I saw this article yesterday (http://www.engadget.com/2014/01/05/cobra-jumpack/) and thought it would be really funny if people on bikes had them on hand to help stranded motorists, especially if it could be connected to a hub dynamo to recharge it.
I believe it’s LiPo (LiFe) based battery and as a result it’s extremely sensitive to temperature. 7,500mAh is just 7.5Ah. It’s 7.5 amp per hour at around 13 v. Or around 7.5*13*3600=351,000 Jouls. And it charged in 3 hours. So it’s going to be around 14V and 2.5A. We cannot charge it dynohub — too much effort. But it looks like it is 100C or even 200C batteries (could be dischared with 100 or 200 times charging amperage without causing damages to the battery). The best I saw is http://www.hobbyking.com/hobbyking/store/__19134__Turnigy_nano_tech_3300mah_4S_65_130C_Lipo_Pack.html — with 65C constant and 130C peak. And it’s 14.8v and 3,300mAh. So 3 of them connected in parallel should provide 14.8v peak of around 230A and 9,900mah with price of $151.