Personally, I spend a lot of time on the road. I work in the cash register biz, covering 3 states, so there are a lot of long drives. Here's what gets me thru the day:
First off, I've got a DC converter for my truck that has 2 USB ports. (WalMart, I think it was around $20) One has my USB passthrough, the other is either charging my PCC or my cell phone. Using the passthrough when "in transit" really, REALLY helps a lot.
I usually carry at least 3 batts, one I'm using, another in my shirt pocket, and the third in my PCC. 2 could get you by, tho. The trick here is that you keep the PCC on that charger in the vehicle as much as you can, and swap out batts often... you don't have to wait until a batty is "dead" before charging it (L/I batteries don't have a "memory" like the old Ni-Cads). If I stop for an hour somewhere, I take the batt I was using in the store and swap it with the PCC as soon as I leave.
Oh, and it helps that I vape 20-25 mg juice

Stronger puffs = fewer puffs. At that strength, a 510 batty lasts me usually around 1.5-2 hours... so with even just 2 batteries + PCC, and using my passthrough... even if I crash "Bob the Truck" and/or kill the truck battery, and even with one of my 2 batteries dead, I've got 1+4 charges, or 7 1/2 hours minimum to get to a USB port before I have to really worry. That USB adapter turns "Bob" into a charging station for my PCC and a source for my passthrough at the same time. I usually end up back at home after a 14-16 hour day with everything fully charged
Just personal experience... hope that helps. My bosses wife just got an Ego, and she smokes about half a pack of Marlboro reds a day. I fully expect one of those batts to last her a day and a half, maybe more. Ego might be a better option, especially if you spend a LOT of time away from your vehicle.