LiPo charge termination when full

Hi,

yesterday I wondered why my SODAQ SARA R412 AFF board would not terminate the charging process despite the battery being fully charged. The charging indicator LED would not turn off and there were permanently flowing some milliamps into the battery.
I wondered quite a long time about the functioning of the charger IC but could not find anything. After digging through the board schematics I found that R29 was placed and not R28 like said in the schematics.

With the resistor placed like this the current out of the charger will never drop below the charge termination threshold. I moved the resistor from R29 to R28 and now it is working as expected.

Maybe someone else has a board with a misplaced resistor.

Regards
Thomas

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Hi @thomas,

Thanks for letting us know. I will send this to our hardware department.

Best regards,
Jan

Hi @thomas,

The hardware guys came back to me. The schematic isn’t up-to-date.
The R28 is for the N2/R410
The R29 is for the R412 and others.

The reason why you see that the lipo is always charging is that when the modem is enabled, you charge the battery, but the battery is directly feeding the modem. So there is always current going in and out of the lipo.

R28 or R29 only matters when you use the R412Ms 2G.
2G has high current spikes, the battery can handle this.

In short:
When you use only LTE-M or NB-IoT, you can keep it like it is.
The performance won’t be affected, you actually spare the battery. So this is good :slight_smile:

If you want to use the 2G of the modem, please revert back the resistor. You will run into issues with 2G connections.

Best regards,
Jan

Hi @Jan,

I see what you mean. I did not consider that the battery is cut-off from the LDO_IN net when the USB is plugged in. But in consequence the CHG_STAT indicator is of no use when R29 is populated.
Maybe I can monitor USB_DETECT and the battery voltage.

Best regards
Thomas

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