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BQ24080: Deep discharge

JayLC
Prodigy 30 points
Part Number:

The deep discharge of a LiPo batterypack with integrated PCM does not recover with the BQ24080, but instead shows strange sawtooth discharge behavior.

We slowly discharged a LiPo battery pack with integrated PCM. Once the battery was discharged, we enabled the charger, but we are unable to reinstate the battery voltage above 3V..

Has anyone seen this before and have an explanation/solution please? (see images)

  • 0
    TI__Guru* 75906 points

    Hello

    When battery voltage is below 3.0V (V-LOWV) unit is in a precharge mode and output current is limited to 20% of fast charge current (ISET).  If the battery has a load on it during this time the current may not be enough to charge the battery above 3.0V and start fast charge.  It looks like a load is applied at about 2.8V and turns off about 2.5V.

  • 0 in reply to Bill Johns
    Prodigy 30 points

    Thank you very much for the insight Bill.

    The load is indeed always applied and consumes 4.7 mA in the set mode. 20% of fast-charge mode would mean a 8-9 mA charge current of which half would go to the load.

    Is there a solution for this? What I can think of is: switch of loads (if possible) or increase ISET. Any other options? Possibly there are better components for this use case?

    Thanks!

    J

  • 0 in reply to JayLC
    TI__Guru* 75906 points

    Hello

    Take a look at the BQ24040, several parts in the family.  The precharge to fast charge transition point is reduced from 3V to 2.5V. 

  • 0 in reply to Bill Johns
    Prodigy 30 points

    Thank you Bill. Our battery spec requires a precharge below 3V in their application notes so I will check with them.

    One thing I still do not understand is why there is a discharge of the battery occurring in the sawtooth and no charging behavior.

    From what I understand, the PCM disconnects the battery at 2.5V, after which the charger goes into precharge mode. Why is it that this only occurs very briefly until 2.8V, after which a long discharge reoccurs every cycle until 2.5V is reached once again and the cycle restarts?

    discharge 2.5V -> PCM disconnects -> charger activates I_PRECHRG  -> voltage increases to 2.8V? -> charger turns off? -> discharge to 2.5V (this loops)

    Am I understanding this correctly?

  • 0 in reply to JayLC
    TI__Guru* 75906 points

    I suspect the system load connected to battery will start to operate (try to operate) at about 2.8V.  The circuit has some hysteresis, on about 2.8V and off about 2.5V.  The 2.5V may be controlled by the PCM.  The system starting at 2.8V then loads battery and starts discharge until PCM disconnects or load shuts off.  You mentioned a load switch to isolate the load from battery, this would probably work.  We have used TPS22914 and others from the family.