I was looking at the Mophie Juice Pack for the iPhone 4 and thought it has a very attractive design. I am not familiar with how juice packs work. So please indulge me. Will the phone first use the phone battery and then switch to the case battery after it runs out? Is there a way to toggle it on and off so that I only use the case battery when needed? Also I put a juice pack case on my iPhone, will it affect the iPhone battery's long term life in any way by always using the battery from the pack and not discharging the iPhone battery at all unless I go really long without charging?