This is my 4th drone and my opinion/experiences.
I have the EVO I and have 4 batteries...
On this drone, I have never opened the box for two of them.
I used to travel a lot and liked to take my drones when I went places because, well you never know!
I have car chargers for all and very seldom use them.
The vehicle has to be on and preferably be running for a good charge.
On my EVO I batteries it takes about 1 1/2- 2 hours to charge a battery with the car running on a trip,
too long IMO to worry about charging a battery.
I get about 20 Min+/- with my EVO and for most of the time feel this has been enough time to fly and see what I want to film.
Lets face it, with the Drones range, how much area can you film in one flight?
If you make a video to post, how much time will someone watch it before they get bored?
Average video on these forums is about 4 minutes.
NOW...
If you are using the drone for business, roof inspections, mapping, real estate, ect on a daily basis,
probably at least 4 to meet the drones time uses.
If you travel a lot and are not near a good power source, I would think three GOOD batteries would be enough.
If you fly from home and just go to the lake for a day, two would be sufficient.
I think a batteries would last about 1 1/2 years before it would have to be replaced for a long flight time, then would be the time get get more.
I would like to hear other's opinions to also help ME decide whe nit time to get my
EVO II