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BQ20Z45-R1_BQEASY-SW: bq20z45-r1 jumps to ZERO SOC at freezing

Part Number: BQ20Z45-R1
Other Parts Discussed in Thread: BQEVSW, BQ40Z50, GPCRB, BQSTUDIO

I have a battery pack that uses a bq20z45-r1 that jumps zero SOC when the temperature drops to 0C or lower. 

I did not have anything to do with the design of this battery pack but several years ago I did some work with a BQ 27510 and I have used BQ software (I have bqEVSW 0.9.90) software and the necessary hardware to interface to the bq20z45-r1 gas gauge in the battery pack.  All the settings I see in the BQ software seem to only protect the battery against over and under temp when charging but I do not see any register that specifics a low temperature cutoff on discharge.  

What we saw was the battery was reporting around 75% SOC at 1C and a couple min later the temp dropped to Zero C and the SOC started reporting zero SOC but the battery was still running in our system as expected based on the battery being around 75% SOC actual.

We would like the battery to operated at -20C so is there a setting or a register I could change to get the gas gauge report the actual SOC down at these lower temps?



  • Hello Doug,

    For new designs we recommend the bq40z50 family of products. There is a GPCRB tool that can help optimize for low temperature performance.

    This tool can be used for bq20z45 but the file format that is output by GPCRB may not be compatible with bqEVSW, so there may be no way to program it now.

  • Is this a know problem with the bq20z45-r1?  We are using the bq20z45-r1 in our current product design so this part was pick some time ago and we are not current in a design cycle and are able to easily pick a new part.

  • Hello Doug,

    Discharge cannot be set to terminate at a specified low temperature unless it triggers a safety event. You can use that workaround.

    The bq20z45 is old and due to new features requiring additional data, the file formats have changed. The issue with programming the GPCRB data is caused by incompatibilities introduced because of advances in technology. The old tools are not likely to be updated to handle the new data.

  • The battery does not stop discharge when it gets cold, it just suddenly reports a state of charge (SOC) of Zero when it gets cold even though it has 80% charge remaining.   The software thinks the battery is dead when it can still run for a while based on the actual charge the battery still has and can still supply to the load.  The problem is the BQ is suddenly reporting a SOC of 0.  Is this a known problem in the chip that requires the BQ you recommend? 

  • Hello Doug,

    The bq20z45 can be optimized(configured) for low temperature operation. Low temperature optimization(GPCRB) is an additional step that is required for operation below 0C. It was available for the bq20z45 and worked. Later, the data format changed and bqEVSW was not updated to support the new format.

    This additional step is still required on new products like bq40z50 and will solve the issue. bq40z50 uses bqStudio which supports programming the new data format

  • I searched for GPCRB and found this:

    Would this explain why the SOC suddenly goes from 80% SOC to zero SOC when the battery reaches freezing temps?  



  • Hello Doug,

    Yes, that is the tool to use for low temperature optimization. Without that optimization SOC will be usually be incorrect at freezing temps as there is no reference point for how the battery characteristics vary at low temperatures