My Nexus 4 can be about 2x to be charged to the External Battery to the end of its capacity approaches, in normal use, I invite the mobile phone but only in the evening on the train on when the battery is still almost empty and I but during the drive home still wants to play.
Although the fixed micro USB cable is not long but it is sufficient to connect the mobile phone, thereby the battery for its 5.000mAh is quite small and light, it can be then also hold together seamlessly to mobile phone.
For me, a perfect solution after I once found the train I got my USB / micro USB cable at the office can be ....