Instead of an external lap top battery, keep a 12 volt car battery on a trickle charger and keep a 500 watt AC inverter handy, you will be able to use your laptop for several hours.
Thanks. I didn’t think of that. I have one in the car for the kid’s games.
And when the spare one drains, you can start your truck, switch out the batteries and let your truck charge your drained one, infinitely.