I find my phone uses about 25% battery per flight. That gives me 4 flights at a push, 3 if we're being realistic. Yet, I see people ordering 4+ batteries. How do you intend to solve this problem and get the most consecutive flights?
The battery on your phone or tablet will be subject to change from device to device... My device will fly 5 or 6 times... and I have a usb power pack to charge the device. I just ordered a ipad air 2 and hope the battery life is better. so I will have 2 devices also.
But 3 flight batteries are said to be the sweet spot where you can charge one, fly one and have one ready/on charger...
I will add to this and say it really depends on what you do with your Phantom. Most people that have 4+ batteries will go on an adventure and dont plan on going home within 2 hours. A tablet/phone can easily be charged in a car as well. It all depends on what you want to do with your Phantom. I will tell you that I worked at Apple and we advised customers to not charge their iOS device in a car as the voltage is different and can damage your device. To me I dont care to charge my phone in the car. But, I won't risk charging my P3 battery in a car. If the battery does get damage your looking at $1,300 falling from the sky. Where as if my phone battery gets damaged from charging in the car. I can easily just replace the battery
Before I had extra batteries, I bought a 12v 10ah battery from a hardware store and a power inverter to give me a couple outlets so that I could charge in the field if I had no spare power. Running out of power is always a scary or annoying thought.I usually plug my tablet in to charge between flights. I think it's a habit formed after the WiFi extender for my P2V ran out of juice mid-flight once. Flying back without the OSD was more nerve-wracking than it probably should have been and I don't want to take a chance again.
I usually have a source of A/C power near where I often fly. Otherwise, I have an automotive "jump pack" and a couple of USB power packs handy.