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

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?

• I don't see the need for the laptop model. The laptops' power specs are there and my question has to do with the USB PD specification. Can less than 2.25A be drawn at 20V from a USB-C PD 45W power supply? Oct 10, 2019 at 23:43

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.

• Is it ok? a laptop requires 19V but USB-C provides 20V. Doesn't the laptop burn out because of overvoltage? May 27, 2021 at 7:22
• 19V is the nominal voltage. The input can be slightly lower and higher and that is perfectly fine. We use one of these adapters with an asus ux32vd without any problems. The selection of maximum 19V voltage on laptop adapters is purely based on some old certification requirements. electronics.stackexchange.com/questions/31618/… May 28, 2021 at 8:45

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.

• The plug cannot switch down to 5V at lower power needs since the 20V DC Trigger I'm planning to use will always ask for 20V, hence my question. If my laptop 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? Oct 10, 2019 at 23:47
• @Morrow If the device complies with electrical code, it should just shut off at a certain threshold. Jun 10, 2020 at 22:09