Cox Sportavia Batteries?
Pierre,
Yes indeed, it is a Cox Sportavia. I have one that is very similar to it, except mine is powered by a Cox QRC .049 glow engine. I bought mine on eBay a year or so ago purely for the nostalgia rush... I taught myself to fly RC with a Sportavia in 1980. When I saw one on eBay, I couldn't resist.
Incidentally, the plane is a 1/10th scale model of the RF-5B Sperber (sparrow) motorglider, built by Sportavia in the 70's. With respect to scale accuracy, the model is actually quite close to the original, with the exception of the wing planform... the prototype wing had quite a bit more taper.
Sorry, I can't help you with the battery question, but I'd bet someone over in the Electic forum would know exactly what you need.
Gary