Wiring Tesla BMB's for fewer than 6 cells

[robertwa]

I'm planning to use tesla s bmb's and simpbms with my Chevy Bolt pack. It's 96 S with 8x10S and 2×8s modules. Each module will need 2 bmb's.

I am doing a test with a 9s pack from a hoverboard pack to understand cell wiring. This way I can simulate wiring for one half of the 10s module and one half of the 8s module. For this test I have 5s on one bmb and 4s on the other.

So far, I have simpbms reading correct 9s voltage but I am unsure if my wiring is correct. For the 5s bmb, I jumped pin 5 to pin 6. I then jumped pin 6 to pin 0 of 2nd bmb to get it a reference voltage.for this 4s bmb, I jumped pin 4,5,6 together.

On Termite, it mostly looks OK but bmb2 shows a cell 9 at half the true voltage and a phantom cell 10 showing the other half. This is where I am stuck. Any ideas?

[tom91]

Yes it will potentially create wierd cell voltages. Makes sure all your high cell taps are properly connected together.

You can also set the a lower limit where the voltages are ignored, ignore cell voltage or something it is called.

Also please change the title to include Tesla BMBs for clarity

[robertwa]

Title changed - thanks, I see that was confusing.

I'll try resoldering today and see if I can get the readings to behave.

On a related note the blue connectors were hard to source and looks like only comes in a header style configuration. I'm wondering if I can change out the pins for a wire crimp version

[robertwa]

After more messing around, I got it to work. Turns out I have a couple of BMB's that are not happy wired in a 4S config, the other 18 BMB's work great. I tested the two offending boards multiple times with 4S, 5S and 6S packs. Perhaps a tolerance issue, or maybe my sketchy ESD protocol had a play in this. I will delegate these two BMB's to a 5s configuration and move on. I'll also make sure they are easily accessible.

To summarize, to wire a Tesla S BMB for less than 6S, take the highest number cell tap from your module, and connect to each remaining unused position on the BMB, For example to use as 4S, connect battery cell 4+ to BMB position 4, 5 and 6 all together. Hope this is helpful to someone in the future.