Yes, a powerbank. The cost depends on the capacity - how much charge they hold.
For a laptop you will need one with a large capacity. It would be much better to replace the internal battery of your laptop
Sorry, Andy, I got the wrong end of the stick. You will have to check with the airline that a high capacity power bank will be allowed on the flight.
I have an Anker and it is superb, but heavy. I wouldn't buy a cheap unknown brand, I just would not trust them.
You may have mistakenly identified your laptop battery as an internal battery. Be sure to check for battery release latches on the underside.
If your laptop battery is removable, then you can (with the laptop’s model #) purchase a second one for around £30 and bring two fully charged batteries with you when you fly.
If your laptop battery is not removable then go with Barry’s power bank recommendation.