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I am using AP9221SA-CC-HAC-7 PCM IC with 1-cell Li-ion battery (4.2V). I have battery charger (RT9471) after PCM IC and load connected after battery charger. The PACK+ and PACK- signals go the the battery charger supply pins. Load is never disconnected in this system. The part of schematic is as shown in the image attached.

enter image description here enter image description here

Now, my battery is discharged to 2.92V. According to the datasheet of AP9221SA-CC-HAC-7, it is between Over discharge upper threshold (2.97V) and lower threshold range (2.85V) so it should be in Over-discharge state. When I measured voltage across PAC+ and PACK-, I observed a voltage of 1.78V.The system was not functioning as input supply to battery voltage is low in this case (1.78V).

I inserted a USB Type-C Charger which supplies the charging voltage to the VBUS pin of Battery charger. Still the battery is not charging (when voltage measured across CN2). At this time, when I measured the voltage across PACK+ and PACK- the voltage was fluctuating between 2.3V and 2.4V.The voltage at REGN pin of battery charger also fluctuating at 0.5V I left the Charger connected to the system for about an hour and then measured voltage across PACK+ and PACK-, it showed 3V. The battery voltage (across CN2) also increased to 3V. So I left the system for another hour with charger inserted. Again when I measured the voltage across CN2 (Battery voltage) it still remained at 3.0V.The battery is not charging.

I also bypassed PCM by connecting VSS and PACK-. At this time battery started to charge. I again connected a battery of 2.92V and inserted charger (USB Type-C). Now also the battery charged fine and the voltage at REGN of battery charger did not fluctuate. enter image description here

Now my questions are:

  1. Why is the PCM IC not allowing my battery to charge even though the voltage on the battery is 3.0V?
  2. Why there was fluctuation on REGN pin when PCM IC was present and no fluctuation observed when there no PCM IC protection?
  3. Why there was a voltage of 1.78V (across PACK+ and PACK-) when the voltage across CN2 was 2.92V? (Without inserting charger)

Please help me on this one. Also I chose AP9221SA-CC-HAC-7 as there were internal FETs present and my board has space constraint issue.

AP9221SA-CC-HAC-7 Datasheet: https://www.mouser.in/datasheet/2/115/DIOD_S_A0012884324_1-2543822.pdf

RT9471D Datasheet: https://www.richtek.com/assets/product_file/RT9471=RT9471D/DS9471D-02.pdf

Vedashree
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