http://www.moltenrock.co.uk/These? Seem to work. Batteries very small, and slow but all electric off road devices have the same issues.
What I really object to is that everything from a tankchair to this, calls itself a "wheelchair". Its not. Its an outdoor buggy/car. It cannot work indoors or as a wheelchair and so why struggle with battery power offroad where its really up against it?
Battery power doesent = wheelchair!
Have you looked at quiet 4 stroke quad bikes or off road buggies? Much better performance, and range, and cheaper. Many conversions around.