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BQ24040: BQ24040 not reach 100% of charge

Joaquim Martinez
Expert 1270 points
Part Number: BQ24040
Other Parts Discussed in Thread: BQ51013B

Dear all,

I am using the BQ24040 charger in order to charge a LiPo battery (800mAh) and I can't reach the 100% of charge. My setup is:

1. Iset is around 500mA (R Iset = 1K)

2. R preterm = 1K. 

3. Built in NTC in the battery (1K+-5% - B = 3435+-5%).

4. TS pin is only connected to battery NTC.

5. CHG ans PG pins with a pullup of 680K to +3.3V (from a LDO after the charger). 

As I am not reaching the 100% (max 91-92%) of the charge (even after 8 and 9h of charge) now I am monitoring the CHG pin so as to know what battery voltage and current there are when the charger asumes it reaches the end of the process charge (CHG to "0"). Additionally, the temperature in the battery seems OK (which must be under 45ºC according with manufacturer) and we want to keep this range of temperature for the charge. The BQ24040 temperature seems good also. 

My questions are:

1. Will the CHG pin go to "0" once the battery is charged even if is not a full charge? What is the minimum battery capacity or Vbat/current taper so as to this pin goes from "1" to "0"? The datasheet says "The CHG terminal is low (LED on) during the first charge cycle only and turns off once the termination threshold is reached, regardless if termination, for charge current, is enabled or disabled".

 2. What means "the first charge"? I think each every time power good voltage is detected or performing a disable/enable TS terminal, isn't? Is safe to pull to GND TS pin with the NTC of the battery connected to perform a reset for a new first charge cycle? This is important for me because I won't disconnect the battery.

3. As for the TTDM mode, is it enabled if TS pin is connected to the battery NTC? This can explain my problem? I don't understand why if battery charger is in TTDM mode, safety timer and termination are disabled (chapter 8.4.9 of the datasheet)

Please, can somebody help me with this?

Thanks.

Joaquim.

  • 0
    TI__Guru* 75856 points
    Hello Joaquim
    My questions are:

    1. Will the CHG pin go to "0" once the battery is charged even if is not a full charge? ---Bill J - Yes, the precharge timer is 30 min and charge is 10 hr, if the timer expires the /CHG pin will go to high impedance.

    2. What means "the first charge"? --- Bill J - This is the first charge after power is applied or battery connected. Re-charge will not show on the /CHG pin, this is when the battery voltage drops and charge restarts.

    3. As for the TTDM mode, is it enabled if TS pin is connected to the battery NTC?---Bill J -- If the TS pin goes high, no connection unit will enter TTDM mode and timers are disabled.
  • 0 in reply to Bill Johns
    Expert 1270 points

    Hello Bill,

    Thanks for your quick answer.

    Refering to the question 2, my scenario will be recharge the battery with QI BQ51013B, battery won't be disconnected, so I understand once the battery is fully charged ()CHG = "1"), if RX QI is still present over TX, CHG won't be changed (from "1" to "0") when battery voltage drops, it is needed to remove the RX in order to have a new fisrt charge state, isn't? This is important for me, because I want to have a log of the entire charge cycles.

    The TTDM functionality is not clear for me. According to the datasheet, there are 4 scenarios:

    1. Floating TS terminal or pulling high, the TTDM is active (two timers and taper current detection) and disables TS monitoring.

    2. Pulling TS low charge is disabled. 

    3. No NTC sensing, 10k resistor at TS pin to GND.

    4. Preventing IC entering to TTDM mode when battery is removed, 250K resistor at TS pin to GND.

    The 2 and 3 scenarios are clear for me.

    The 1st, just clarify, chapter 8.4.9 specifies: "The battery charger is in TTDM when the TS terminal goes high from removing the thermistor (removing battery pack/floating the TS terminal) or by pulling the TS terminal up to the TTDM threshold." I understand that if TTDM is active, timers and termination current detection are disabled, is it correct? 

    If yes, I am missing a 5th scenario, when the internal NTC of the battery is connected to the TS pin of the charger (my case). What happens here? are TTDM disabled (timers and termination current detection are running) and TS monitorin enabled? In my hardware I have the NTC of the battery connected to the TS pin in order to perform a secure charge (in the range 10-45ºC). Based on the different tests that I performed, it seems that TTDM is disabled because when the current charge reach the current termination and battery voltage is 4.09V, the pin CHG goes from 0 to 1. So, it means that TS monitoring is disabled? Are TTDM mode and TS monitoring compatible?

    Finally, what condition or situation may need the 4th scenario? For me it doesn't make sense disable TTDM mode if battery is not present, isn't?

    Thank you.

  • 0 in reply to Joaquim Martinez
    TI__Guru* 75856 points
    Hi Joaquim
    I have added more info to your TS and TTDM outline.

    1. Floating TS terminal or pulling high, the TTDM is active (two timers and taper current detection) and disables TS monitoring.
    *TTDM mode is similar to an LDO mode and would be used to power the system when battery has been removed.
    *The output voltage will be 4.2V and current will be limited to fast charge current.
    * In this mode TS pin is above 1.6V and monitoring of V-Hot and V-Cold is disabled.

    2. Pulling TS low charge is disabled.

    3. No NTC sensing, 10k resistor at TS pin to GND.
    *Voltage on the TS pin will be 500mV (50uA X 10k) between V-Hot (178mV) and V-Cold (1230mV), safe condition.

    4. Preventing IC entering to TTDM mode when battery is removed, 250K resistor at TS pin to GND.
    *If TTDM mode is not desired, no voltage present on output when battery is removed connect 250k resistor at TS to GND.

    When the battery is inserted with NTC that is connected to TS charger will enter charge mode. Voltage on the TS pin will change due to resistance of the NTC, unit exits TTDM mode and battery detection routine is run then appropriate charge mode is entered.
  • 0 in reply to Bill Johns
    Expert 1270 points

    Hi Billy,

    Thanks for your answer.

    Joaquim.