I did a series of stress test without battery. Here’s what I found. 7840U running Arch Linux, on Performance power profile.
When using a 30W power supply, you can boot into BIOS but that’s as far as you can get, attempting to continue booting to OS leads to immediate power cut. This behavior corresponds to:
When using a 60W power supply, the current is limited to 2700mA, however the power can only reach 33W, far from 54W. Both the instantaneous and sustained CPU power are 25W
readings
$ sudo ectool chargestate show
ac = 1
chg_voltage = 15400mV
chg_current = 0mA
chg_input_current = 2700mA
batt_state_of_charge = 0%
When using a 65W power supply, the current is limited to 2924mA, however the power can only reach 37W, far from 58.5W. Both the instantaneous and sustained CPU power are 28~30W
readings
$ sudo ectool chargestate show
ac = 1
chg_voltage = 15400mV
chg_current = 0mA
chg_input_current = 2924mA
batt_state_of_charge = 0%
This corresponds to
When using a 100W power supply, the current is limited to 4500mA, The power from USBC goes from 57W(still lower than 70+W in Performance mode) to 46W(the same as Performance mode with battery installed). The instantaneous CPU power is 45W(lower than battery installed 53W), the sustained CPU power is the same as battery installed, 35W.
Conclusion: For some reasons the presence of the battery itself enables higher utilization from the USBC input even when the battery is charged to 100% or the charging limit you set.
Battery is used for “stabilizing the grid”, if 30W charger is used, flipping when overall system load(NOT CPU load) <27W, discharging when overall system load >27W.
If 60/65W input, charging when <33/37W, flipping when 33/37W<overall system load<54/58.5W, discharging when >54/58.5W
If 100W input, charging when <57W, flipping when >57W, it’s not possible for overall system load to reach 90W
We can limit the CPU power to 25/29/45W when using 60/65/100W power supply to prevent the battery from flipping.