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bq2002C interrupting fast charge after a couple minutes

abraham gonzalez
Prodigy 70 points
Other Parts Discussed in Thread: BQ2014H, BQ2002C, BQ2002

Hi,

I'm using a set of bq2002C and bq2014H for charging and measuring (respectively) the capacity of 6 (2000mAh) Ni-MH batteries.

The batteries are being charged at the expected rate for C/2, the LED is on, reach their full charge (or very close) and the LED go off normally.

BUT

Sometimes the led (and also the fast charge) go off after a couple minutes of plugging in the 15V input, no matter of how charged those batteries were.

I'd check the temperature and it's not higher than the 47~53°C by the time it cuts off.

When the supply voltage is removed and plugged in again it starts the fast charging again and hopefully reach full capacity.

My questions are:
- Is there a way to know exactly what is causing the IC to stop the fast charging?
- Is there a way to know when the batteries have reached their full capacity?
- Sometimes the led blinks when its trickle charging and sometimes it doesn't, what could be wrong?
- Does anyone has an app note for these ICs?


Best regards

Abraham Gonzalez

  • 0
    TI__Guru*** 145990 points

    Looking at the state diagram answers most of your questions I believe.

    Fast charge is stopped by voltage, temperature, voltage change, or time.  There is also the INH pin.

    Only with your gas gauge can you know if the batteries are full.  You can see if fast change has terminated by checking if you are in the trickle charge state.  The LED being off shows this.

    You will need to look at the various inputs to the IC to see if the LED is actually flashing (at 1 Hz) like it should be or if it is simply transitioning between the various states in the state diagram.

    For other collateral, this tells you if you have selected the correct IC based on your needs: http://www.ti.com/lit/ug/sluu229/sluu229.pdf  There is also a user's guide that discusses the evaluation circuit design: http://www.ti.com/lit/ug/sluu007b/sluu007b.pdf

  • 0 in reply to Chris Glaser
    Prodigy 70 points

    My mistake, I was looking at the wrong datasheet for the bq2002/f,

    Looking at the state diagram and measuring the behavior of the thermistor, I'm able to see the moment when it overheats and everything looks fine there.

    On the other hand, it seems like the only condition that get this IC back to fast charge state is VBAT coming lower than 2V but, unless I'm wrong (again) that will never occur with the voltage divider used in the DV2002L2/TL2 (499Kohms and 100Kohms for 6 cells) which gives us the average voltage of one cell, the voltage of the pack should be 12V to have this voltage on VBAT?

    During fast charge, the voltage on VBAT is about 1.70V and during trickle charge VBAT is about 1.43V, these values are by far lower than 2V so i don't see the transition to lower than 2V on VBAT.

     

    I need to know if it's ok to expect  the VPACK/6 to be higher than 2V for 6 Ni-MH cells or I have to change this divider

  • 0 in reply to abraham gonzalez
    TI__Guru*** 145990 points

    No, the voltage should never go above 2V for a nickel pack if the divider is properly chosen.  The equation of page 2 ensures this.

    But yes, once you're in the final trickle charge state, you need to remove/reinsert the batteries or cycle the input power to restart the state diagram.  Removing the batteries would typically cause Vbat to go above 2V and it would then go back below it when the batteries were reinserted.

  • 0 in reply to Chris Glaser
    Prodigy 70 points

    Lets say I have an almost discharged battery pack and plug the input power;

    If batteries are ok, the fast charge starts at C/2, in my case the bq2002c feed the pack with 1.35A for as long as the thermistor is "cool".

    Eventually, after 30 minutes, the thermistor (and batteries) reach high temperature (about 50°C) and the charger goes trickle at C/32

    So, with no intervention; If the batteries get hot in 30 minutes, the charge cycle will take many hours to complete the full charge with trickle?


    I know that nickel at 50°C hits the lifespan and shouldn't happen but have no room for a bigger heat sink (or a fan) in this design.

    Is it possible in this or any other (pin compatible) charger to go back to fast charge once the battery pack recover a normal temperature and complete the charge in a couple of hours?

    I need to charge fast and complete, can you give me any suggestions?

    Chris thank you very much, your help is invaluable.

  • 0 in reply to abraham gonzalez
    TI__Guru*** 145990 points

    I'm afraid not.  The issue is that you are hitting the temperature cutoff threshold which makes the charger go into a safety mode to prevent an extreme overheating that could damage your pack or worse.  It's odd that you hit this level with such a low charge current.  You could raise the TCO temperature by adjusting the TS pin resistor values or reduce the fast charge current to some slightly lower level (0.4C) but otherwise there's no way to override this built in safety feature of the device.  Thicker wire to the pack might help draw more heat away from the pack and would create less heat in the first place.