Short changing a BMS?

[Tex]

Ok, up to my old tricks again, this will be my last power tool conversion, all others working fine.
I already fitted the old Makita body with one of these converter bases as I have done with other tools, but an 18 volt battery is a bit of overkill for the poor old 7.2 volt Makita, so I bought the closest BMS I could find with a Milwaukee connector, 2P 5S. I would like to set this up as a 18650 3P 2S giving me 7.4 volts, (see below).
I also bought a 9.6 volt LifePO4 -3S - 100 amp BMS which I note comes in numerous configurations, the 3S just having resistors soldered over Bat1 & Bat2 terminals. I'm going to build a 2P 3S 26650 cell pack with that for testing.
Can I get some suggestions as to additional connections possibly required for Milwaukee connector, 2P 5S to 3P 2S configuration please? Or can I just ignore Bat3 & Bat4?
I also note these Milwaukee converter bases only have 2 pins, .i.e. Pos & Neg, no cell balance detection. Both BMS have balance so I guess they take care of it.
I have also set one of those bases up as a variable voltage charger station.
Looking forward to your thoughts

 

[Tex]

Nobody?

 

[OffGridInTheCity]

Out of my wheel house - but I am following along Don't see a 'risk' in trying with just 1 and 2? - will either work or not?

 

[Tex]

I think you're right, nothing ventured no special smoke!

 

[Mark_T]

I think it's not going to work...
Thеse pins b1+,b2+,b3+,b4+ are for balancing = discharge (if it is implemented on this board) and for voltage monitoring. Charge voltage only comes to b1- and b5+.

 

[Tex]

Ok, when in doubt ask AI right, I mean what could possibly go wrong??
She says:
4S BMS for a 2S battery pack,
Standard Conversion Method
To use a 4S BMS for a 2S battery pack, you must bridge the voltage sensing pins for the missing cells to the highest active cell's voltage.
Wiring Scheme:
B-: Connect to the main negative of your 2S pack.
B1 (3.7V): Connect to the positive of the first cell.
B2 (7.4V): Connect to the positive of the second cell (this is your 2S total voltage).
B3 & B4 (and B+): Connect these pads together and wire them directly to B2 (the positive of the second cell).
Result: The BMS "sees" the full voltage across its upper sensing circuits, satisfying the logic that the upper cells are fully charged and balanced.

Also, I had a look at a bunch of BMS's I have in the box, and I noticed a number of them have what looks like a little (resistor?) bridging unused connection pads, like this.

 

[Korishan]

To use a 4S BMS for a 2S battery pack, you must bridge the voltage sensing pins for the missing cells to the highest active cell's voltage.

Actually yeah, that's what you do with the some of the earlier JK units. Generally what you did was any wires after series connections, you'd short them out. Or, you short out with the last wire. IE. for a 7s bms, connect cell-wire #3 to cell-wire #7 which also connects to Pack Positive. Later on they did away with this issue.

But shorting the cell wires that aren't used together shouldn't really have any negative effects

looks like a little (resistor?) bridging unused connection pads

Yeah, those are probably 0-Ohm resistors, or pretty close to it. it's easier to drop those than try to make sure solder is bridging the connection on every board. They probably cost about as much as the solder too.

 

[Mark_T]

I hate to make that comment, but that chip BM3451 specifically designed to be used for
3,4,5 sells configuration (NOT 2!) and have dedicated pin (16) to do that.

 

[Korishan]

https://uelectronics.com/wp-content/uploads/2021/02/BM3451-BYDMicroelectronics.pdf

Checking out the documentation, it looks like they don't show anything less than 3 cells. However the way they do it is opposite of how I suggested in my previous comment. They short cell-connections 1-2 and then use 3-5 to get their 3s connection.

I do not know if you could include V3 in the shorting or not. I would imagine it would just cause instability and/or not power up at worse case. But that's just a guess on my part.

 

[Tex]

Thanks guys. Ok that settles it, it's not really the BMS I need (I have plenty of 2S for both 18650 and 26650 cells, I just really need the connector.
So the plan now is just cut off the pin section of the board and marry and wire it it to a 2S BMS, too easy!