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Re:PocketBeagle – A $25 open-source USB-key-fob computer by pragmaddux on Thursday September 21, 2017 @12:51PM (#15305301)
I don't think the battery life was there, but people used to be really into turning Motorola Lapdocks, originally intended for their Atrix phones, into Raspberry Pi machines.

A Chromebook might be able to do what you require for 9-10 hours, with a little tweaking.

Re:PocketBeagle – A $25 open-source USB-key-fob computer by ac29 on Thursday September 21, 2017 @10:42PM (#15309069)
I have a Baytrail based Chromebook running GalliumOS (Ubuntu + XFCE) that is good for about 10 hours of usage (in an 11 inch form factor, a larger laptop could accommodate a larger battery). Apollo Lake is rolling out now and should be even better (~30% performance increase should lead to more time idling with the same workload). I'd expect firmware/software support for running generic Linux on it in 6 months or so.

Probably wont hit 20 hours, but I could imagine 12-14, which is pretty respectable.