Misleading electric car specification, Kilo-What?

Wed, 2011-03-02 15:19 -- wprothero

While reading the news about a new Chinese electric car, I read the meaningless statement that:

"CODA's battery can carry a 34 kilowatt charge, versus with 24 kilowatts for Nissan's Leaf, with a comparable increase in range, Murtaugh said."

Why is this meaningless? It's the incorrect use of the units of power, the kilowatts. It's very common to mix up the rate of energy use with the actual energy use, which is what our electric meter reads, and that we pay for each month. Let me explain with an analogy.

Let's imagine that we have a truck that carries around a water tank on a tower. Let's also imagine that the water drains, through a valve to a water wheel that turns the wheels of the truck. Don't worry how this works, it's not important for this explanation. But, we know that the higher the water tank, the more pressure at the water wheel, which is at truck level. Also, we know intuitively that if we open the valve further, more water will flow out. It also seems reasonable that when the water is all gone from the tank, the truck will stop moving. The speed of the truck will depend on the water pressure and flow rate at the turbine (or whatever) that turns the truck's wheels. More flow without varying the pressure: faster turning. More pressure, but same flow: faster wheel turning. It's the pressure times the flow rate that determines the power. In electrical terms, the pressure is the voltage, and the flow of water is the current. Volts times amps gives you watts, just like pressure times water flow is proportional to the "horse" power of the truck. Stick with me, I'll get there!!

BUT, the truck moves only until the water in the tower is gone. To get more range, you have to increase the size of the water tank, or fill up again. So, there are two factors at work. The first is the power and that's what gives you forward motion and acceleration. Think "horse-power" in muscle cars. There's also duration, which is how long the power lasts.

So, suppose the water tank holds 1,000 gallons of water and the flow of 100 gallons per hour will make the truck move 2 miles per hour. The water will be gone in 10 hours. You can see this by multiplying (10 hours)x(100 gallons per hour) = 1,000 gallons. So, if the truck runs for 10 hours at the aforementioned 2 miles per hour, we multiply (10 hours)x(2 miles/hour) and get 10 miles.

Finally! The capacity of the water tank is what matters, as far as range is concerned. This is analogous the capacity (or total energy storage) of the battery. The power only gets the truck up to speed and up hills, etc.

So, the answer is: kilowatt-hours, NOT kilowatts.

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Comments

Submitted by CathyK. on

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