"Tim Lamb" wrote in message
...

Generator efficiency will presumable vary with load

It does, but not much. Microturbines rev at 30,000 rpm. High speed gennies
are smaller and more efficient. Microturbines are looking good for range
extenders - new Jag to have one. The size of a large can of beans.

and I wonder if the overall performance tails off as the battery
approaches full charge?

The charge will be used by the driving motor and depending on the management
software, either preserve the battery, or use both or either.