BQ76952: Raw current data conversion and more

Hi Dan,

The values reported using the CC1 Current(), CC2 Current(), and CC3 Current() commands already have been calculated using the CC Gain values, so you do not need to do further math on those.  You do not necessarily even need to use the Current Counts data unless you want to do your own calculations - the data here is included for customers who want access to the additional 8-bits of data for their own analysis.

Note that the CC2 Current() data does have moderate noise level, which is why you see the value jumping around. The noise level versus the speed of the conversion are a tradeoff, so noise can be lowered if the conversion takes more time.  However, since some customers do care about having fast data, we provide CC2 Current() every ~3ms when FASTADC=0, or every ~1.5ms when FASTADC=1.  Note that noise will be higher when using FASTADC=1.

This is also why we included the CC3 Current() reading, which is an averaged value of multiple CC2 Current readings.  So if a customer does not need such fast data, they can instead simply read CC3 Current() and set the Settings:Configuration:CC3 Samples to the number of samples to average.  For example, if using FASTADC=0 and CC3 Samples = 100, the device will average 100 samples from the CC2 Current and provide a CC3 Current result every ~300ms.

Additional comment - you mentioned the 12V across the 16 200-Ohm resistors on the EVM.  Note that this current through those series resistors does not flow through the sense resistor, so the EVM is not measuring that current.

The Current Counts data is 24-bit data that is represented using a 32-bit format.  Each LSB of the Current Counts data will have units of VREF2 / 5 / (2^23) = 29.56nV using the typical VREF2 value of 1.24V.  So if you read a Current Counts value of, for example, 147, this represents a voltage across the sense resistor of 147 * 29.56nV = 4.35uV.  If your sense resistor is 1mOhm, this represents a current of 4.35uV / 1mOhm = 4.35mA. 

The values in 0x0071~0x0074 are raw Voltage Counts and Current Counts taken at the same time for each cell.  Some customers want to analyze the impedance of each cell individually, and for that they need the simultaneously measured voltage and current on the cell.  If you do not need to analyze this information, you can ignore this data.

The Cell # Voltage Counts data has each LSB in units of VREF1 * 5 / (2^23) = 722.4nV using the typical VREF1 value of 1.212V.  In order to calculate the cell impedance, you can convert the Voltage Counts into volts, the Current counts into amps, then divide to get resistance in Ohms.

Thanks,

Terry

Hi Dan,

You can use the Cell # Voltage Counts and Cell # Current Counts from 0x0071~0x0074 to determine the simultaneous voltage and current of a cell.  The Cell # Current Counts represents an ADC conversion of the differential voltage between the SRP and SRN pins of the device, which are effectively the voltage across your external low-side sense resistor.

If you need to measure currents as large as 300A, then you need to select your sense resistor such that the maximum voltage across it is within +/-200mV.  If I assume your maximum current is 300A, then the sense resistor must be less than 667uOhms.  

For example, assume your sense resistor = 500uOhms, and Cell 5 Voltage Counts = 5500000, and Cell 5 Current Counts = 200000.

I can convert the voltage counts into volts by:

Cell voltage = (voltage counts) * VREF1 * 5 / (2^23) = 5500000 * 1.212V * 5 / (2^23) = 3.9732V

Cell current = (current counts) * VREF2 / 5 / (2^23) / Rsense = 200000 * 1.24V / 5 / (2^23) / 500uOhms = 11.826A

Regarding cell impedance, this gets more complex and is not practical to explain in depth here.  Our battery gauges integrate cell impedance calculations and use this information together with the measured voltage, current, and temperature, to calculate the battery state-of-charge, state-of-health, and many other parameters.  You can see more information on that at  .

What is generally of interest is the "small signal resistance" of a cell, which is represented by the delta-V / delta-I of a cell (note that it changes with temperature).  Our battery gauge products which use our Impedance Track algorithm calculate the cell impedance of each cell during operation.  You might consider looking into those products in more depth, if they can accommodate the number of series cells you have in your system.  The bq40z80 can accommodate up to 6 series cells.

Thanks,

Terry

Hi Dan,

The expectation is that the cells are all in series with the sense resistor, so the current through every cell and the sense resistor is the same (except when balancing is active).  So it is only necessary to measure current at the sense resistor.

However, the complication when calculating a cell's impedance is the timing of the voltage vs the current measurements.  It is important for the analysis to use voltage and current data captured at the same time on a particular cell.  Since the voltage ADC is muxed between all the cell voltages and other measurements (such as thermistors, stack, and diagnostic measurements), the cell voltages are measured at different times.  So the coulomb counter captures a current measurement at the same time as each cell voltage is being measured, and stores that together in the Cell # Voltage Counts and Cell # Current Counts.  So these represent data captured at the same time.

Thanks,

Terry