Charge laptop with USB PD at 20V even if it draws less than 45W?

Question

I want the possibility to charge my Laptop (19V/3.42A, Acer Aspire 5 A515-52G) from a power bank. Since my laptop only has the ordinary barrel power connector but power banks usually come with a USB-C PD 45W (20V/2.25A) port, I'm planning to use a USB-C to 20V DC Trigger (this thing requests 20V constantly from the USB PD charger.

The USB PD rev. 2.0/3.0 source power rules however define 2.25A as the minimum current for 20V. If my laptop is in a low power scenario and requires less than 45W, only the amperage may go below 2.25A. How does a USB PD charger/power bank behave in this scenario? Will it deliver lower amperage or refuse to work entirely?

Answer 1

There is no minimum current limit for 20V output in PD specification. The charger will give 20V even if your laptop use 0A. There is a minimum advertised current value for charger to be considered eg. 45W, which is not what you are thinking.

This is how my native USB-PD powered laptop works. You can already buy Type-C plugs with barrel or square (Lenovo etc.) ends which give out 20V output (I have a few of those as well). So you can use your favorite USB PD charger with your laptop easily. Actually, you can measure the voltage at barrel/pd adapter when it is not connected to laptop and you will find that it has ~20V with 0A current.

The table at Wikipedia at the link you provided is flawed, as it is there without any context. The current values in the table are minimum/maximum advertised values by chargers. A 45W charger should advertise supporting 2.25A at 20V. It is PDF/20 which is in this case 45 / 20 = 2.25

You can get the full specification from USB site (page 602 lists the actual table): https://www.usb.org/document-library/usb-power-delivery

Here is a screenshot of the actual table from specification with explanation of the values (including the 2.25v etc.): USB PD Table

Although, you would be well advised to use at least a 60W PD charger. Because charger will enter overcurrent-protection if your non-PD laptop exceeds the charger's maximum current value.

Answer 2

The specs allow 20v at 5 amps max or 100w.

Basically, the more amps available the faster it will charge. There's a critical threshold after which it either won't charge at all or will be so slow it might as well not be charging.

Further there's a even lower threshold where it will drain the battery faster and faster as you amps decrease.

Your only chance of using the minimum is if your battery is 100% charged, and your laptop is off or near idling.

If you plug a USB type C into the laptop for charging, and its using super low power, there is a chance it might switch from 20v down to 5v where it can get 5v at 5a or 25w.

The barrel plug expects 20v (19v) no matter what.