Bypass mode is present in the current 13in mainboards.
The mosfets are rated correctly for bypass mode.
But we never enabled the switching in firmware. It is somewhat complex to implement, and we did implement bypass mode on 16in, I do not think there is a lot of value in implementing it.
There are a few downsides to enabling bypass mode as well.
- During peak current events from the CPU, you will need to draw power from the battery, but the VBUS voltage needs to stay at around 20V, it cannot droop down to the battery voltage.
This requires the buck boost to start boosting the battery power back to the Vsys rail.
If the battery is charging, it takes a few hundred us to turn all this around, so during this time peak loading from the adapter is really high.
Eg - battery is charging at 60W, and system load does a load step from 2 to 80W. On a 100W adapter you will draw around 140W for a very brief time.
When running in NVDC mode this does not happen, as the battery will immediately supply the needed power.
My statement was regarding the previous comment suggesting to turn off the BFET while still running in NVDC mode, In this case, the Vsys rail will be lower, and the battery will provide power to Vsys during peak events through the Bfet body diode. This would risk damage to the Bfet body diode as it is not rated for the full system load.
Regarding margins between charge current and our measured current. Based on the charger max ratings for current accuracy + sense resistor, we need to have a margin for the input current limit so that the charger measured current draws less than the adapter limit + measurement accuracy margin.
Your unit may work fine if you increase your current limit, but a different unit may not, just due to hardware tolerance differences.