I charge my T1/350/650 on the go with a PowerMonkey, charged from the mains, (3-ish hrs from zero) which holds it for around 6-8 months, and will still charge the readers fully then.
However, you can get a solar panel for it instead, thereby enabling you to charge the reader by solar energy....with an extra step in t' middle.
But it amounts to the same thing, dunnit ........
I know there's another charger like the Monkey, the name of which I can't recall at the moment, but it isn't as well designed - but I think the panel they sell is a bit quicker (and bigger) than Monkey's own. Presuming it fits, which it must with all the tips they include with everything .............
Incidentally, the solar-powered calculator's been with us for many many years - I think the first one I had was a dual power source one around 20 years ago.