LiIon 12V battery. Schematic and model of housing.

Sometimes you go to the flea market and spend some amount of money on some scrap. After you get back home you start thinking of what to do with all these things you have bought. The same happened to me: I have bought batteries for some unknown portable device. There were four 3.7V 3100 mAh cells. Unfortunately one of them was completely dead. Fortunately, three cells is enough for building the pretty powerful yet small rechargeable battery providing 12 volts.