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Original question:

BQ25710: Where is battery depletion threshold for 4s battery

BQ25720: battery depletion threshold setting

Amit Raushan
Prodigy 100 points
Part Number: BQ25720

Hi,

My configuration is 2S LiFePO4 where my battery can discharge from 7.2v to 5V or say 5.2V.

So as per understanding from the datasheet(section 11), my battery depletion threshold should be 5.2V

With adapter absent my battery is connected to VBAT and load is connected to VSYS.

As per the datasheet (section 11) and above explanation, should my VSYS_MIN be 5V?

And if I let VSYS_MIN be 6.6V (which is default for 2S) ,  should discharge through VSYS be discontinued once overall voltage drops below 6.6V?

  • 0
    TI__Mastermind 18845 points

    Hi, Amit,

    The battery is connected to the system through BATFET (with a body diode). You can set VSYS_MIN to 5V to fully utilize the battery capacity. Even if VSYS_MIN is set to 6.6V, the battery may be discharged if adapter power is unavailable. It is suggested to disconnect the system load once VSYS_MIN is reached. 

    Regards,

    Tiger

  • 0 in reply to Tiger Zhou
    Prodigy 100 points

    So, the statement "battery depletion threshold should be greater than the VSYS_MIN so that the battery capacity can be fully utilized for maximum battery run time" specifically applies to the condition when adapter is present and LEARN mode is set (EN_LEARN=1). am I correct?

  • 0 in reply to Amit Raushan
    TI__Mastermind 18845 points

    Let me elaborate on this statement  "battery depletion threshold should be greater than the VSYS_MIN so that the battery capacity can be fully utilized for maximum battery run time". 

    In battery only mode or learn mode, the system relies on the battery as the supply. Battery depletion threshold is set by the smart battery, it stops discharging when the battery voltage reaches the depletion level. The VSYS_MIN is set by the charger, the system collapses if below VSYS_MIN. 

    If the battery depletion threshold > VSYS_MIN, the possible battery usage is 100%, from battery fully charged to battery fully depleted. 

    If the battery depletion threshold <= VSYS_MIN, the possible battery usage is from battery fully charged to VSYS_MIN, <=100%, however, the battery may continue discharging to the depletion threshold.  

    Regards,

    Tiger

  • 0 in reply to Tiger Zhou
    Prodigy 100 points

    Hi,

    If the battery depletion threshold <= VSYS_MIN, the possible battery usage is from battery fully charged to VSYS_MIN, <=100%, however, the battery may continue discharging to the depletion threshold. 

    What I can understand from your last statement as mentioned above is that anyway battery is going to deplete (because of BATFET diode) and won't depend upon VSYS_MIN.

    So, I guess that particular statement mentioned in the datasheet is misleading upto some extent.  

  • +1 in reply to Amit Raushan
    TI__Mastermind 18845 points

    Correct! Even if the battery is fully depleted below VSYS_MIN, the battery is only utilized partially, assuming the system won't operate below VSYS_MIN.