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BQ27520-G4: external temperature jumps with sudden load current increase

Daniel Bode
Prodigy 60 points
Part Number: BQ27520-G4

Testing the fuel gauge at higher ambient temperatures (~55C) I noticed that the reported external temperature (battery's thermistor) measured by the fuel gauge suddenly increases by 3.5C (in 5sec) when there is a sudden current jump (load increases from ~500mA to 1300mA).

I replicated that jump at my desk at room temperature and found a ~1.5C jump within 1s when increasing the load from 0 to 1300mA.

Probing the thermistor's resistance directly without using the fuel gauge, it all looks reasonable. The resistance slowly drops (battery heating up) when a higher load is applied. Probing the bias voltage and thermistor's resistance with the fuel gauge connected doesn't work because the fuel gauge network seems to be doing something that constantly changes the resistance and voltage. So no reliably reading is possible.

Is that temperature jump normal? Is there a way to minimize that jump, maybe by changing the temp model coefficients of the fuel gauge?

  • 0
    TI__Expert 8245 points

    Hi,

    The temperature jump you see here is normal and expected given the current jump from 0 to 1300mA. Changing the temp model coefficients will not change the behavior of this jump. Since it is only a few degrees and soothes out over time, there should be no concern here.

    Thanks,

    Jackson

  • 0 in reply to Jackson DiGiovanna
    Prodigy 60 points

    Hi Jackson,

    I agree, at room temperature it is acceptable. But at higher ambient temperatures the temperature jump is huge which makes it difficult to use the reported temperature to trigger a battery temperature warning. Here is an example where the battery temperature is quite high and when the load is reduced by ~700mA then the temperature drops by >10C. 

    Thanks.

  • +1 in reply to Daniel Bode
    TI__Expert 8245 points

    Hi Daniel,

    This behavior of large jumps in temperature will certainly be amplified by the fact that the ambient temp is probably near the abs max spec for your test here. This device is spec'd to work at up to 85C ambient. When you are at or near this ambient temperature and also are drawing a large amount of current, these large jumps may occur.

    Thanks,

    Jackson