I chose for my HTC Desire HD also place a backup battery for the PowerTube because I not only can thus charge my smartphone again, but also (almost) all other mobile devices. The application is more than easy and self-explanatory. My completely empty battery from HTC is the PowerTube recharged in approximately 2.5 hours to 88% again, but the phone was there in standby mode, ie, WLAN, etc. was active during charging, so I think he it could have been fully charged when switched off. And maybe I have the PowerTube also only a few train times, I had only the second time fully loaded. The only thing that would be in need of improvement if MiPow would incorporate an indication of the current charge status of PowerTube, at the moment, unfortunately, you never know how full or empty the Powertube. But it is not so important, probably only costs in addition, valuable energy and therefore there is still full 5 stars from me!