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bq24251 - charging issues with "protected" li-ion battery

Other Parts Discussed in Thread: BQ24251

Greetings. We are using a bq24251 (in i2c mode) in a device that is in production. Due to an unrelated issue, a small percentage of these devices are discharging too quickly. The permanently connected 18650s we are using in these devices have over-discharge protection circuitry - the small percentage of our production lot that are discharging too quickly will discharge the battery to the point where the over-discharge protection circuitry has tripped. We have found that a few (not all) of these defected/over-discharged units will not recharge when a USB connection has been made. We confirmed that the issue follows the PCB and not the battery - in other words the bq24251 seems to be at fault, not the over-discharge protection circuitry. I am wondering if the bq24251 is not sensing the presence of the battery since the over-discharge protection circuitry has tripped (bq24251 and i2c connected MCU are powered down), but I can't explain why this appears to be occurring on only a small percentage of boards. Can someone explain the battery detection process if the bq24251 is brought up from dead? Do I need to prompt (over i2c) the "AnyBoot" process upon startup? If so, why do most units work without this prompt? The datasheet appears to be lacking in this department.

Any help will be greatly appreciated.

-Chris

  • Please see attached diagram of anyboot operation.  The current source into the battery is typically enough to recharge the battery cell enough and close the protector FETs.

    Despite, we are aware of anyboot failing to identify a battery with very high impedance (>250mohm in series) that is close to full charge.  But, once the battery drops below the recharge threshold, it should start charging. Do you see the battery detection square wave on SYS and BAT?

    With D+/D- SDP port and a good battery (V(BAT)>~3.6V), the original BC1.2 spec required that charging be disabled.  We have had no issues reported with this feature.  Can you confirm that D+/D- is correctly identifying the port?  To do this, you need to do an I2C read of the appropriate register with the IC still in standalone mode (i.e. disable watchdog timer signal - not the watchdog timer itself and wait 50s to do the read).AnyBootDiagram.pdf 

  • We have no issues recognizing the port, we enable charging if a SDP port is recognized (yes, we are contradicting the spec, but it is pretty rare to find a USB port that can't supply 500mA). Thank you from posting the Anyboot diagram, it is not clear (to me at least) how the bq24251 operates from a dead battery condition upon connection of a USB source (note that until the USB is connected, both the MCU and 24251 have no power source). In this situation, the 24251 must wake up first to provide power to the MCU which will then query the 24251 regarding the USB port present and set the charge current accordingly. When the 24251 "wakes up" in this use case, does it automatically run "AnyBoot"? If not, should I force it to?
  • Battery detection starts at USB attach. You can force battery detection via I2C.
  • Correction: Battery detection starts at USB attach unless there is a TS fault. Is it possible that the TS pin voltage is indicating HOT or COLD?
  • Jeff - I know I can force battery detection, but do I need to if I am using the bq24251 in i2c mode? If so, then any ideas about why most units start charging without an issue if the battery protection circuitry has been tripped, but a few do not?
  • Does the force battery detection work? If so, then it is not the battery detection circuit itself that is failing.
  • Jeff- Thanks, I will try this, but do you know if I should be forcing battery detection in my application (bq24251 in i2c mode), or should it be prompted automatically upon USB connection regardless?
  • I just confirmed on the bench that battery detection starts up with USB insertion unless there is a TS fault.

  • Jeff- Thanks, I was looking in to the temp fault angle. I am measuring a Vts/Vldo that is within the operational range.

    We have been having board assembly issues in production, but they are usually exposed through elevated idle currents. That is not happening on the boards in question. I can't see/measure anything on these boards that is indicative of an assembly issue. Do you think it is possible that elevated temperatures during reflow or extended duration at a given temperature could be damaging the bq24251, resulting in this problem? Or would temperature damage be exhibited in another fashion?
  • We have not seen reflow issues from other customers. If the ICs function normally otherwise I would not expect high temperature damage.