Mostly for my phone I rely on a Powerocks Magic Cube 6000mAh extended battery which I’ve carried around with me for the last year or some. But at some point you still have to charge that up too.
Which is where solar recharging kits like the Goal Zero 41022 Guide 10 come in. The name is quite a mouthful but basically it’s a 7 watt charger with AA batteries to store up a charge. If those batteries are fully charged it is capable of giving your tablet a 50% battery life boost or charging your smartphone a couple of times. Put the solar panel out in a sunny location and it will recharge in a couple of hours which is not too shabby.
While technically portable (you could put it in your computer case for example) it’s probably awkward to be carrying this around everywhere. But I could definitely see myself setting this up in the car to recharge.