[notatoad]

the nice thing about USB-PD is that you can connect any PD-compliant charger to any PD-compliant device, and get some charge.

so even without any software or clever switches to slow-charge your laptop, if you took a USB-PD laptop that prefers a 240W charger and plugged it into your 50W tablet charger or 20W phone charger, it would charge. it would just do it a lot more slowly than witht he high-power charger.

I suspect your lenovo works the same way - if you plugged it into a 65W thinkpad X1 charger, it'd probably slow-charge your laptop.

[crote]

This is not a given, though: it has guaranteed forward compatibility, not backward compatibility.

Charging a laptop from a 20W phone charger usually doesn't work because the laptop will have a minimum voltage/current requirement, but charging a phone from a 240W laptop charger will always work.

[kiwijamo]

That has been my experience as well. I've not come across a laptop that accepts a charge from a low power USB-C PD charger. I'm aware the standards allow for falling back to lower voltages but it seems laptop manufacturers specifically go out of their way to stop this. I presume to avoid issues with their battery charging system.