Power tool batteries are already very useful. If they can be a backup power source for small electronics, they become even more cost-effective.
I bought a Greenworks 60V 2.5ah battery (reconditioned) for 89 USD, that is 1.7 Wh/USD. It is close to the base cost of 18650 batteries.
The output of the battery is 60V DC. Firstly I got a DC-DC converter to convert voltage to 20V. The converter accepts input voltage of 10V-75V. So it also works with 20V drill batteries. The output can be adjusted between 0V-60V. As a adjustable DC power supply, it can directly charge car batteries and drill batteries.
The next component is a power delivery converter. It can negotiate output voltage with the device and output 5V/2A,9V/3A,12V/2A,15V/2A,20V/2.25A.
I also added a usb-c tester to monitor the voltage and current.
With these converters, the 2.5ah Greenworks battery should be able to run my laptop for 5 hours.