And in other news, it seems it's a perpetual motion machine in London:

"Transport for London (TfL) said the bus recharged its battery
using energy from its brakes, but on long journeys had to run on
diesel."

http://www.bbc.co.uk/news/uk-england-london-16244634

But I'm glad I diagnosed the problem correctly (there's some value in an
engineering degree after all!):

"However when the bus undertakes long, non-stopping motorway
journeys, such as its journey to Bedfordshire this morning, it
can lose charge and the driver is briefed to pull to the side of
the road to allow the battery to recharge."
--
Roland Perry