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BQ40Z60: Current Scaling - how does it affect Gas gauging / Impedance Tracking and protection

Steven Shenker
Intellectual 930 points
Part Number: BQ40Z60
Other Parts Discussed in Thread: BQSTUDIO

We need to use current scaling as our application is over 32A, we have a 0.001 Ohm sense resistor for R43 and thus theoretically could operate up to 100A with a 0.1V drop over R43.

If we calibrate current using 2x scaling, the actual current will be 2x higher than the reported current. The question is how will this affect:

1. Parameters which depend on the discharge current value. Do we need to halve everything? e.g OCD (over discharge current threshold)? CUVC (compensated undervoltage protection - assuming double current)? Gas gauging Dsg Current Threshold e.t.c? 

2. Charging, would we need to halve all the current values for charging as well as charge overcurrent protection values similar to the avove?  

3. Gas gauging. I assume that 50% RSOC will now mean 0% RSOC?

4. IT configuration. If the impedance track learning cycle is run with this 2x scaling, will the learning cycle result be incorrect/inaccurate because the current is actually double the value stored in the register? How would this scaling affect the measured battery resistance values e.t.c? Do we need to turn off any kind of learning cycle features with the current scaling active?

Thanks in advance for your help,

Steven

  

  • 0
    TI__Mastermind 42535 points
    Hi Steven,

    I'll have to check with fw to confirm scaling effect on IT. Allow me a week to get back to you.
  • 0 in reply to Batt
    Intellectual 930 points
    Thank you - also if you could let us know the effect on the other functions - as far as I understand AFE functions will not be affected.
  • 0 in reply to Steven Shenker
    TI__Mastermind 42535 points
    Steven,

    AFE will not be affected as it's hw based off voltage thresholds.

    Other than that

    1. Yes, current thresholds will be affected so protections will need to change.
    2. Charging currents have to be halved or used with whatever ratio you are using.
    3.Capacity will scale but RSOC thresholds will remain the same.
    4. Your accuracy resolution will change because of the values. Ra values will double as you are halving currents. No you don't have to change IT features.
  • 0 in reply to Batt
    Intellectual 930 points

    Thank you for your reply, this is helpful but there is still some issues which are not clear. Please see my comments to your answers below:

    Thank you!


    AFE will not be affected as it's hw based off voltage thresholds. This is now clear.


    Other than that

    1. Yes, current thresholds will be affected so protections will need to change. This is now clear.
    2. Charging currents have to be halved or used with whatever ratio you are using. I halved the charge currents in the advanced charge algorithmn parameters but the currents were half what was expected when measured with a multimeter. My conclusion is that these currents are not affected by the scaling as the current controlled over Rchg and not over Rsns. Only the value which is fed back into BQstudio is half the actual charge current.  
    3.Capacity will scale but RSOC thresholds will remain the same. What do you mean by this - should we halve all capacity parameters? e.g design capacity/Qmax cell e.t.c?
    4. Your accuracy resolution will change because of the values. Ra values will double as you are halving currents. No you don't have to change IT features. 1. Do we need to change any paramters regarding resolution? If so which ones? For example, LCHG current resolution should be unaffected since it is only based on the external resistor values.

    2. Should we double all Ra values in the Ra Table?

     

     

  • 0 in reply to Steven Shenker
    TI__Mastermind 42535 points
    2. Yes, if you calibrated your current as half of what you apply, you will see the same exact current in the opposite direction when charging. Same magnitude, different symbol.
    3. RSOC just depends on FCC, so when you halve currents you are also halving the coulomb counters passed charge. So, in effect half the passed charge would get you to zero capacity. You are correct, if that's what you implied.
    4. Your error is literally going to be doubled because your resolution is halved. 1mA off calibrated current literally means you're 2mA off in reality. That's what I meant.

    Finally, no. Don't tweak Ra values. Just do a learning cycle again just for safety. We never recommend tweaking Ra values outside of gauge computed values or the defaults from the gauge chemistry.