Would a rental battery pack be a better choice than a super-long-range electric car?

When you buy an electric car, you can often choose among various battery sizes. The larger your battery, the more range you have -- and you may get some extra performance -- and the longer your pack lasts, but the extra capacity is very expensive and adds weight to the car. The truth is, most people only need the extra capacity of a long range car when doing road trips. A modest 150 to 200 mile range car is sufficient for driving around a town, depending on the town.

A good answer would be to design the car to allow an additional rental battery pack. This pack could go in a special mounting rack in the existing battery rack, or it could have a place to mount in the rear or front trunk. It would need wiring and cooling tubes.

A 15kwh pack -- good for about 60 miles of extra range -- would weigh about 80 kilograms (175lb) so this has to be a deliberately designed system. Connection might need an authorized service team if you want to be sure it's right. The pack would cost around $3K at today's prices. But you could rent it for a more reasonable rate only when you want to do a road trip. If it's $100 to rent it takes 30 road trips before renting cost you any money, and the value of not carrying that weight the rest of time.

It turns out that getting closer to 300 miles of range does make a difference on long road trips, especially in the west. It opens up more places you can go, more ability to do diversionary side trips and the ability to explore more beyond the established charging networks.

For the shorter range cars -- the ones with 70-150 miles of range, this is also possible, but the pack would be quite large and heavy, and would take more significant space in the trunk, if that's where it goes. With that size and weight, a service center install/remove are likely, and the rental fee would be higher. However, if you consider how much cheaper these short range cars are, it's likely that rental fees would still be quite a savings compared to buying a longer range car as long as road trips only take place rarely. In addition, the car without the pack will be much lighter, more efficient and have extra space.

Even people with long range (300 mile) cars might even consider this option. Adding 60-100 miles of range would offer tremendous flexibility on road trips, and access to roads that simply are not practical today, such as U.S. 50 across Nevada. There, the distance between Tesla superchargers, at 550 miles, is too large even with an extra pack, but a large pack and a stop of a few hours at an RV park could make it doable.


The main option that people have explored in this direction has been generators. Most plug-in hybrid cars are "parallel" hybrids where the gasoline motor drives the wheels directly. The BMW i3 REX is an exception -- it has a small generator with a motorcycle engine built in, and a 2 gallon gasoline tank. The generator doesn't drive the wheels, but it provides enough electricity to barely drive. In the USA, the car is programmed to only use the generator once the battery is low, leading to anemic driving and very poor hill climbing. For the driver, it's much nicer to offer the European option, where you can set the generator to run at the start of a long trip to keep the battery topped up, and then switch it off when you are in range of your destination.

Towing a trailer makes driving a pain, so this solution isn't too great.

In the era of all cars being short range, I wrote about towable trailer generators and packs which had been proposed. Towing a trailer just for that is a burden on road trips, as it makes backing up and parking a pain, but they are much easier to rent.

One could design a car so that a temporary generator/tank unit could be inserted in it, again probably at a service station. The simplest approach might be to make the generator live on a trailer hitch style mounting, making it easy to deal with heat and exhaust. A 20kw generator could support sustained 55mph. The BMW's, at 34hp/25kw, is slightly larger. It could also be possible to go much smaller. If you consider a 10kw generator, which might sustain only 30mph, you could turn it on at the start of a trip with a 260 mile battery, and 4 hours later it would have added 120 miles. That's a nice boost for non-stop driving, but real driving usually contains many stops where the generator could run. The generator could also fully recharge the battery at night. This might not strike you as a very green option, but it's actually lower in fuel consumption than almost any gasoline car. Combined with the ability to use DC fast superchargers, and occasional overnight charges at hotels, it could produce a very low emissions trip from a vehicle with the "go anywhere" flexibility of gasoline powered cars. A 10kw generator could be small, cheap and light -- its main issue as that smaller generators tend to be less efficient.

As a simple calculation, if you specify a maximum daily range of 500 miles, and your battery has 250 miles, and your travel day is 12 hours, then you need about 70 kwh. If you did it perfectly, 6kw could do the job, though when you are stopped or otherwise putting power into the battery rather than the wheel motor, you suffer battery losses, and so might want 7kw, or around 12hp mechanical energy.

In theory, a Stirling engine generator could be a highly efficient and quiet choice, but in practice these are not made in mass volume anywhere. Stirlings are no good for driving cars because they take time to fire up and can't accelerate, but they're great for providing constant power like this. They can use almost any fuel that burns, too.


It should be noted that regulations might stand in the way of a generator pack, because they would convert the vehicle from a pure electric to a plug-in hybrid. In some cases that might invalidate your ability to use things like carpool lanes, though in practice, plug-in hybrids are still treated like BEVs but that won't last forever. As long as the generator were truly a temporary add-on, one would hope the regulations would be kind as this is still much greener than a full-time gasoline car or hybrid.

If necessary, software in the car could prohibit the number of days per year the generator could be attached.


My issue with rental packs/generators/etc. is demand tends to be strong on weekends and especially holidays, but very low otherwise. This is bad for rental economics.

My pet solution is dynamic charging. Wire up one mile out of every ten on interstates and major state highways. Honda demo'd a cool system with wires in a standard guardrail. Roughly the cost of overhead wires you see in some European trials, but trucks, buses and cars can all use it. 100 mile BEVs could go cross country non-stop.

Trucks are the biggest winner. Musk's 500 mile Semi will cost a lot more than their 180k announced price, but a 75 mile truck would cost LESS than the diesel equivalent. It would also weigh a lot less than the Tesla Semi, so you don't have to sacrifice revenue-generating payload.

You need high C rate, long cycle life batteries for this strategy. Something like LTO or maybe LFP. You also need a federal government with a clue.

Wiring highways and guardrails is infrastructure. That's always your last choice if you can avoid it. Overhead wires and guardrails are hugely expensive and greatly add to the cost of maintaining the roads, and you can only go where they build them -- ten years from now when they are finished. Trucks are another story because they go along specially selected routes, unlike private cars.

I would not expect there would be a one day rental of these packs, probably minimum of a week, more commonly 2 weeks.

Roads are infrastructure, too, and much more expensive than wires. We pay for roads because it's cheaper in aggregate than making every vehicle robust enough to travel at speed on unpaved paths and ford creeks and rivers.

Anyway, this wouldn't be a lot of infrastructure. 1 mile out of 10 on 50k miles of interstates and a similar amount of major state highways. EVs still have their battery for "last mile", i.e. last 75 miles. They just don't need to buy and lug around a half ton of excess battery to have 300+ miles of range. Unless they want to, of course.

Not that I think there's a chance in a million we'd ever build this. By the same token, try renting one of your extend-a-packs Thanksgiving week. Or Christmas, Memorial Day, etc. The only way to meet holiday demand is to have excess packs sitting around the 48 weeks other weeks of the year. If you've ever rented low-utilization equipment you know the expense is extreme. More than buying used and re-selling in some cases.

Are you going to Tesla's big self-driving bash on Monday? You probably know too much about self-driving to be on their list, but it'd be great to hear your thoughts.

The purpose of range extending packs is to go off the beaten track. Tesla has already shown they can put in a supercharger network to cover the freeway network. You won't find any new infrastructure out on these rural roads.

As for rental on Thanksgiving, remember that the alternative is that everybody who wants to drive at Thanksgiving buys the pack up front and carries it at all time. Not everybody road trips that weekend, many stay in the homes people are driving to, so it's far less battery to be built. However, if there is a shortage, well, that's where gasoline cars come in. Electric car buyers don't want to drive gasoline cars, but they will drive them rather than not go.

You could even trade out cars in less time than waiting to recharge at a supercharger.

For $100, you could rent a whole car, for multiple days.

I'm starting to think that the way of the future might be to have a very low range personal EV (or even a cheap gas guzzler) and then rent out longer range vehicles as needed. At least for people who are concerned about reducing waste.

Indeed, a rental car is often the better choice. However, when I suggest this to other electric car owners, they express that they love their electric car so much they will put up with its (shrinking) downsides in order to have their electric car on their trip. It is not yet possible to rent a long range electric car economically. There should eventually be a market for that. (Though I think car rental companies lose money on cars rented for road trips, and make it up on all those business rentals where you only put 20 miles on the car during the rental.)

The option to rent batteries is a good idea. People who don't like it don't have to rent them. I would like to see electric cars designed to accept a half dozen or so battery packs. That would allow people to choose what they need. Ideally, whatever battery packs are left out would provide additional cargo space. The next step beyond that would be a range extender similar to the BMW design, except that it could take the place of several battery packs. Then you could configure your car for short, medium or long trips as may be needed. Another possible advantage is that the car could be told to favor the oldest pack, so that it would wear out first. It would be an economic convenience to have battery replacement costs spread out over several years. The easier it is to plug in the packs, the more likely a rental market would flourish.

It would be great if a standard was established for these 15kwh packs, enhancing interchangeability. It would then make it plausible to "add" capacity to my vehicle for long trips, but then also use it as storage for my habitat on most other days, aka Power Wall.

This could also extend to a Town / Village / Hamlet Urban model, with a core Power Wall bank that serves as a reserve for N habitats, but can also broker these packs out for long trips in vehicles that accept these standard packs.

The problem with standardization is it interferes with innovation. Each company has their own innovations in how to wire the packs for charging and discharge, and how to cool them, including feeding coolant through the packs. So you don't want those standardized.

Though you could allow standardization if you support the idea that the rental pack is less capable than native packs. Ie. because it doesn't have the cooling of the native pack, it can't be charged as fast, or you can't draw energy from it as fast. The latter is probably not an issue, but the former would be, since Tesla owners would not want a pack they could not supercharge.

The next question is standardization of mounting hardware. That's possible but mainly means it has to be in the trunk and you can't have a special space under it etc. You need to make sure that there is no fire risk from the rental pack.

Agreed, if this is an area where manufacturers are indeed differentiating themselves, then market forces prevent standardization/convergence. With that said, it would appear that the objective functions are common across manufacturers (they all want more efficient cooling for example.)

In terms of range / kwh, has any one manufacturer maintained a competitive edge as of 2019? There seems to be an opportunity here to seek a "container" standard for these packs, given that the core cells from Pana, CALT etc. are largely commoditized too, at least with the current chemistry.

I suspect this would have already happened if it weren't for form factor / packaging limitations. I have no evidence to support this, but I suspect manufacturers, especially the German ones, seem to really like creating bespoke stockings to stuff with battery, to optimize for range, even lower CofG and other factors.

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