Battery works but won't charge. Cells seem fine, bms issue?

[jimmyjabs]

Hello, forum, I'll try to be brief but will include all the info I can. Thanks in advanced.

Issue

- I've got a 10s2p Lithium Ion battery that when plugged in to the charger the charger indicates that the battery is charged even though it is not.
- I accidentally got it wet so there is some rust (or corrosion?) on some terminals, but the BMS doesn't look like it got wet.

Photos




I got it charging once but it only went to about 36v and stopped charging

This is what I did to get it charging:

- At this point, the battery was about 35v give or take 0.1-0.2v
- Charged via the discharge plug for a few minutes, then unplugged
- Unplugged the balance lead for about 30 mins, then plugged back in
- Plugged the charger to the charge plug and it started charging
- Once it got to 36v it stopped charging
- Now it's back to square one, where it won't charge. I tried discharging until 35v again, and it still won't charge that either.

Tests I've performed; and its current state

- Charging via the discharge plug: This works. The charger indicates it charges, and it does actually charge.
- Resetting the BMS by unplugging balance plug and putting it back in. I left it off overnight. This doesn't help anymore.

- Voltage reading from discharge plug: 35.4v
- Voltage reading from charge plug: 35.1v
- Voltage reading from individual cells: 3.55v except one where it was 3.56v

- Based on this video https://youtu.be/6i8i6h1LkBw?t=435 which notes that I shouldn't have any voltage between the negative terminals on the BMS (C-, P- and B-), I can confirm that I do. I get about 0.01v -- I'll need to recheck this to confirm what it is exactly, but I can say that it's not 0.

What I'm after

- Any help further troubleshooting?
- If it's the BMS, any help in buying a correct replacement

Other misc info:
- While testing voltage for individual cells, I don't know what happened, but it began smoking at the positive terminal. Looks like the smoke came from the casing of the battery being burnt. I still successfully tested that battery and was the same as the rest.
- It's a battery for an electric skateboard

 

[russ18uk]

The soldering on the balance connector looks horrific. Can you check you have voltage on the 2kR resistors. Also the connector is out slightly. Push it back in.

 

[jimmyjabs]

Can you check you have voltage on the 2kR resistors.

I should have mentioned that I'm not very experienced with this stuff. I've tagged each of the sections in the following pic so I can reference it easier.



Can you confirm which ones are the resistors?

- I tested the components in section C and B
- I tested them by applying the multimeter probes to the ends of each component
- For components in section C, they all read 3.54v - 3.55v
- For components in section B, they either didn't have any voltage, or the film over it prevented me from doing so.

 

[russ18uk]

Yes, I mean in Section B. FYI: 202 = 20(+2 zeros) so 2000R.

Do you have very sharp probes? You should be reading a voltage on the resistors as they are connected to each series set. Have you fully pushed the connector together because it looks clearly out to me.

 

[jimmyjabs]

I don't get a voltage from any resistor by placing the multimeter probes on each end of a single resistor.

If that's not what you meant, I did try placing the probes between the resistors which gave me something.

So from left to right (not including the first un-numbered black component, I placed my first probe on the top of the first resistor and my second probe on the bottom of the second resistor. I did this all the way down.

All but one reading was 3.55v. With one probe on the top of the 4th last resistor, and the second probe on the bottom of the 3rd last resistor, I got 3.52v.

Again, this is above my experience level so let me know if that's not correct. Cheers

Have you fully pushed the connector together because it looks clearly out to me.

Yep, it was only like that in the photo because I had recently tried to reset again by removing it.

 

[docw009]

I worry about batteries that start smoking and it wasn't something that I did. Be careful! 20700's are 3.7V nominal and charge to 4.2 volts max? I think your battery is half charged at this point.

Also think that video has some terminology issues. A BMS uses circuits that connects the negative lead of the battery (B-) to the negative returns C- and P- on the charge and power connectors. You'll get about 0 volts if your probe them, and the BMS is working, but you can also get that if something is not working.

I agree that the BMS probably got wet and one or more of the voltage checking circuits is dead. A battery tech would probably verify that the cells are still OK, and then replace the BMS. If you find one with the same balance connector layout (B10 on left and B0 on right per your photo), it's easier.

 

[jimmyjabs]

Yeah, it got to about half charge, until I ran down the battery a bit when I was testing it.

I think at this point, I'm just going to try a new BMS and see how it goes. I use this battery for my main mode of transport so would like to get it sorted soon.

If you find one with the same balance connector layout (B10 on left and B0 on right per your photo), it's easier.

Are there any other concerns regarding a replacement BMS? The below BMS I can get pretty easily from eBay that is a 10s 36v 20A BMS. It does look like the balance connector is reversed, though?