>Have you considered a laptop with two battery bays?  Or a separate battery?

This is probably the way to work. My Dell can replace the CD drive with a 
removable battery or another hard drive.

The CD drive is USB, and I believe the hard drive caddy also does a USB 
interface to a standard 2.5" drive, but there must be a separate set of 
contacts to use a second battery in there. OK it won't have the capacity of 
the primary battery, but anything that gets you extra life has to be good.

The second battery slot appears on the battery management panel that comes 
up to show charge status, even with no battery in there. 

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