I think what you are looking for is advanced battery management. You can give the operator of the vehicle the perception that the battery is near depleted and

I think what you are looking for is advanced battery management. You can give the operator of the vehicle the perception that the battery is near depleted and have say 10% reserved to get to the next charging station. The owner should probably have the option to specify what "reserved" is based on their location and driving habits or by default the car could learn this and set it dynamically. Modern batteries can be monitored for age/capability/c-rating near real time with the current battery management systems even down to individual cells. It is up to the car manufacturer to implement this and do something with the information. It is unlikely there would ever be extra physical batteries, as the battery pack would have to contain the same current capacity as the primary battery pack to meet the current rating of the motors. The battery pack makes up most of the weight of an EV.