That what interested me. If you can go for 20 miles on battery you would have plenty of time to get the fuel cell up to temperature. The FC could charge even when the car is off if the batter is below some threshold and not plugged in or you could tell it how long you intend to drive before your next plug in.
UK-based Ceres Power Holdings, a spin-out from Imperial College, is leading a consortium that includes Nissan Motor Manufacturing (UK) Ltd and M-Solv to develop a compact, on-board solid oxide fuel cell (SOFC) stack as a range extender for an electric light commercial vehicle (van). (Earlier post.)
£772,000 (US$1 million) in funding for the work comes from Innovate UK and The Office for Low Emission Vehicles (OLEV); of that, £573,000 (US$755,000) is allocated to Ceres. The SOFC stack is based on Ceres Power’s unique SteelCell technology, which is able to work with a variety of high efficiency fuel types (including biofuels) applicable to the automotive sector.