Because in the time it takes to physically swap some batteries and charge others, you could have just charged them all and that would be simpler and easier. On

Because in the time it takes to physically swap some batteries and charge others, you could have just charged them all and that would be simpler and easier. One of the things that's counterintuitive about batteries is that if a battery is twice as big it still takes the same time to charge, not twice as long. (That's assuming you can supply enough power to charge at the fastest rate the batteries can handle.)