A French company recently rolled out its EP Tender, a trailer with a 600cc engine that can recharge your battery while driving. The EP Tender monitors the car's battery pack and when the power gets low enough, the engine kicks in to recharge while driving continues. The trailer can deliver up to 22 kilowatts of power, and the maker is looking at a 900cc engine that could produce 35 kW, too.
Right now, the Tender is still in the prototype testing phase, with additional usage envisioned for an electric motorcycle or a plug-in hybrid car. Another interesting side note is that the 600cc engine comes from the Tata Nano.
As Autoblog Green has reported, there are a few EV trailers out there. Germany's Ministry of Economics and Technology helped fund the first "ebuggy" prototype. Satellite navigation technologies in the Stuttgart-based ebuggy were impressive enough for it to win the European Satellite Navigation Competitoion 2012 for the Hessen region. Electric Motors and Vehicles Company (EMAV) has tested out another e-trailer powered with its Power Regeneration Unit (PRU). EMAV described the PRU as a "self-propelled electric vehicle power regernatiuon system that substantially extends the range of any electric vehicle."
Of note: Today's mainstream, modern EVs aren't actually designed to drive while charging. This charger trailer solution would seem to require either a hack or DIY conversion, then. Of course, wireless charging is waiting in the wings, as well.