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BQ78350-R1: RSOC jumps to 99% during REST

Kevin Knicker
Expert 1091 points
Part Number: BQ78350-R1

Tool/software:

TI Team,


We have a battery pack that uses the bq78350-R1.  The chemistry is LiFePO4.

We perform a full cycle on the pack and then charge it to 25%.

After a period of REST, the RSOC has jumped to 99%.

This has never happened before so it appears to be a new issue.

Any ideas as to what could be causing this?

I can PM the gg file if need be.

Thanks,
Kevin

  • 0
    TI__Guru 61536 points

    How long was the rest period?

  • 0 in reply to Dominik Hartl11
    Expert 1091 points

    They had been sitting for about four weeks waiting to be shipped.

    We ship packs at less than 30% RSOC.
    When we checked them again, we noticed the RSOC had jumped to 99% on some of the packs.

    We performed a RESET and that set the RSOC back to a value that makes more sense.

    Thanks,
    Kevin

  • +1 in reply to Kevin Knicker
    TI__Guru 61536 points

    LFP has a flat OCV so any error in voltage measurement in relax can have an outsized impact on SOC error. 

    This is especially problematic if you stop charging in this flat zone. The gauge has an automatic block of 48 hours where it will not use voltage measurements to update its cell state (and SOC).

    It looks like some of these packs read a few mV higher than what's expected for 30% RSOC (it's actually about DOD, depth of discharge, where the gauge uses OCV measurements to re-sync with the state of each cell in the battery) and because a small voltage error leads to a large DOD error for some LFP cells, SOC can change significantly.

    I don't think that there's an automatic method to make sure that RSOC doesn't change that much for long relax and some LFP chemistries.