That seems like a fantastic idea. This way, the car's internal battery can be smaller and lighter for day-to-day use (and therefore more efficient), and whenever longer range is needed you can just rent a fully-charged tow-a-battery, which you can swap out like propane tanks, or recharge yourself at home. These tow-a-batteries would work with any electric car with standard secure hook-ups.

Since these batteries would be rather expensive, I imagine that you would have to pay a refundable deposit to the tow-a-battery network (Shell, Esso, etc.) that covers the full cost of the tow-a-battery in order to rent one.

The biggest down-side is that a rear-end crash could be rather dangerous. I don't see a way around that without making the tow-a-battery a vehicle in itself.

by the time you've added enough protection (ie, extra mass and drag) to the towed battery, it seems like you would be pretty deep into diminishing returns. a large amount of the extra energy would be spent on moving the trailer itself.

Not really. Yes batteries add lots of weight. But electric motors have lots of torque and are known to climb hills easily. Like how Tesla’s forthcoming truck can tow up to 14000 pounds. I’m not doing the calculus, but I suspect the power characteristics of electric motors more than make up for a towable battery.

Edit-here’s an article about a company doing this

https://electrek.co/2020/02/19/french-startup-proposes-batte...

energy, not power. I'm sure the motors have enough torque to tow the battery, what I'm doubting is how efficiently that extra battery capacity gets converted to range.

as an aside, the cybertruck is (probably) pretty bad at towing. see this video for an interesting argument: https://www.youtube.com/watch?v=S4W-P5aCWJs

Very slow video. Wish it were transcripted.

It also allows for faster retrofitting of old vehicles with new battery tech.

Most apartment sized AC units top out at a few kW - that's the peak power, just maintaining the already cooled apartment is going to be consuming much less power.

A car cruising at highway speed is lconsuming something on order of 10x as much power. Given how much smaller cars are than apartments, it's an even greater difference.

So to answer your question: not much. Heating in cold weather is much worse on range, as it's not just the cabin but also the battery that needs to be maintained at an adequate operational temperature.

It does sound like a very intriguing idea, and one that I believe requires some research.

However, in practice I think this would be equivilent to towing a small tanker trailer full of chemicals or gasoline. It would require plaques indicating the contents, special insurance, and all kinds of BS at the RMV. Imaging if one decoupled on the highway and drove into oncoming traffic at 60mph? It would essentially be a rolling bomb. This happens quite often with camper trailers.

I think engineers would need to emphasize redundancy, failsafe measures, and automated attachment/detachment capability that does not require human interaction to safely accomplish. Then you will convince CA that it doesn't need a ream of paperwork to register and then the rest of the country will adopt it.

So here’s a company doing it for fleet vehicles in China. Granted, it’s not a guarantee it can get through the US regulatory system, but there are definitely more dangerous factors on the road.

Personally I see this as completely new tech from the ICE. That means it really ought to open up new usage and user experiences than the outgoing tech. And those changes should be perceived as better in some way to really take off.

Edit-forgot the link: https://electrek.co/2020/05/06/mobile-ev-chargers-and-vans-g...

Towing would annoying but dropping a block into the Cybertruck bed or the Y back cargo area would be pretty neat.

Most drivers aren't competent to tow a trailer, especially not at high speed or reversing.