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BQ24401: Early charge termination

Luca Robertson
Prodigy 30 points
Part Number: BQ24401

Hi,

We've had a 6S NiMH charge circuit designed externally using the BQ24401. As far as I can tell, the circuit matches that in Figure 7. of http://www.ti.com/lit/ds/symlink/bq24401.pdf with the VBAT resistor divider changed to match our 6 cell battery pack.

For the most part, the circuit works fine, however about 20% of the time it will exit fast charge early after less than an hour. From the datasheet, fast charge is only exited from timeout, the battery being too hot, or the battery temperate rate of change exceeding -Vcc/161.

Our MTO is set to 5.5hrs, so it shouldn't be terminating from timeout.

Our Vcc = 5V, so the temperature cutoff is 0.225*5 = 1.125V. In all cases of the early charge termination, Vts has been around 1.6V, so it shouldn't be terminating from the battery being too hot.

Finally, I've recorded Vts using an external ADC at 1Hz and have seen the temperature rate of change be far from the limit. A plot of this is below. The low spikes in the plot are an artefact of the ADC I'm using and they do not appear on an oscilloscope. Similarly, the early termination happens with and without the external ADC connected, so the ADC connection is not the cause.

I've also added a filter circuit on Vts as shown in Figure 11 of http://www.ti.com/lit/an/slua064b/slua064b.pdf. This change made no difference.

As far as I can tell, there's no reason it should be terminating fast charge early, so any help or advice would be appreciated.

Thanks,

Luca

  • 0
    TI__Guru**** 154276 points

    Hi Luca,

    To confirm, you used the equations below from the app note to compute your TS pin resistors?

    If so, does the LED status (hi Z) and the RC pin status (below) confirm that the IC thinks charge is complete?

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 30 points

    Hi Jeff,

    The resistor values are correct and the pin behaviour does show it moving from fast charge to top-off charge.

    Fortunately, I believe I've found the issue to be with the 5V zener regulator supply. The original designer was powering a 555 timer oscillator with the same supply as the BQ24401, causing the supply to be overloaded and drop a couple hundred millivolts periodically. This was propagating to Vts as well.

    I'll have to do more testing to verify but I'm confident this is the issue, so I'll mark this as resolved.

    Thanks for the help,

    Luca