You've probably seen the battle going on between Elon Musk of Tesla and the New York Times over the strongly negative review the NYT made of a long road trip in a Model S. The reviewer ran out of charge and had a very rough trip with lots of range anxiety. The data logs published by Tesla show he made a number of mistakes, didn't follow some instructions on speed and heat and could have pulled off the road trip if he had done it right.

Both sides are right, though. Tesla has made it possible to do the road trip in the Model S, but they haven't made it easy. It's possible to screw it up, and instructions to go slow and keep the heater low are not ones people want to take. 40 minute supercharges are still pretty long, they are not good for the battery and it's hard to believe that they scale since they take so long. While Better Place's battery swap provides a tolerable 5 minute swap, it also presents scaling issues -- you don't want to show up at a station that does 5 minute swaps and be 6th in line.

The Tesla Model S is an amazing car, hugely fun to drive and zippy, cool on the inside and high tech. Driving around a large metro area can be done without range anxiety, which is great. I would love to have one -- I just love $85K more. But a long road trip, particularly on a cold day? There are better choices. (And in the Robocar world when you can get cars delivered, you will get the right car for your trip delivered.)

Electric cars have a number of worthwhile advantages, and as battery technologies improve they will come into their own. But let's consider the economics of a long range electric. The Tesla Model S comes in 3 levels, and there is a $20,000 difference between the 40khw 160 mile version and the 85kwh 300 mile version. It's a $35K difference if you want the performance package.

The unspoken secret of electric cars is that while you can get the electricity for the model S for just 3 cents/mile at national grid average prices (compared to 12 cents/mile for gasoline in a 30mpg car and 7 cents/mile in a 50mpg hybrid) this is not the full story. You also pay, as you can see, a lot for the battery. There are conflicting reports on how long a battery pack will last you (and that in turn varies on how you use and abuse it.) If we take the battery lifetime at 150,000 miles -- which is more than most give it -- you can see that the extra 45kwh add-on in the Tesla for $20K is costing about 13 cents/mile. The whole battery pack in the 85kwh Telsa, at $42K estimated, is costing a whopping 28 cents/mile for depreciation.

Here's a yikes. At a 5% interest rate, you're paying $2,100 a year in interest on the $42,000 Tesla S 85kwh battery pack. If you go the national average 12,000 miles/year that's 17.5 cents/mile just for interest on the battery. Not counting vehicle or battery life. Add interest, depreciation and electricity and it's just under 40 cents/mile -- similar to a 10mpg Hummer H2. (I bet most Tesla Model S owners do more than that average 12K miles/year, which improves this.)

In other words, the cost of the battery dwarfs the cost of the electricity, and sadly it also dwarfs the cost of gasoline in most cars. With an electric car, you are effectively paying most of your fuel costs up front. You may also be adding home charging station costs. This helps us learn how much cheaper we must make the battery.

It's a bit easier in the Nissan LEAF, whose 24kwh battery pack is estimated to cost about $15,000. Here if it lasts 150K miles we have 10 cents/mile plus the electricity, for a total cost of 13 cents/mile which competes with gasoline cars, though adding interest it's 19 cents/mile -- which does not compete. As a plus, the electric car is simpler and should need less maintenance. (Of course with as much as $10,000 in tax credits, that battery pack can be a reasonable purchase, at taxpayer expense.) A typical gasoline car spends about 5 cents/mile on non-tire maintenance.

This math changes a lot with the actual battery life, and many people are estimating that battery lives will be worse than 150K miles and others are estimating more. The larger your battery pack and the less often you fully use it, the longer it lasts. The average car doesn't last a lot more than 150k miles, at least outside of California.

The problem with range anxiety becomes more clear. The 85kwh Tesla lets you do your daily driving around your city with no range anxiety. That's great. But to get that you buy a huge battery pack. But you only use that extra range rarely, though you spend a lot to get it. Most trips can actually be handled by the 70 mile range Leaf, though with some anxiety. You only need all that extra battery for those occasional longer trips. You spend a lot of extra money just to use the range from time to time.

This is part of the justification of the plug-in hybrid, though the Volt and Prius Plug-in are not selling super well. With a PHEV, you really make full use of that battery. Almost every trip exploits all its range, then the gasoline engine kicks in on those medium and longer trips. The cost however, turns out to be also high -- you have to build a full electric car and include a capable (but smaller) gasoline engine. Most PHEVs have very short electric range -- like just 10 miles -- and they also pay a lot (a whole gasoline engine system) to do those medium and longer trips.

To top it all off, when you use batteries, you carry around all their weight if they are empty or full. If you want that extra long range, you pay for it with a lot of weight you carry all the time, not just on those few trips which are longer.

We now see some of gasoline's huge advantages. It has a really great energy density, can be refueled quickly, and as you burn it up its weight goes away too -- though of course this is also one of gasoline's big problems; the weight goes into the atmosphere as emissions. This discussion is on the cost of the system to the owner (or taxpayer.) Electric cars can be greener (if the local grid is not coal-based) and particularly good if they are small and light. The Tesla, Leaf and others are not particularly small and light so far.

McKinsey forecasts L-Ion batteries will be $200/kwh by 2020, making a 40kwh pack cost just $8,000. Put into a small, light car would produce a cost-effective car with no metro-area range anxiety when the decade ends.

Can a trailer be the answer?

This makes me more enamoured with the concept of a temporary "range extending trailer." I discussed versions of the trailer concept in an earlier post. There are a few variations of such a plan. Some have the trailer just contain extra batteries and a thick power connection -- here you pay to rent the extra range only when you need it. This is mainly of appeal to those who want to be pure electric. Other options include a modest generator in the trailer. I think the physically simplest plan is a "pusher trailer" that just has a conventional motor that gives you a push from behind. You can even recharge your batteries this way by engaging the car's regeneration mode, with some cost in efficiency over a direct link, but you don't need to do this if you use something like this well. Such pushing is unstable and needs computer assist on the steering. It would only be done on long stretches of flat highway, and be halted in the event of steering or braking. Alternately, the trailer could be attached at two points on the rear, able to bounce up and down but not articulated laterally.

It could also not be a trailer but something designed to bolt into a special mount (if it's under a few hundred pounds.) Such designs are more likely to be proprietary -- the advantage of trailers is it's easier to develop a standard that many vehicles can use, and allow innovation in the trailer design.

Trailers are not speculation. In addition to the company cited above, EV pioneer Alan Cocconi's firm AC Propulsion actually built some for their small EVs.

Unlike Tesla's fancy rapid charge stations or Better Place's even fancier battery swap stations, trailers could be located anywhere there is some spare parking space. Gas stations. Rental depots. They could be self-service. They could be robotized with independent steering, allowing you to steer naturally and back up more easily. They could automate a lot of the docking.

Instead of pulling over to one of Tesla's rare rapid-charge stations and waiting 30-40 minutes (if there is not a line) your car could automatically plot a short detour to a rental trailer station and you could get hooked up in a couple of minutes, perhaps faster than a gasoline fill-up. Rather than limping up to a trailer with your battery low, your car's computer would plan the trip for maximum efficiency, planning to use electric power for all urban stop-and-go activity and only use the trailer's engine in its most efficient cycle on the highway, with electric assist where needed. This highly efficient use requires more advance planning than a gasoline trip, but you could also be spontaneous at the cost of burning a bit more fossil fuel or getting a more powerful trailer on those trips.

You don't want to be fetching (and returning) such a trailer every day. You would size your electric battery perhaps to need the trailer just a few times a month. However, if there were trailer stations along your route to the highway, or in easy-on/easy-off highway stations, you might find it quite acceptable to use one more often, especially if it can attach itself to your special receiver. The trailer, to be cheap, would be quite simple -- a stock car engine, drivetrain and wheels, with perhaps even no transmission. Its fanciest features would be computer control, fancy steering and perhaps a liftable 3rd wheel for docking operations and assist when reversing. It could be quite compact, and with a smaller car fit into parking spaces by moving up the tow bar -- which is just a metal bar, no electronics. And like the original proposer suggests, some models could come with extra trunk space.

You could make the trailer just extra batteries. This has a number of advantages and disadvantages:

• It's a 100% electric solution
• You would need a high-power electrical connection between vehicles. But none of the instability of pushing.
• The only motors needed in the trailer would be steering motors (for easy reverse) and possibly a lower-power motor for docking operations.
• It's more expensive and possibly worth stealing. There's $15K of batteries in it.
• You need charging at all the rental locations (though possibly just L1) and each unit has to go out of service for recharge.
• You still probably only get another 100 miles range out of the unit, but you can stop and swap easily.

You could also own a trailer and hook it up at home when you knew you were taking a longer trip. Owning the battery trailer is not that useful an idea, it only saves you some weight: Just get the bigger battery car.