I bought one of these https://m.ebay.co.uk/itm/12V-BATTERY-ST ... SwLYBaHCWg
As it was the cheapest I could find at the time, I think I paid £32 for it, it did not last long and it got knocked off the workbench when it was in a discharged state which didn't help it either. So I took it apart and bought a new battery for it with a higher AH rating: 20 instead of 16 I think.
I now use a £12 solar charger to keep the booster topped up. https://www.ebay.co.uk/itm/Ring-RSP150- ... xyRhBSz7V5
The solar charger was a cheap job from ebay, it only really does anything when in direct sunlight and is only rated at 1.5w. I am sure there are better value ones around that produce more power, or maybe I could double them up for dark days like we have at this time of year.
Overall a bit of a waste of money buying a booster, a new booster battery and a solar charger. However this situation evolved over 18 -24 months and was not by design. The booster saved me quite a few times when the car I had would suffer from bad starting when hot. The battery on the car would eventually run down but the booster pack saved me (when I remembered to take it with me).