So both Rivian and Tesla have or say they are going to have range extenders for their trucks, but in both cases even if they are removable and rentable they are huge as trucks are huge. In teslas case it seems to be a permanent change though.
What about commuter cars though?
One thing we really need is cheaper in city commuters and those don't need a long range. That brings costs down and gets more people into EVs, but those will get relegated to 2nd cars in many cases.
If those commuter cars could go to a shop and get an extender added in the trunk though that would make them much more capable of longer trips as well while keeping costs down.
If the battery rental is similar or less to renting a car for the same period then people would opt to use their own car for the longer trip and all the personal comforts that provides.
The batteries would be much smaller as well for a smaller vehicle.
All the extra hardware required to make that work and the time and cost of installing and removing the extra battery pack means it'll probably never cost less than just hiring a longer range vehicle for the weekend. People who really need the range wouldn't bother, they'd just buy a longer range vehicle in the first place. Or one with the extra battery pre-installed and never remove it.
Not to mention that battery prices are continuing to plummet - we're quickly approaching the point where cars are just coming with as many batteries as they can easily fit. Short range EVs are likely not going to be a thing in a few more years when it doesn't cost significantly more to make a longer range EV.
Lithium ion batteries are huge though, one that would fit in a trunk wouldn't add very much range. Maybe 60 miles at most. And then you've lost all your trunk space, which is usually pretty needed on car trips to begin with. Far more important is reliable and plentiful fast chargers along major highway routes. Even an EV with a modest range can handle long car trips if there are fast chargers available and in the right locations.
They would also somehow have to plug into a temperature control system which would be very tricky and involve connecting hydraulics. Good EVs have heating/cooling systems dedicated to their batteries. You would need temp control with the battery which adds a whole level of fiddliness.
Tesla has been saying for years that they are going to make swappable batteries an option, but it just doesn't make any sense compared to just having more and better fast chargers available.
I'm not so sure they'd be as huge as you might think though.
It's huge on the trucks because the trucks are huge. The CT extender is almost as big as a standard range vehicles battery.
The extenders size would be proportional to the size of the vehicle and i think battery size doesn't grow linearly so smaller could be substantially smaller, but i might be wrong on that part.
Tesla ditched swapping batteries as a drive up service ages ago and hasn't talked about doing that since. This would be different as it'd be a service appointment, not a park it for 5 min and swap it out.
Both Rivian and Tesla supposedly solved the cooling problem on their trucks,so it's really just how much does that increase the size, and would the option to even do it make it prohibitively expensive on an inexpensive car.