In reality they don’t have one big battery. Their big battery is a battery of batteries very similar in form to the ones you put on your toys. All those batteries need to work on unison, and need to have the most similar voltage, capacity and charge discharge cycles. If you could swap those individual cells you would create as many problems as you solve. And accessing them would be difficult because those packs are cramped between sandwiches of cooling layers. But hey, loading your car with batteries like bullets into an A10 plane ammo belt would be cool as F