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BQ78350: "Impedance track" vs "Compensated End-of-Discharge Voltage"

Albert Galofre
Intellectual 600 points
Part Number: BQ78350
Other Parts Discussed in Thread: BQ34110, BQ34Z100, BQ40Z50, BQ4050, BQ76940,

Hi there,

My question is really easy,

What is the best algorithm, the "Impedance track" (IT) or "Compensated End-of-Discharge Voltage" (CEDV)?

I have read several pdf regarding this topic, at seems that the IT is better because take in account 2 key points more:

- Aging

- Self-discharging

I have to choose one of them. In my application the batteries achieves the requirements but on the limit. I mean that the nominal specs are enough; but I'm pretty sure that after some charge/discahrge cycles the batteries will lost some capacitive, and consequently won't accomplish the requirements.


Acording to TI pdf, seems that the IT algorothim is the better option because "meas" the impedance batery, which means that I can get all the capacity available in the batery. On the other hand the CEDV makes some "estimation" and consequently there are always some safe range wasted, I mean some remaining capacity on the battery which is not use.

What do you think? IT or CEDV?

Thanks a lot

  • 0
    TI__Guru* 81034 points
    Hi Albert,

    You selected BQ78350 as the device number for your question which is a CEDV gauge. There are some other devices like the BQ4050/BQ40z50 that have both options to choose from. The BQ34110 (CEDV) and BQ34z100 (IT) are also very popular as stand-alone gauges that measure only the stack voltage in applications where individual cells cannot be measured directly.

    Impedance Track has the potential for the best accuracy. When accuracy is the highest priority, this should be your first option to consider. However, it is a little more work to set up the battery gauge and there are certain application conditions that are not ideal: highly pulsed loads and applications that do not allow rest periods. Impedance Track is also not ideal for rarely discharged applications because it needs to take voltage measurements at different capacities to update the impedance occasionally.

    CEDV can also be very accurate, but it does need the battery to discharge down to 7% occasionally to learn the capacity as the battery ages. It is the better algorithm for the applications where impedance track is not ideal (highly pulsed loads and no rest periods).

    Best regards,
    Matt
  • 0 in reply to Matt Sunna
    Intellectual 600 points

    Understood.

    My application has to manage up to 100A during 0.2 seconds. 14 cells, which means 50V. 5KW a long 0.2seconds.

    Really high pulsed load. After some pulsed load, a relax time up to several minutes even more.

    So in this scenario you suggest me the CEDV algorithm?

    I this case I have to discharge the battery down to 7%. In order that the algorithm update its internal aging estimation, right?

    Thanks a lot.

  • 0 in reply to Albert Galofre
    TI__Guru* 81034 points
    Yes, for this application a CEDV gauge would likely be the best choice. You might want to look at the BQ76940 + BQ78350-R1 CEDV gauge. There are multiple TI reference designs based on these devices (see www.ti.com/.../TIDA-00792).

    The CEDV algorithm will update the Full Charge Capacity when it sees a qualified discharge down to 7%. For best accuracy, it should update the Full Charge Capacity occasionally.

    Best regards,
    Matt