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BQ25731: Charge Controller intermittently overheats NiMH battery pack

John Pierson
Prodigy 105 points
Part Number: BQ25730
Other Parts Discussed in Thread: BQ25731

We have implemented a battery charger based on the TI BQ25730 charge controller. We have written firmware for an MCU to communicate with the charge controller. Most of the time this is working OK. However, we have had two incidents that have caused our battery packs to severely overheat to > 110 C. This results in the plastic film the battery pack is packaged in to melt and battery cells to blow their pressure valves. Additionally, smoke and out-gassing occur.

What I am looking for is some guidance from TI on what might be causing this issue. Here are some details:

Battery Chemistry – NiMH
Cell Configuration – 12S2P
Integral NTC in battery pack – Our MCU monitors this temp every 10 secs with an ADC measurement

Charge termination conditions we are looking for in firmware:
- dT/dt (> 1 deg C per minute rise rate),
- Absolute temperature (40 deg C for charging 50 deg C for dis-charging
- 10-deg C increase above temperature when charging started
- VBATT max – set to 18.2 VDC – charge termination if VBATT exceeds this
- Safety Timer

Observations on two PC boards that overheated the battery pack
- We have watched the charging firmware with debug messages reach a number of charge termination conditions, which then lists it is stopping charging – but the charger continues charging
- We set the charge controller to 3.6 A rapid charge current level – but measure 8.2 Amps of charge, other current levels into the battery pack
- Charge current levels drifting around over time – We only have three charge current levels: 3.6 A, 768 mA and 256 mA
- This problem is intermittent

I haven't been able to determine what may be making the BQ25730 charge controller behave this way. This is a critical issue.
Thank you for your support. John

  • 0
    Prodigy 105 points

    Correction: We are using the BQ25730 charge controller - Not the BQ25731. My mistake. John

  • 0 in reply to John Pierson
    TI__Genius 11385 points

    Hi John,

    Reading your descriptions, the first actions regarding the charger I would recommend would be the following:

    1. Make sure to terminate when the system condition calls for termination. The MCU has to tell the part to terminate.
    2. The charger will never drive more or less current than what is programmed by the MCU - need to find out why your measured charge current doesn't match your expected current level. This tells me that something is not working properly in the system.

    Thanks,

    Peng

  • 0 in reply to Pengzhao Zhu
    TI__Genius 11385 points

    Hi John,

    Matthew moved it to email so I will be closing the thread.

    Thanks,

    Peng

  • 0 in reply to Pengzhao Zhu
    Prodigy 105 points

    Hi Peng,

    Regarding your response, We are terminating charging when the system condition calls for termination. We have checked this thoroughly. However, this normally works correctly and charging terminates. But as I mentioned, we are seeing times when we communicate to the charge controller to terminate charging - but it doesn't stop charging. This is the issue we are trying to resolve.

    This has not resolved my issue. But since Matthew has moved it to email. this case can be closed.

    Thank you for your response Peng. I appreciate your support.

    Regards, John