BMS wiring question

fstop · 12-23-2021, 11:33 AM

In taking a look at my 24S BMS wiring (pre-wired by factory, not something I put together) it seems that I'm "missing" a connection compared to most wiring diagrams I've seen.

It seems that generally there are X + 2 wires connected to the BMS; where X is the number of cells. Usually there is a pack total +, a pack -, and one wire for each cell.

My BMS has a 14 pin connector (using all 14 pins) and a 12 pin connector (with one not connected) - see below - for a total of 25 connections -vs- the more "normal" 26 I usually would expect.

I think usually there are 2 connections to the BMS for the total pack voltage +, with one being the "last" cell, and the other going to the BMS total + connection. Maybe in my wiring the BMS is using the same connection and not needing 2 separate wires (which would be the same connection anyway, if I'm reading things correctly)?

What is a little odd also is that my "missing" connection isn't at the beginning or end - it is #4 of the 12 pin connector.

There is no issue with anything and it has worked perfectly for over a year, but I'm just curious.

w = white wires to cells, BLK is -, RED is pack + (...presumably)
So there are 23 cell wires, but 24 cells in the pack. Maybe RED is total and cell #24?

14 Pin connector
BLK w w w w w w w w w w w w w
12 Pin connector - no connection at X
w w w w w w w X w w w RED

Pat911 · 12-23-2021, 03:37 PM

It is correct. There should be n cells +1, not +2. So for 24s there are 25 balance leads. One pack negative and one for each cell positive. The 24th cell balance lead is also the pack positive.

fstop · 12-23-2021, 05:19 PM

Quote:

Originally Posted by Pat911

It is correct. There should be n cells +1, not +2. So for 24s there are 25 balance leads. One pack negative and one for each cell positive. The 24th cell balance lead is also the pack positive.

So maybe what I'm seeing is that some BMS units can use less cells than they have cell input channels, meaning the "last" cell isn't always also the total voltage.

So the image showing a separate total voltage lead right next to cell 24's lead I guess in case someone wants to set it up as a 16s unit maybe.

If that is the case it appears that my BMS may be capable of 25s operation with that extra / unused pin - seems odd but who knows.

Thanks for your take on it. I guess if I ever have problems with it I'll just have to put a meter on it to figure it out definitively, but I'd guess it would be as you suggest.

Pat911 · 12-24-2021, 05:04 PM

Correct, some, not all, BMS’s can be used for multiple configurations, say 10s-16s. In this case the last cell may have two wires.

12-23-2021, 11:33 AM	#1
fstop Gone Wild Join Date: Oct 2019 Posts: 646	BMS wiring question In taking a look at my 24S BMS wiring (pre-wired by factory, not something I put together) it seems that I'm "missing" a connection compared to most wiring diagrams I've seen. It seems that generally there are X + 2 wires connected to the BMS; where X is the number of cells. Usually there is a pack total +, a pack -, and one wire for each cell. My BMS has a 14 pin connector (using all 14 pins) and a 12 pin connector (with one not connected) - see below - for a total of 25 connections -vs- the more "normal" 26 I usually would expect. I think usually there are 2 connections to the BMS for the total pack voltage +, with one being the "last" cell, and the other going to the BMS total + connection. Maybe in my wiring the BMS is using the same connection and not needing 2 separate wires (which would be the same connection anyway, if I'm reading things correctly)? What is a little odd also is that my "missing" connection isn't at the beginning or end - it is #4 of the 12 pin connector. There is no issue with anything and it has worked perfectly for over a year, but I'm just curious. w = white wires to cells, BLK is -, RED is pack + (...presumably) So there are 23 cell wires, but 24 cells in the pack. Maybe RED is total and cell #24? 14 Pin connector BLK w w w w w w w w w w w w w 12 Pin connector - no connection at X w w w w w w w X w w w RED
fstop Gone Wild Join Date: Oct 2019 Posts: 646

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12-23-2021, 03:37 PM	#2
Pat911 Gone Wild Join Date: Dec 2020 Location: Melbourne, Australia Posts: 1,934	Re: BMS wiring question It is correct. There should be n cells +1, not +2. So for 24s there are 25 balance leads. One pack negative and one for each cell positive. The 24th cell balance lead is also the pack positive.

12-24-2021, 05:04 PM	#4
Pat911 Gone Wild Join Date: Dec 2020 Location: Melbourne, Australia Posts: 1,934	Re: BMS wiring question Correct, some, not all, BMS’s can be used for multiple configurations, say 10s-16s. In this case the last cell may have two wires.