Sag Rising

Notes to Myself

Electric Car Snake Oil?

Posted by Richard Cockrum on December 16, 2008

I dislike snake oil. It's entirely too common in our world. It is especially endemic in times of 'crisis', such as we are experiencing now.

One of the current crises of the day is energy, which for some reason is the focus of a crisis every few decades. One of the proposed cures for the energy crisis and climate change is to begin using electric vehicles.

I'm not against electric vehicles. I'd love to have an electric car for my day to day use. They are less polluting than internal combustion cars. They have fewer parts and less to go wrong. An electric motor can run almost forever if properly maintained. For example, I have a movie projector with an electric motor that is over 60 years old, and still working well.

I am against snake oil and misleading claims. A recent example is an article I read in Wired about a new multi-fuel car Mercedes Benz is designing, which they are calling the Concept BlueZero.

On the face of it, the BlueZero sounds pretty cool. The same basic model car can be equipped either as a purely electric vehicle that has to be plugged into an external source to charge the batteries, an electric vehicle with a one liter liquid fuel motor as a charger, or as a fuel cell powered electric vehicle. I want to talk about the electric model.

In it's purely electric configuration the vehicle is to contain liquid cooled lithium-ion batteries with a capacity of 35KWH. It would have a range of around 240 miles according to the press release or 125 miles according to the Wired article, and a 70KW motor (approximately equivalent to 94HP). An efficient small car can easily get 40MPG. In energy terms, gasolene has around 125,000 BTU, which equates to around 36.6 KW.

Most internal combustion engines have an energy efficiency of around 20%. To be generous, assume the car getting 40MPG has an efficiency of 25%. That means it is using around 9.1KW of the energy in a gallon of gasolene to go 40 miles.

Compared to that, the BlueZero going 125 miles on 35KWH of electricity doesn't sound outrageous, since it equates to around 32.5MPG, which in terms of expense works out to an energy cost of $5.25 to go 125 miles if electricity is available at $0.15/KWH, whereas the efficient small internal combustion engine powered car costs $6.25 to go the same distance if gasolene only costs $2.00/gallon, which gives a slight dollar saving to the electric car. On the other hand, if gasolene costs $4.00/gallon like we recently experienced in the US, the internal combustion automobile would cost $12.50 to go that same 125 miles, resulting in quite an expense savings on the part of the electric car. That same $5.25 would take you 240 miles with the press release figures for the BlueZero, at the equivalent of over 62MPG.

The worrisome part comes in when talking about charging the BlueZero. To quote the Wired article:

Mercedes says they charge in four hours when plugged into a typical wall outlet.

Imagine that. You could plug the BlueZero into your living room receptacle and in four hours transfer 35KWH of electricity from your incoming power line to your car's batteries. 35KWH in four hours. 8.75KWH in an hour. In the US, where we have 110V power, this means 79.5 amps of electricity are coursing through the walls of your house. I don't know about your house, but mine uses wiring that will safely pass 15A, less than a fifth of the 79.5A. That turns a four charging time in 20 hours.

Mercedes Benz is a European company, though. In Europe electricity in the home is usually 220V. At that voltage only 39.7A would be travelling through your living walls to the typical wall socket to charge those car batteries in four hours. I don't know about you, but I doubt the typical European house is built to safely handle almost 40A of electricity travelling through its wiring.

To be accurate, the press release just says the battery can be charged in 3 - 4 hours. It says nothing about using a typical wall outlet. Still, that calls for circuitry that can safely pass 40 - 80 amps of electricity at normal voltages, which would require special wiring to be installed in your house to handle the charging. I wouldn't mind that. I just wish people would try to be accurate rather than misleading. We need truth, not snake oil.

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