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Capacity calculation on bq27501

Alex Gutica
Prodigy 70 points
Other Parts Discussed in Thread: BQ27501

Hi,

I have setup the bq27501 with a single Li-Ion cell with a design capacity of 1220mAh.

When configuring the gas gauge, I entered the design capacity as 1220mAh as I proceeded through the bqEASY wizard. I also set the minimum operating voltage as 3.0V

The wizard put the battery through several discharge cycles where it brought the voltage well below 3.0V and it finally determined the chemistry to be code 0147 with an error of 1.78%

During my subsequent testing and cycling I noticed that FullChargeCap is only 990mAh and has remained at this value through 10 discharge cycles. Full AvailCap however has changed from 990 to 995mAh but it does not actually add to the capacity from which the gauge measures. My questions are as follows:

1. What is the difference between FullChgCap and FullAvailCap and their uses?

2. How are these variables calculated or determined by the gauge when I specific design capacity to be 1220mAh? The gauge reaches 0% SoC well before cell voltage reaches 3.0V.

3. How are these values updated during normal operation (after initial calibration) as a result of aging etc?

4. What is the difference between RemCap and NomAvailCap and their uses?

Thank you.

 

  • 0
    Prodigy 70 points

    Any information regarding the above questions would be greatly appreciated.

  • 0
    TI__Expert 4050 points

    1. FCC is capacity under load. FAC is capacity under no load or light load

    2. design capacity is not used. The Qmax is the one really determine the FCC and FAC.

    3. at R update, temperature change or start of discharge, the gauge will run the simulation and update these values

    4. RMcap is under load and NAC is no load or light load.

     

    Ming

  • 0 in reply to Ming Yu
    Prodigy 70 points

    Thanks for the clarification Ming. Another question:

    How does the bq27501 determine 0% SoC? Based on what criteria?

    I also noticed that once it reaches 0% SoC, if discharge current is set to 0, the gauge will suddenly update/jump to show 7-8% SoC after resting for around 5+ hours. This is without energy being added or removed from the battery. Why does this happen? It does not seem correct that it reach 0% and then jumps to 7% with the battery just resting.

    The discharge current in these tests is C/5.