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I recently got a BMS model "bw-6022-5s" and planned to use it with my 4S5P (21V) battery pack. However, since this BMS looked different from the ones I'm used to, I did some research and found conflicting wiring diagrams.

The original wiring diagram shows that the first (B1) and last (B6) wires should be connected to the outer terminals of the battery pack, as illustrated in this picture Here But when I looked online—specifically on YouTube and Google—I saw several examples where people cut off those outer balance wires and only used the inner four balance wires (B2, B3, B4, B5), as shown in this video at the 5:13 mark. Video

This has left me confused, especially after reading through the few comments on the video. One person mentioned that their battery pack exploded when they connected the outer leads (B1, B6), which has me even more concerned.

While I do know how to safely build a battery pack, I'm not as experienced with the inner workings of every type of BMS. Could anyone here clarify what the purpose of those outer leads is, and why some people recommend using them while others suggest cutting them off? Any insights would be greatly appreciated!

Here is another wiring diagram, that doesn't use the B1,B6 balance wires. Other wired diagram

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  • \$\begingroup\$ " but it resulted in his battery pack blowing up." Because he wired the cells wrong. \$\endgroup\$
    – Davide Andrea
    Commented Aug 22 at 13:57
  • \$\begingroup\$ From where you bought it? Was there any instructions or a manual with the product, and is the documentation credible? If you answer "from a random seller on an e-commerce platform" you have no guarantees what you bought, if you got a manual for the correct product or a manual at all. Also about the Youtubers, while there is a chance that the explosion was just for the viewers, it is also possible that the batteries were connected incorrectly by accident so it was a mistake, or, the BMS was damaged or had a solder bridge or something and it exploded even with batteries connected correctly. \$\endgroup\$
    – Justme
    Commented Aug 23 at 5:34
  • \$\begingroup\$ I bought the BMS from Ali express. It was a cheap one, the stock photos gave away how to use all the wires. But i got confused when i looked up on the internet and saw that people were using different diagrams. Anyway, i have asked the seller for a refund. I don't feel safe using the BMS anymore, because there isn't a lot of documentation or people around that understand the BMS. \$\endgroup\$
    – Wastef
    Commented Aug 23 at 16:06

2 Answers 2

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The wiring diagram is correct and the video is wrong.

And, by the way, do not solder directly to Li-ion cells! Buy cells with spot-welded tabs. Or get a spot welder.

what the purpose of those outer leads is,

Which connection do you call "the outer lead"? If you mean the purple wire at the top left of the wiring diagram: that is the wire that carries the high current from the B- of the string of cells to the protector switch in board of the BMS. The same network is connected to the BMS through the balance connector, but that lead is tiny and can't carry the high current. That's why there is a second wire, of larger gauge, between the B_ of teh cells and the BMS.

and why some people recommend using them while others suggest cutting them off?

"Some people" are the BMS manufacturer, and you should listen to them.

"others" is random YouTubers who are it it for the views, not for the safety, and you should be skeptical of what they say.

Here is another wiring diagram, that doesn't use the B1,B6 balance wires

That is so messed up! Do not follow that diagram! It doesn't protect the most positive cell, which actually doesn't matter because the BMS will not see the top voltage and shut off.

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    \$\begingroup\$ Thanks, I used a spot welder with tabs. You’ve made a great point about following the manufacturer’s diagram. But then again, there's this guy in the comment section asking/stating what the outer wires (B1, B6) were meant for. He mentioned that he used those wires as you are suggesting, but it resulted in his battery pack blowing up. Also, implying that the YouTuber did it for views rather than safety doesn’t make a lot of sense to me. You have to wonder why some choose not to follow the manufacturer’s diagram. There has to be a reason other then "just for views" \$\endgroup\$
    – Wastef
    Commented Aug 22 at 14:09
  • \$\begingroup\$ I believe I already answered this question in a comment below your main question. I copy it here: "Because he wired the cells wrong". \$\endgroup\$
    – Davide Andrea
    Commented Aug 22 at 14:14
  • \$\begingroup\$ How would you know that he wired the cells incorrectly? The person in the comment section explicitly stated that he connected the outer balance leads where they should have gone, and it still resulted in an explosion. Besides, this doesn’t explain why different distributors are providing different diagrams. My distributor provided a diagram that shows the outer leads connected, but at this point, I’m not even sure which diagram is actually from the manufacturer. Again google the BMS and you will find a lot of different diagrams. \$\endgroup\$
    – Wastef
    Commented Aug 22 at 14:31
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The BMS measurement system should "see" the cell voltage as precise as possible.

Different BMS behave different if you omit the first and/or last wire. Some contain safety resistors, that provide a weak path to the main battery connectors and are still able to estimate the cell voltages.

But these values then include all voltage drops across shunt resistors, PCB tracks and wiring.

This introduces measurement errors under load or charge conditions and wrong balancing decisions may be a consequence. Also the precision of overvoltage and deep discharge thresholds is affected.

Other BMS may just measure random voltages and cannot establish pack safety at all. You may not immediately see an effect of this issue, it is really dangerous.

Summary: If you need to leave these wires unconnected, there is something very bad going on.

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  • \$\begingroup\$ Read this thrice, and i couldn't conclude anything if i should use the BMS or not. Anyway i contacted the seller, and asked for a refund. I don't know what to do with a BMS that has conflicting diagrams and instruction al over the internet. \$\endgroup\$
    – Wastef
    Commented Aug 23 at 16:07

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