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BQ27520-G4: The RM value changes when the battery is inserted or removed during discharging.

Part Number: BQ27520-G4


Hi Team,
The RM value changed when the battery was inserted and removed while the battery was being discharged using the application.
Before the battery was inserted and removed, the RM value was updated by the Coulomb count, but when the battery is inserted and removed, the RM value is predicted by OCV measurement, so it is estimated that the RM value has changed.

Does the accuracy of the RM value measured by OCV when inserting and removing the battery deteriorate as the discharge rate of the battery increases?

Would you please let me know if there is a possible cause?

  • When you insert the battery and the gauge detects that the battery was inserted, the gauge takes a current, voltage and temperature measurement and it will try to determine DOD=function(I, V, T).

    This function uses a temperature compensated OCV table and a temperature compensated resistance table (with the resistance being a function of DOD, hence this is circular and the gauge makes a guess) to calculate DOD by reverse looking up the OCV table (it assumes that OCV = V + I * R_estimate)

    The higher the current, the more impact the inherent inaccuracy of the estimated cell resistance has and the worse accuracy becomes. Transient effects (dynamic behavior of the battery) also add to the uncertainty and inaccuracy.

    --> Only a true OCV leads to good initial accuracy.

  • Hello Dominik,

    Thank you for your answer.
    Does the calculation error of DOD by OCV measurement increase because the cell resistance changes more drastically as the DOD is closer to 100%?
    Does the uncertainty and inaccuracies due to transient effects (battery dynamic behavior) increase as the DOD approaches 100%?

    Sincerely,
    Shun

  • At higher DOD, cell resistance will increase exponentially, hence the error due to the voltage drop over the cell resistance will increase. However, the voltage curve also becomes very steep, which will counteract that error.