Thinkpad X1 carbon gen 9 battery not charging / possible incorrect voltage_min_design?

My X1 carbon 9th gen laptop has worked great for the 5 months since purchased new old stock. The battery life is great etc. I am familiar with tlp and have used setcharge 70 80 for most of its life to hopefully prolong the battery life and to some extent that has been working, it still claims to have 104% of new capacity.

Except yesterday it hit 20% and when I plugged it in the laptop kept running but it never charged up above 20%. I've tried two different USBC adapters and both keep it running, but neither will add any charge to the battery. It doesn't say "pending-charge," like in one other question; it says "charging." If I take it off the power supply it keeps the laptop running, and over the last couple days I've drawn the battery down a little more so now it's down to 18%.

I tried rebooting and I even tried going in to the bios, telling it to disconnect the battery, unplugging, then holding down the power button for 10s, plugging back in and rebooting which shows up as the best answer to a similar fault on this question. https://askubuntu.com/questions/1051209/battery-not-charging-but-detected

But that had no effect.

In my case I wonder if it has to do with the

Showing that the firmware thinks the battery is already below its min design voltage, which I can see inhibiting charging. The thing is I'm pretty sure that min design voltage is wrong. I haven't yet opened it up to confirm the wiring but from what I've read x1 gen 9 batteries are 4 cell so the voltage_min_design should be something like 12000000. The nominal voltage should be the 15440000 voltage that's listed.

Does anyone know where the acpi system gets told that min_design number? I didn't delve this deeply into the config when I first got the laptop so it could be that min_design has always been that way and this is a red herring.

Here's the output of everything I could think of that might be relevant:

acpi -b
Battery 0: Charging, 18%, charging at zero rate - will never fully charge.

# sudo tlp-stat -b
--- TLP 1.8.0 --------------------------------------------

+++ Battery Care
Plugin: thinkpad
Supported features: charge thresholds, recalibration
Driver usage:
* natacpi (thinkpad_acpi) = active (charge thresholds, recalibration)
Parameter value ranges:
* START_CHARGE_THRESH_BAT0/1:  0(off)..96(default)..99
* STOP_CHARGE_THRESH_BAT0/1:   1..100(default)
 
+++ ThinkPad Battery Status: BAT0 (Main / Internal)
/sys/class/power_supply/BAT0/manufacturer                   = SMD
/sys/class/power_supply/BAT0/model_name                     = 5B10W13973
/sys/class/power_supply/BAT0/cycle_count                    =    105
/sys/class/power_supply/BAT0/energy_full_design             =  57000 [mWh]
/sys/class/power_supply/BAT0/energy_full                    =  59730 [mWh]
/sys/class/power_supply/BAT0/energy_now                     =  11190 [mWh]
/sys/class/power_supply/BAT0/power_now                      =      0 [mW]
/sys/class/power_supply/BAT0/status                         = Charging
 
/sys/class/power_supply/BAT0/charge_control_start_threshold =     70 [%]
/sys/class/power_supply/BAT0/charge_control_end_threshold   =     80 [%]
/sys/class/power_supply/BAT0/charge_behaviour               = [auto] inhibit-charge force-discharge
 
Charge                                                      =   18.7 [%]
Capacity                                                    =  104.8 [%]