New Battery Build - BMS installation problems

[Lightasair]

I have assembled a 36v pack 3P and 10S of new Samsung 3000ma cells. When I soldered in the new BMS correctly and plug in the charger (class 2 generic charger) the BMS gets really hot right near the sense wires plug and the charger light stays green and won't charge the pack. The voltage going into the BMS from the charger shows 42 volts. I have 2 of these chargers, which have wolrked fine up to now and get the same result with both.
This isn't my first build, and did not have this issue before, but I have been working with used cells on the previous build.
At this point I have tried 3 new BMSs.
Any suggestions is greatly appreciated.

 

[Gruesome]

Taking you at your word that all is wired correctly, maybe the BMS is trying to balance, bleeding off some of the higher voltage cell groups? Can you measure the ten 3p groups with a DVM, and post the ten voltages here? Ideally with three digits behind the decimal point, i.e. measured to the nearest mV.

 

[docw009]

In the past, I've been careless and what I thought were correctly wired BMS were wrong. Usually, it's a pair of balance wires switched. One time, I got a different BMS that used a red wire to show the most positive balance pin but I thought that was B0. That was an epoxy filled BMS, and it got hot by the bakance connector. Melted the epoxy.

Best thing to do, even if you are sure it's correct, is to start at B0 with the balance connector not connected to the BMS. Walk a voltage probe up from B0 to B10, making sure the voltage increases appropriately with each pin, I must have wired up three dozen BMS by now, and I still do that last sanity check. Once in a whike, I do find a wiring error I made.

 

[Lightasair]

Taking you at your word that all is wired correctly, maybe the BMS is trying to balance, bleeding off some of the higher voltage cell groups? Can you measure the ten 3p groups with a DVM, and post the ten voltages here? Ideally with three digits behind the decimal point, i.e. measured to the nearest mV.

Voltages 1 through 10 are 3.62 - 3.57 - 3.57 - 3.59 - 3.60 - 3.66 - 3.63 - 3.63 - 3.68 - 3.69

When I received the new cells they were all bang on 3.44V, so after I went through a couple of new BMSs I tried charging the pack directly and brought the voltage up, but they didn't charge equally, which I assume isn't surprising.
I have gone over my sense wires and they are all in order with the black on negative #1 and the next sense wire on positive #1 and then sequentially through to the final sense wire on #10 positive.
On the BMS negative wire to negative on #1 and then the two negatives out to the charger and the output. I did run a wire from positive #10 also to the charger.
Its frustrating, but I'm sure the batteries out of balance isn't helping.

 

[Lightasair]

In the past, I've been careless and what I thought were correctly wired BMS were wrong. Usually, it's a pair of balance wires switched. One time, I got a different BMS that used a red wire to show the most positive balance pin but I thought that was B0. That was an epoxy filled BMS, and it got hot by the bakance connector. Melted the epoxy.

Best thing to do, even if you are sure it's correct, is to start at B0 with the balance connector not connected to the BMS. Walk a voltage probe up from B0 to B10, making sure the voltage increases appropriately with each pin, I must have wired up three dozen BMS by now, and I still do that last sanity check. Once in a whike, I do find a wiring error I made.

Tomorrow I will check the voltages through the sense wires. I have another BMS coming and hopefully will get this sorted. I have another pack of 21700s that I want to have this BMS figured out before I connect them.

 

[Gruesome]

Voltages 1 through 10 are 3.62 - 3.57 - 3.57 - 3.59 - 3.60 - 3.66 - 3.63 - 3.63 - 3.68 - 3.69

So they are not fully charged at all, and also roughly about 15% unbalanced (3.69V-3.57V= 0.12V, versus about 3.5V to 4.2V for 90% of the useable stored energy). Would the BMS you have currently wired in try to balance all the time, or only at/near full voltage?

When I received the new cells they were all bang on 3.44V, so after I went through a couple of new BMSs I tried charging the pack directly and brought the voltage up, but they didn't charge equally, which I assume isn't surprising.
I have gone over my sense wires and they are all in order with the black on negative #1 and the next sense wire on positive #1 and then sequentially through to the final sense wire on #10 positive.

That sounds straightforward.

On the BMS negative wire to negative on #1 and then the two negatives out to the charger and the output. I did run a wire from positive #10 also to the charger.

Is this how it supposed to be? Maybe double check against the BMS wiring instructions.

Its frustrating, but I'm sure the batteries out of balance isn't helping.

 

[docw009]

Most negative point of cells goes to B- pad on BMS. If it is a two port BMS, the negative leg of charger jack goes to C-. The negative leg of the discharge plug goes to P-. Both the positive end of the charger jack and the the discharge plug goes to the most positive terminal.

Are you reading a little over 36 volts on the battery plug? You should, if the balance wires are in order.

 

[Lightasair]

Most negative point of cells goes to B- pad on BMS. If it is a two port BMS, the negative leg of charger jack goes to C-. The negative leg of the discharge plug goes to P-. Both the positive end of the charger jack and the the discharge plug goes to the most positive terminal.

Are you reading a little over 36 volts on the battery plug? You should, if the balance wires are in order.

That is exactly the way the BMSs is laid out, and yes I am getting a 36V reading out of the pack. I still think that hot BMS might have been trying to balance the cells. Even after I unplugged the charger it stayed warm for a while. The only reason I'm not totally convinced of that is that my issues were from the very beginning, but I may have had a wire wrong then.
Some things just take a little time to figure out.

 

[Lightasair]

I have assembled a 36v pack 3P and 10S of new Samsung 3000ma cells. When I soldered in the new BMS correctly and plug in the charger (class 2 generic charger) the BMS gets really hot right near the sense wires plug and the charger light stays green and won't charge the pack. The voltage going into the BMS from the charger shows 42 volts. I have 2 of these chargers, which have wolrked fine up to now and get the same result with both.
This isn't my first build, and did not have this issue before, but I have been working with used cells on the previous build.
At this point I have tried 3 new BMSs.
Any suggestions is greatly appreciated.

****An update to my BMS dilemma. I checked the BMS with another battery pack and it was functioning fine. So I disassembled my new pack completely including all the spot welded strips. Then reassembled it carefully and all is fine now. The BMS charges the pack.
All I can think of that could have been my problem would be a loose weld or something like that. I use a battery powered spot welder which seems to work fine as the welds were not easy to separate.
For anyone else that uses one of these portable spot welders I think the welds are not as good when the welder battery gets discharged below 50% and works best when fully charged.

 

[BlueSwordM]

Yeah. A lot of these welders have relatively capable cells, but their interconnects have massive voltage drops, resulting in poor welds as they discharge further.

Putting another cell in parallel and making the interconnects beefy is what helps a lot.

I personally just went the ballsy route and am using 105Ah LFP cells for this use case
The cells don't care at all