That's as silly as expecting gas stations to pump the gas out of the ground and refine it right then and there as each customer pulls up. No, you just have the fuel ready to use in a big storage tank that you refill in bulk.
Similarly, in this scenario you'd just have enough spare battery packs lying around to service all your potential customers, and charge the rest when it's not so busy. It's not like there's going to be a steady line of customers every 3 minutes waiting to swap. Charge your battery banks during the day with wind and solar, and keep doing it in the dead of night when electricity prices are lower, and have them ready when customers pull in the next day.