A superb slimline 5000mAh power bank charger (similar in size and depth to a modern smartphone).
This item can be personalised with Pokémon Go logo of your choice. You can optionally add further personalisation to the rear, or if you’re not into Pokémon personalise both the front and rear. Of course, you don’t have to personalise at all!
On my phone I achieved 2 full charges whilst the phone was turned on with WiFi and GPS enabled and full brightness. With these turned off you may get more, and a quicker charge. You will always get the quickest recharge whilst the phone is powered off.