However, the really practical helpers are on already on BlackBerry devices with a required output of 12 volts, tested on a BlackBerry Torch 9800 and a Blackberry Bold 9790. With smart phones from other manufacturers with the same power requirements the device is also likely to have its problems.
By the way, the consistently lit LED in the inserted condition robs unnecessary power so that the adapter in energy-conscious people and those who do not trust their car battery completely, should be unplugged when not in use.
All in all you could from the idea of a USB adapter equal to consign two corresponding ports, can do more. Unfortunately, the mumbi USB adapter is too weak for this purpose on the chest. I recommend rather spend a few more euros and to acquire a of the numerous competitors, who does his job better.