Does your battery drain that quickly when not in use? I have a Braun 7 myself and often take it with me when travelling, and it easily does 6 shaves on a full battery, probably can do 10+.
I have an older Braun, and it may last three or four of my shaves. This means that it lasts too much to bother charging every time, but it drains too often to become a nuisance. If it only could work while plugged it would be perfect.