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LTC1760 not charging undervoltage protected smart batteries


I'm using the LTC1760IFW in a product that has one or two Li-Ion battery packs (4 cells in series) with integrated BMS that communicates with the LTC1760.

All is working as expected (the power path, communication of an external µP with the packs, normal charging, etc. ).  There is just one situation that does not work as I expect. It concerns batteries that are in protection mode (cell undervoltage protection, is no voltage on the pack's "+" pin):
As soon as such a pack is connected to the device, the LTC detects the battery and the PRESENT_BAT1/2 and CHARGE_BAT1/2 bits of BatterySystemState() are set. There is indeed activity on the I2C bus towards the pack. However, I do not see any voltage applied to the "+" pin (i.e. the output of the charge mux). I don't see how the battery is charged in this situation and as long as no voltage is present on the + (constant voltage, nor any pulses), there is no charge current and the battery will not wake up (as is does when I manually apply 12.5V / 200mA to the pack for 10 seconds.

Looking at the LTC1760 registers, I see (with 1 battery pack connected) :

  • SystemState = 00xx.0000.0001.0001
    => BAT1 and BAT2 present, both charging (?), batteries do not provide power
  • SystemStateCont =
    => AC present, "Power is good"
  • SystemInfo = 0b0000000010000011

There is a voltage of 16.8V at the input of the "charge mux" (U17 and U18 in my schematic, attached as PDF) but, as mentioned, no voltage on the other side of the mux, i.e. on the battery.

So my question is why doesn't the LTC apply a voltage the battery when I has detected the battery and the 'charging bit' is set ? As I mentioned above, charging is working correctly when the battery is not in undervoltage and provides a voltage on its + pin.

Any help is appreciated.

Best regards,