Link to the product: https://www.fuel-rod.com/

We purchased one in the States, its a little tube containing a small battery and short cables for backup power for your phone.  It also offers a swap out feature but that isn't of interest to us here in NZ but was quite cool, if your fuelrod is low you can just throw the battery into one of their vending machines and out pops a charged one.

It's small and fits into your pocket easily, and having a tube to hold both the batter and the cables is fantastic.

Anyone got anything similar they can point me to?