10Ah battery you should be OK charging at 0.5C rate (a.k.a half of the capacity) which means 5A charging at most.
 
With 10 cells you should have 11 wires...
Not 9
I thought about that as well. That's why I included a picture of the white wire connector that goes from the cell junctions to the BMS. Since the cells are all connected in series, and the B- and B+ wires connect to the - and + ends of the series of batteries, I am guessing the BMS was designed with that in mind, because there are 11 connections to the battery pack in total 9 white, B-, B+. I made a simple PS schematic of what I'm seeing.
 

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Lithium ion polymer? As in LiPo ?
I would run as fast as I can... It's a bomb waiting to explode!

Well, maybe I'm exaggerating, but that Lithium chemistry is one of the most dangerous ones. A friend of mine accidently made some sort of short while working on his hobby drone, and the battery simply exploded into a big ball of fire! It took 5 large fire extinguishers to put that fire out.
Battery was a LiPo...
Please be careful, especially if your not sure what you're doing!

Anyways, you didn't mention at what current you are charging this 10Ah battery with?
Maybe you are charging too fast? What Ah rating is the charger?

For a 10Ah battery you should be OK charging at 0.5C rate (a.k.a half of the capacity) which means 5A charging at most.
Thanks for the heads up. I was an electronics tech so I'm familiar with basic electronics, but I'm only a newbie when it comes to batteries.
My charger is 2A.
 
Ok, it looks like the bms is positive linked, so start at main positive, measure to the first white wire, then heck each cell from there down the line.
I made a quick PS showing what I'm seeing, one cell appears dead, is it correct to think that a 37V battery should charge to a few volts higher than that? I'm measuring 4.16V to 4.17V per cell, but one cell appeqars to be dead, and as my 48V bike battery charges to a few volts higheer, I'm thinking that I do have a dead cell and my fully charged voltage should be close to 41.5V. Does that appear to be the case to you?
 

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I made a quick PS showing what I'm seeing, one cell appears dead, is it correct to think that a 37V battery should charge to a few volts higher than that? I'm measuring 4.16V to 4.17V per cell, but one cell appeqars to be dead, and as my 48V bike battery charges to a few volts higheer, I'm thinking that I do have a dead cell and my fully charged voltage should be close to 41.5V. Does that appear to be the case to you?
Yes.
 
Thanks. I'll see if I can find a replacement cell.
I spoke with a guy who had a bad cell in a lithium battery pack, he said by charging that individual cell he brought it 'back to life'. Have you ever heard of or done this?
 
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With 10 cells you should have 11 wires...
Not 9
I can't find an exact match to replace the cell that appears dead. I talked to a guy who said he charged a dead cell and 'brought it back to life' and it works normally again. Is that possible? If so, any advice, besides have a fire extinguisher handy and do it outside in the clear away from buildings, lol.
 
I can't find an exact match to replace the cell that appears dead. I talked to a guy who said he charged a dead cell and 'brought it back to life' and it works normally again. Is that possible? If so, any advice, besides have a fire extinguisher handy and do it outside in the clear away from buildings, lol.
Sometimes you can bring back a cell to life. It really depends on how low it was and for how long.

I haven't had much luck with a few 18650 Li-ion cells from an old laptop battery pack. Some of the cells were fine, others were dead (0.5v) and I couldn't bring them back to life.
Same went for a few pouch cells I tried.

Basically the procedure is to slowly charge them up to around 2.5v, using only a few mA of charge (depending on the rated cell capacity). Then slowly charging up to around 3.2v. If that works well, you can then charge them up to 4.2v using the rated charging C rate of the cells.
If they don't hold thier voltage at any of these steps, then thier practically dead. Check periodically for heat also. If they heat up at these low charging rates, that's not a good sign.
 
Sometimes you can bring back a cell to life. It really depends on how low it was and for how long.

I haven't had much luck with a few 18650 Li-ion cells from an old laptop battery pack. Some of the cells were fine, others were dead (0.5v) and I couldn't bring them back to life.
Same went for a few pouch cells I tried.

Basically the procedure is to slowly charge them up to around 2.5v, using only a few mA of charge (depending on the rated cell capacity). Then slowly charging up to around 3.2v. If that works well, you can then charge them up to 4.2v using the rated charging C rate of the cells.
If they don't hold thier voltage at any of these steps, then thier practically dead. Check periodically for heat also. If they heat up at these low charging rates, that's not a good sign.
Thanks meetyg, very helpful post! I'll give it a try.
 
Sometimes you can bring back a cell to life. It really depends on how low it was and for how long.

I haven't had much luck with a few 18650 Li-ion cells from an old laptop battery pack. Some of the cells were fine, others were dead (0.5v) and I couldn't bring them back to life.
Same went for a few pouch cells I tried.

Basically the procedure is to slowly charge them up to around 2.5v, using only a few mA of charge (depending on the rated cell capacity). Then slowly charging up to around 3.2v. If that works well, you can then charge them up to 4.2v using the rated charging C rate of the cells.
If they don't hold thier voltage at any of these steps, then thier practically dead. Check periodically for heat also. If they heat up at these low charging rates, that's not a good sign.
I bought a CC CV pwr supply and turned up the current until I could register voltage, at .5A I saw 0.1V, so I left it that way for 2 days, no observable change, so now I'm trying 2A which gets up to 0.4V. So far no detectable heat from the cell, doing it outside in case it blows up. Is it safe to say this cell is permanently dead?
 
Where are checking voltage at? The cell or what the PSU displays?
 
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