The solution is to either turn off Wi-Fi sync, or use an external battery pack that has an on-off switch so that you can control when it is actually sending current to the phone.
[kirkmc adds: I guess this makes sense. Apple says that Wi-Fi sync occurs when "The iOS device is plugged in to power." But this should only occur if two other conditions are met: "iTunes is open on the computer," and "The iOS device and the computer are on the same Wi-Fi network." So I would think that if you're not on the same network as your Mac, then nothing should happen. Perhaps the phone is constantly searching for that Mac?]

