Do you need to use the 15V or 20V modes to supply 18W to a laptop or can the laptop just use the 2A@9V?

I can plug my Macbook into a 5V only USB phone charger and it says it's charging (ok, it doesn't really keep up with the power usage, so the battery still drains, but it drains slower than it would if not plugged into anything at all)

Lots of laptops require 20v because they are re-using circuitry from the days of 19.5 volt proprietary charging bricks.

Also, having both step-up and step down convertors in your laptop is going to make everything a bit heavier, bulkier, and costlier, for the very tiny proportion of users who want to charge from a 5v phone charger overnight.

Since the USB-C port often supports bidirectional power, it may not actually make it that much larger, if at all, as the battery charger IC can often operate in "reverse buck" mode to turn the battery voltage into 5V, which when using synchronous rectification (almost mandatory for efficiency now) means it has all the parts for forward boost, and just needs a slightly cleverer control IC. This is like how many modern power banks can use the same inductor for buck (5V->3.7V) charging and boost (3.7->5V) discharging.

Check this app note from Intersil/Renesas: https://www.renesas.com/eu/en/doc/whitepapers/power/usb-c-bu...

(of course, it may be slightly less efficient in boost charging, but since the 5V usb source has limited power, this may not be a concern in terms of component thermal limits etc).

So this reuse is why almost no power banks support simultaneous charge and discharge, I am guessing.

It is super annoying that I can't use any of my power banks as laptop-like batteries for Raspberry Pis, Arduinos, and the like.

Take a look at OmniCharge, e.g.:

https://www.omnicharge.co/products/omni-20-usb-c/

The Xiaomi Powerbank supports simultaneous charge and discharge.

Many laptops require higher voltage. My XPS 13 does require 20V, for example.

It's very mysterious that it's so particular about the charger. The XPS 13's battery is only 7.4V, so that's not the reason (a simple charging circuit needs a higher input voltage than the top voltage of the battery it's charging).

It's hard to say that the poor implementation of USB-PD ruins the XPS 13, because it's overall a great laptop. But it is very annoying to travel with this computer, realize that you brought only an 18w or 30w charger, and contemplate how nearly every other device you have works fine with it. And every year since 2016 I decline to upgrade my XPS 13 because they haven't fixed this (moreover they probably don't even consider it a problem).

Don't you risk of overheating the charger?

The device and charger are supposed to negotiate an appropriate charge rate. If the charger agrees to supply more than it's capable of, that sounds like a defect.

http://www.chargerlab.com/archives/539.html This one is for laptops. Very compact and provides 60W. I use it with Lenovo X1 Carbon.

What laptop? My XPS refuses to charge in any way below 45W, but any cell phone charger I've seen can trickle charge my Macbook Pro (slowly, or with lid closed, or something)

That's interesting, my XPS 15 9575 can charge from seemingly almost any power supply, including those 5V 2.4A USB wall warts, as long as it's off or the lid is closed. I do have to connect the charger after it enters sleep mode though.

XPS 13 here, it does happily accept any 45W+ charger but the "charging" light does not turn on and in Windows, does not display anything like "plugged in but not charging" for anything less. I'm not sure why this is or if it actually does trickle charge while completely powered off, just not showing it

For power ratings up to 27W, only the 5V and 9V modes are required.

What does the "PD" in 'PD 2/3.0' mean?

Power Delivery which is a recent part of the USB spec.