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BQ2000: Charger cutting out at random times before charge level is complete

Tony Crocker
Prodigy 110 points
Part Number: BQ2000
Other Parts Discussed in Thread: BQ24610, BQ24753A

I am using the BQ2000SN-B5TR to charge a 7.4v 2 in series 2 in parallel battery.  The circuit is designed to allow a device to continuously run on battery by placing one battery in charge mode for 8 minutes and then swap that battery for the one running the circuit.  This cycle continuously repeats while the device is on.  I have used the BQ2000 in a 4 cell battery with only one in the circuit with no problems.  However, in this circuit I am having glitches where at time the BQ2000 continuous to charge and other times it stops charging and the voltage is not above 7.2 volts.  The timing is random in that it may charge for an hour or it may charge for 15 minutes.

I would like to have the circuit continuously run until 7.4 - 7.5 volts is reached.  And when the other battery swaps in the BQ2000 charges it back to 7.4 to 7.5 volts and on and on.  Attached is a pdf of the charging portion.  I need to change the current sense resistor to .03 ohms to get more current.  But what I am missing?

Attached is a schematic

Thank youbattery_supply.pdf

  • 0
    TI__Guru 56650 points
    bq2000 is not only for LI-ion battery. It is also for NiMH battery. When you swap the battery, you may make bq2000 run into other mode which has early termination. Also, you required a high charge current.
    Please change it to a Li-ion battery: bq24610 or bq24753A.
  • 0 in reply to Wang5577
    Prodigy 110 points

    Thank you for the reply.

    I cannot use either of those pinouts.

    Why would the BQ2000 run in the other mode?  I am not changing any of its connections.  The swap is handled by a relay at the battery terminals so the BQ2000 does not know that anything has happened other than a slight voltage drop of .05 to .1 volts.  Also it does not always happen.  Most of the time it will run long periods.  It seems to me to be a stability issue.  Perhaps related to the 220K resistor.

    Current supplied should be handled by the FETs which I increased their size.  Right now I am running at 1amp.

  • 0 in reply to Tony Crocker
    TI__Guru 56650 points

    "a slight voltage drop of .05 to .1 volts"

    The bq2000 uses a peak-voltage detection (PVD) scheme to terminate fast charge for NiCd and NiMH batteries. bq2000 samples, at a rate of MTO/128, the voltage on the BAT pin and triggers the peak detection feature if this value falls below the maximum sampled value by as much as 3.8mV (PVD).

    So, that slight voltage drop may trigger PVD termination.

  • 0 in reply to Wang5577
    Prodigy 110 points
    The PVD drop would be after 11 minutes based on 220K and a .047uf. That timing makes sense. If I change the CMTO to 1uf. that will extend the do not check PVD to 247 minutes. I can also reduce my swap times between battery times such that the voltage drop between the charged battery and the to be charged battery is not as great.

    I will give that a shot and run multiple test times over the next couple days
  • 0 in reply to Tony Crocker
    Prodigy 110 points
    I have had a chance to change out the timer cap to 1.0 uf and the resistor was set at 220K. I ran 3.5 hours without issue and only shut down because I had to leave. I was even able to have one battery completely disconnected. The charger would shutdown on the disconnected battery and then start back up when the connected battery switched over to charge.

    Thank you for your help
  • 0 in reply to Tony Crocker
    TI__Guru 56650 points
    Thank you for your feedback.