For a while I've been running Endomondo to monitor my rides. The problem has been that it takes so much energy to keep the screen, gps, and audio running the battery lasts 3 hours at best. Then I was looking through my junk box and found 2 850mah LiPo batteries I used on r/c helis when I had them, a small car usb adapter, and a data/charging cable. Took the adapter and soldered a deans plug to the lighter connector, wrapped that in tape and then took the charging cable and wrapped that around the whole assembly for neatness. More tape to secure it. Took the batteries and soldered them in parellel to give me 1700mah at 12.4v (full charge). Attached the opposite deans to the batteries. Took the whole assembly and put in the pocket of my small top bag with the smart phone installed on the top under the window.

Tested it all out.. and I get a constant charge going to the phone with the storage batteries dropping by 500ma or so in 1 1/2 hours. This means that even applying the 80% rule on Lipo I still get an extra 3-4 hours of mp3/gps/full screen on the phone and have the original battery (1500mah 4.1v) totally charged - which is worth another 2-3 hours run time on its own. If I were to change out the storage batteries to 2 2100mah batteries - there should be room I can see an all day run time as no problem. I also get to use the phone as .. a phone if I need to.