There are battery-based chargers - to be charged up and carried as a reserve.
Small ones are about the same size as a phone, flat or cylindrical, while some are quite a bit larger, and might power even a PC for a while.
Some articles refer to a powerbank, so I assume it's at the large end of the range, maybe tablet size but a bit thicker, but even so water melon seems an odd metaphor for something that is probably flat.