oops.se
First you need to estimate the max power consumption.
- What devices is also powered by the Pi?
- How much power does each device consume?
- How long time do you want the laptop to operate?
And some general thoughts:
There is DC/DC converters that sits between the battery and the Pi. So no problem to use "any battery", it has an cost as they are from 80% to 99% efficient .
You probably want to be able to charge the battery and use the Pi at the same time and this demands a more tricky circuit.