Keeping all my batteries charged through 4 on track sessions during a single track day has been a real challenge, but one I've finally conquered. My strategy has been to make sure all the devices can carry enough charge to make it through a single day, and not to depend on live charging during use.
I have to keep the following devices adequately charged:
[-] The iPhone itself
[-] XGPS150
[-] 3 GoPro cameras
I use the Kiwi ODBII device, which powers itself off the car, so I can ignore that. The XGPS150 will last all day, but only if you make sure you turn it off when not in use. For a 20-25 minute session on track, I am usually getting everything setup about 10-15 minutes before hand (I like to be first in line, to avoid all those slow Corvettes at the start

, so for an individual session, things are usually powered on for close to 45 minutes. I have not had a problem with the XGPS150 lasting through a single day.
The GoPro's are a PITA, because a single battery charge will last through just over 2 sessions, so at first I bought a set of spare batteries that I would swap out mid day, but I went ahead and invested in the external battery pacs, so each camera has TWO batteries, and lasts all day. I still have a set of spares handy, too.
The iPhone, however, will require frequent recharging (at least my iPhone 4 does), and I avoid recharging that by using a case that includes an extra battery (see
http://www.mophie.com/ for their products -- highly recommended).
This allowed me to FORGET about recharging anything during a single track day, so I can focus on the car and my driving. It also allows me to use everything in the car wirelessly, and not deal with charging cables.
What's really fun is getting everything RE-charged during the evening between 2-day events. Basically, as soon as I hit the hotel room, I get as many things plugged in as possible, and then keep swapping devices out until everything's full. I'm still looking for an economical way to charge a LOT of devices at once without a next of USB cables.