Saw this - It's not available yet but thought it would be cool to include in future designs.
https://news.cnet.com/8301-11386_3-57428080-76/two-weeks-of-smartphone-charging-in-your-pocket/?part=rss&subj=news&tag=title
The perfect auxilery charger for your robot.
Could'nt find any spec's out there yet but I like the idea of charging your robot off butane.
It says it can charge the Iphone 10 - 14 times for $2 worth of butane. The Iphone is 3.7v 1420mah so that sounds pretty good.
It may even be hackable to take a bigger butane tank, however electricity and flamable gasses don't really go down well.....
It's just a regular passive fuel cell , you can use hydrogen , propane , methane ect.