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BQ24166: Battery Detection Error & Initiating New Charge Cycle

Jason Young38
Prodigy 50 points
Part Number: BQ24166

We've implemented the BQ24166 on a small board used simply to recharge batteries that have been on the shelf in storage.  The CE\ pin is tied directly to ground.

When power is applied to the input pin (5V) and a battery is not attached, the PG\ line is pulled low, as expected.  However, the CHG\ line is also pulled low, indicating a charge cycle has started.  Is this the proper state of CHG\ when a battery is not attached?  I would expect the battery detect algorithm to confirm a battery is present before the charge cycle begins.

All stages of charging work correctly (conditioning, constant current & constant voltage).  When the charge cycle completes, CHG\ goes to high-impedance, as expected.  However, when the battery is removed (power at IN pin stays connected) and a new battery is attached, the CHG\ line is not pulled low.  The battery definitely begins charging.  The datasheet indicates that "A new charge cycle is initiated only in the event of VSUPPLY POR or the battery being removed and replaced."    Another section of the datasheet states "During new charge cycles, the CHG\ output goes low to indicate a new charge cycle is in progress or that charge has been suspended due to a TS pin fault or the thermal protection circuit.  A new charge cycle is initiated by removing and replacing the battery or toggling the input power."

Shouldn't the CHG\ line go to high impedance when a battery is removed & replaced?  Am I misreading the specification?  

Thanks for your help!

  • 0 in reply to Jason Young38
    TI__Guru**** 158501 points
    Hi Jason,

    I would have expected operation similar to datasheet figures 6 and7 on page 11 where /CHG goes low when the battery is re-inserted. Do you see the battery detection waveform when the battery is removed? At what voltage is the TS pin when the battery is removed?
  • 0 in reply to Jeff F
    Prodigy 50 points

    Hi Jeff,

    I do not see the battery detection waveform.

    When the battery is removed, the TS voltage is 3.44V.  This is above VCOLD (0.6 * VDRV on page 20).  Could this create a problem?

    As a side note, the TS voltage when a battery is inserted is 2.5V.

    Thanks,

    Jason

  • 0 in reply to Jason Young38
    TI__Guru**** 158501 points
    Jason,

    As per datasheet Table 4, /CHG goes low only for first charge cycle but also for TS fault. At battery removal, if the battery voltage reaches regulation (4.2V) before TS goes high for COLD fault, then you would see the battery detection waveform. If battery detection occurs, the charger then realizes that the different battery has been attached and the /CHG indicator is reset for a new charge cycle. Therefore, you may need to add a (0.1uF-1uF) small capacitor from TS pin to ground in order to delay V(TS) from rising too quickly when the battery is removed. Otherwise a TS fault occurs and battery detection cannot start. Also, more than 33uF on BAT will defeat the battery detection algorithm. Note that termination and therefore battery detection are disabled on USB pin if input current limit is set 100mA.

    The datasheet is incorrect in stating that /CE toggle resets the /CHG indicator. Only removal and replacement of input power or battery (assuming battery detection occurs) resets the /CHG indicator to report a new charge cycle.
  • 0 in reply to Jeff F
    Prodigy 50 points
    Jeff,

    We tried placing a 1uF cap from TS to ground, as you recommended. No improvement was seen. In fact, if the logic is working correctly, we're definitely not triggering a COLD fault because the /CHG line stays high after a charge cycle has completed and the battery is changed. Page 24 of datasheet states that /CHG should go low due to a TS fault.

    To your other points, we only have 1uF (plus minor parasitics) on the BAT line. The USB charge current is set to 1.5A (010 binary on IUSB pins), so that should not be disabling termination or battery detection.

    It seems we're on the right path with the battery detection theory, but I don't see anything in the circuit that would disable battery detection. I believe I mentioned earlier that the datasheet states /CHG remains low until charge termination unless the battery is removed. When we're running a new charge cycle, /CHG is low and remains low even after the battery is removed. This points to battery detection being disabled, as well.

    -Jason
  • 0 in reply to Jason Young38
    TI__Guru**** 158501 points
    Jason,

    Battery detection is only disabled if TS fault occurs first or if input current limit on USB is set to 100mA. At battery removal, if V(TS) hits the fault threshold before one battery detection cycle completes in 250ms, then battery detection will not start.
  • 0 in reply to Jeff F
    TI__Guru**** 158501 points
    Jason,

    I just tested on the bench and found that if the new charge cycle as terminated, meaning /CHG changes from low to HiZ, and then you discharge the battery for recharge cycle and enter a TS fault, /CHG stays HiZ. TS fault has lower priority than new vs recharge for the /CHG indicator.
  • 0 in reply to Jeff F
    Prodigy 50 points
    Thanks for your help, Jeff. It looks like we need to implement a process to power cycle the boards when batteries are changed. That's the approach we'll take and we've proven that it works.