All propane tanks around here have a "do not refill past this date" stamped into them.
There are 2 ways of "refilling" a tank -- the first is to swap your tank for another tank that's filled; the other is to go to a place that can refill your tank.
99% of the time when I do the tank swap they try to give me one with an expiration date that's coming up in the next 6 months. I almost never am offered one with a very long expiration date.
Do you really think that, in a situation where you own your car and battery, you'll reliably get a battery that's as good or better than the one that's in your car? Keep in mind the value of the vehicle is 40-80% tied up in _only_ the battery.
Battery swaps aren't a realistic plan unless someone else owns the battery, and then you'll get all sorts of other nonsense in the ecosystem.