BMS - does it see TOTAL voltage?

[sshibly]

I was peeking at some battery porn at BH ;-)

BH has this Samsung 48 volt module that uses a 15s BMS but has only 13s hooked up,

1. BMS design allows this?
2. BH mentioned higher cut off voltage due to the fact 13s leads are hooked up on a 15s BMS
3. Do a BMS actually see the total voltage? Or does it only sense each cell's voltage?

 

[MisterSandals]

3. Do a BMS actually see the total voltage? Or does it only sense each cell's voltage?

It depends which cells the BMS is connected to. If it is connected to the “end cells”, cell 1 and 15, then yes it will see total battery voltage.
At some point in how its wired, a single wire will show the voltage increase from 2 cells, twice.

 

[sshibly]

It depends which cells the BMS is connected to. If it is connected to the “end cells”, cell 1 and 15, then yes it will see total battery voltage.
At some point in how its wired, a single wire will show the voltage increase from 2 cells, twice.

the BH 48v has

13s7p setup, but the BMS is 15s

How is the BMS leads connected to this 13s setup, you have 2 extra leads, are they just hanging out? or as you mentioned doubled up to the last 2 cells?

 

[MisterSandals]

How is the BMS leads connected to this 13s setup, you have 2 extra leads, are they just hanging out? or as you mentioned doubled up to the last 2 cells?

Oops, early morning posting error. I was thinking 13 leads for 15 cells.

The BMS would have to be configurable for the number of cells. I cannot see how it could be wired so that incremental leads would read incremental voltage doubling up (which would have to ignore low cell voltage).

 

[GXMnow]

I just looked at the listing on BH. The BMS is only configured as a 13S. The board is modular and populated for what they ordered. It only has 13 balance load resistors, but locations for several more. The cell low voltage cut off is probably just set high to prolong cell life. 3.23 is not a crazy high cutoff voltage. I currently have my inverter shut off at 3.6 volts per cell. I doubt you could easily change the BMS to anything other than 13S.

Each BMS has it's own setup. Many of the cheaper ones are hard coded for specific cell count and voltages. Some allow different numbers of cells by skipping or shorting certain cells to tell the chip how many you are using. This one looks like it could be configured for more cells, but components are not even installed on the board for more than 13. Some BMS's are fully programmable like my JK BMS, I can set it for anything from 4S to 24S. I am using it at 14S. It only measures the cell voltages, and reports the sum of the readings as the pack voltage. I know this because I had a balance lead failure which caused 2 cells to report zero volts. When it did that, the total pack voltage reported be the BMS app was about 8 volts less than the actual pack voltage. It essentially has one 1 to 5 volt meter, and it switches it to read each cell one at a time. This is how it can have 0.001 volt resolution on each cell. A BMS that does not report data to a screen or an app is only using the cell voltages for the balancing and high and low cut off. It does not even care what the total pack voltage is. It really does not matter. It is protecting each cell. If a cell is too high or too low, it shuts off, and the cells that are higher than the balance threshold have a load resistor connected. That's it. The balance and cut off voltages can be set by a hard code in the chip, or it may be set with resistors or jumpers on the board, or it could be set with software on a "smart" BMS.