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How can I prevent large RSOC jump down to zero with bq78PL114S12 and high discharge current levels???

John M
Expert 2135 points
Other Parts Discussed in Thread: BQWIZARD

We're configuring a bq78PL114S12-based BMS for use with a 4S, 2.3Ah pack  for a client to evaluate and we're having a lot problems getting the RSOC values to behave when we discharge at high current levels. The discharge current profile alternates between approximately 5A-10A, for most of the discharge, with 50A bursts for up to 30 seconds.

As you can see in the attached plot, RSOC behaved pretty well during the first 50A burst. During the second burst, RSOC was adjusted (that's OK) but then it suddenly dropped to zero and stayed there until we started discharging again. At the end of the discharge, RSOC had only dropped to 23% or so.

Checking the Learned tab, Qmax was around 196mAh and Qrem was around 59mAh (we're using a 1mOhm sense resistor so 0.1x for everything). This certainly doesn't seem to indicate that the pack has reached CUV at just above 2.5V/cell at a fairly low discharge rate, approx. 5A. After clearing the CUV alarm, a 5A discharge ended in seconds with a CUV alarm so the pack was empty. The RSOC value had only dropped to 20%.

How can we prevent this sudden RSOC drop? It wouldn't be too much of a problem if RSOC recovered after resting a bit but previous tests didn't show RSOC being reset even after the pack was idle for an hour (OCV Idle Qualifier = 45 min).

The AUX file for this pack was created at a 5A charge/discharge rate to get as close as we could to the typical 5A-10A continuous and 50A long-burst discharge rates the pack would experience. We can't go much higher than that as AuxGui says it couldn't generate certain Ra values at above an 8A rate during the AUX file creation. There's just not enough capacity available to run the discharge long enough.

The Capacity Algorithm is set to 3 (Average).

How can we stop RSOC diving to zero? Is it better to use as high a rate as we can during the creation of the AUX file? Or is it better to use a C/2 rate to give AuxGui lots of time to create the AUX file data? Are there any  bqWizard settings can we adjust to help prevent this RSOC dive-to-zero? The client's accuracy spec for the SOC reading is +/-10% so even if a changed setting affects overall accuracy, that's OK. We just need to keep RSOC from dropping to zero.

Thanks!

 

 

  • 0
    TI__Intellectual 1940 points

    Thanks for the detailed analysis.  This is a partial response to your questions.

    What is your end of discharge voltage (CUV, PUV)?  This could be the reason for the step change to RSOC=0%.

    What chemistry are you using?  A SOC calculation based on CHEM 400 and a cell voltage = 2500, would likely equate to RSOC=0%.

    The Aux file test procedure should be followed as described in the app note SLUA505b.  Your high discharge pulse currents may be introducing non-linear cell impedance effects that cannot be accounted for in the aux file calculation even if you use higher rates in the aux file run.

  • 0 in reply to Jim M15252
    Expert 2135 points

    My CUV was 2,500 and my PUV was 10,200. I was watching the discharge and, AFAIK, I didn't hit either point. I have the recovery voltages set at 3,400/13,600 so I can catch if it happens.

    I'm using Chem 0400, M1 A123 cells. I never considered that the chem file might have an impact in this situation! This sounds like a very possible cause for the RSOC step down to 0%.

    I also remembered that the AUX file was created using a DC/CUV/PUV in the 2,900/cell range. If the AUX file had any "record" of the capacity reaching 0% at 2,900, I can definitely see it dropping to 0% at 2,550-2,600/cell. I'll create a new AUX file using our new, lower, cutoffs and see if that has any impact. We've currently following the steps in SLUA505B except that our charge and discharge current levels are about 2.5C.

    Is there anything else I can try to help the '114 better handle the very wide discharge current swings?

    - Varying the discharge current rate during creation of the AUX file to impact the Ra calculations?

    - Using lower or high charge current rates or shorter or longer periods during the 10 initial charge on/rest periods when creating the AUX file to impact NDI calculations?

    - Having fewer or more initial charge on/rest periods, during creation of the AUX file to impact NDI calculations?

    - Change the FCC Learn Qualifier setting?

    - Change the NDI Impedance SOC or Gain setting?

    Thanks!

    P.S. I do have to mention that no matter what awful abuse we put these packs through (including surge current levels above 300A and lots of accidental short circuits), the safety settings have worked perfectly!