MC33771C stack voltage vs sum of cell voltages

[tbondar]

I'm working on a 12 cell BMS using the MC33771C. The voltage in the MEAS_STACK register is a couple of volts lower than the sum of the MEAS_CELLx voltages. I can't explain this. I couldn't find much in the datasheet about how stack voltage is measured. Any suggestions?

I convert the register values to mV using the macros in the SDK:

stack_mv = BCC_GET_STACK_VOLT(values[BCC_MSR_STACK_VOLT]) / 1000U;
cell1_mv = BCC_GET_VOLT(values[BCC_MSR_CELL_VOLT1]) / 1000U;

Thanks!

[tbondar · Accepted solution]

Checking the datasheet again, I've realised that MEAS_STACK does not actually measure the stack voltage, but the VPWR input of the chip. This is clear from the block diagram, but the naming of the register mislead me. I think we can consider this problem sorted now. Thanks!

[tbondar]

Hi Rafa,

Thank you for your response. I think the GET_STACK_VOLT() macro does exactly the calculation you referred to, so that cannot be the problem.

If the stack voltage is the voltage measured between CT_REF and CT_14, and for my 12 cell stack CT_5 and CT_6 are connected to CT_4, then the sum of cells should be still equal to the stack voltage. Cell balancing is disabled, but it shouldn't matter really, I think.

I'm going to check the hardware again, maybe it's a soldering problem or something similar. Please let me know if you have other ideas.

[RafaR]

Hello tbondar

Good day!

MEAS_STACK is a single ADC measurement across the entire stack.

MEAS_CELLx values are differential measurements between adjacent cell taps.

The stack voltage calculation is explained in our AN12084, chapter 7.2.2:

This document is a secure file so to have access you first need to has a valid NDA with us to have access

As extra tip, try measuring with cell balancing disabled to see if it affects the readings.

I hope this information has helped you, please let me know if you need help with anything else.

Have a great day and best of luck.