Tag Archives: Efficiency

How efficient is charging?

These days everyone is concerned with how much resources do they use. Tracking things down to the penny/gallon/kWh/etc. When you have an ICE vehicle there are no losses from fueling the tank: The amount of gas that went into your tank is exactly equal to the amount of gas that was extracted from the tank in the ground and is equal to the amount of gas that was in the tanker truck, etc. (within reason). When you plug in your EV its not the same: When you charge what happens? The battery and the electronics heat up. That heat is wasted energy. Not all of the electrons going through the charge cable end up in the battery. But how many? Is there a way to measure this?

Many modern charge stations will report out the kWh value they put into the car (Chargepoint, for one), in addition some home stations (like the Juicebox) also report out the kWh value. This gives us one figure, but to figure out the charge efficiency we also need another number: The kWh the car used from the battery. This value is the amount of kWh the car consumed that the charger replaced. Divide these two values into each other and you’ll get a measure of how efficient the charge process is in the FFE.

For example: This morning my commute in to work consumed 3.5 kWh according to Ford’s online application (one of the few values in the trip history that have been accurately reported here). Once at work I used a local Chargepoint station to top it off. Chargepoint reported that the car consumed 4.14 kWh during charging. This would work out to a 85% efficiency while charging (at least for the top 20% of the battery or so which is what these values amount to).

I’ll have to do the math again with a deeper charge and using the Juiceboxes values for a different comparison (I’m not expecting it to be much different but more datapoints is always better).

Update: The commute home consumed 3.3 kWh according to the trip meter and the Juicebox put in 4.2 kWh to charge it back up (about 79% efficient).

Also note that the amount of energy going in also is used to run the TMS (temperature management system) and thus when its running will lower the efficiency.

Update 2: After a deeper discharge (9.6) charging took 11.7 for an efficiency of 82%

Update 3: A normal commute usage (11.1) and overnight charge (13.0) yields: 85%.

Update 4: Another normal (10.3/12.4): 83%

Update 5: 10.5/12.8: 82%

 

Technology/Efficiency

Taking a break from the story for a bit.

Here I am going to go into a general discussion of the differences between various vehicle propulsion methods: Gas engine, hybrid, and electric motor.

Your normal gas engine operates at about 18%-20% efficiency. Much the losses associated with gas engines are attributed to heat loss through the exhaust and friction in the drive train. This can be increased slightly by adding turbo or super chargers but they only bring the engine closer to its theoretical maximum efficiency (37%).
Electric motors, on the other hand, have an efficiency in the range of 85%-90% thus more of the “fuel” will go to moving the car. This also poses a problem as there isn’t enough waste heat to be used for other purposes (heating the cabin, warming the batteries, etc.). This large difference gives the electric car an advantage in “fuel” economy.

Now lets look at other aspects of the vehicle: What is the one thing that a car does that wastes the most energy? Its not acceleration; its stopping. Think about it: What do brakes do? Brakes turn the forward momentum of the vehicle into heat at the braking mechanism. All of this heat is dissipated into the air–just thrown away.

How can we recover that lost heat? There is no way to feed that back into a gas engine and re-create gasoline. What you can do is spin up a generator and store the electricity. This is the exact principle used by hybrid vehicles: There is a motor/generator that recovers the stopping momentum and puts it back into a battery for use during acceleration (this is what regenerative braking does). Hybrids aren’t designed to run on electricity alone–the battery isn’t large enough–they are only designed to capture and re-use the deceleration energy (for the most part).

Now adding all the equipment for a hybrid increases the vehicles weight by a significant amount (we now have two “engines” and two “fuel” tanks). Manufacturers make a tradeoff between the sizes of the engine, motor, fuel tank, and battery when designing the hybrid car.

An electric car does not suffer from that limitation–only one motor is present and one “fuel” tank. In addition the electric car can make full use of regenerative braking.

Given all these advantages an electric car has one huge drawback to it (as you are probably screaming at your screen about now LOL): The energy density in today’s batteries is nowhere even close to the energy in a tank of gas. There is a ton of electricity storage research (battery, super capacitor, air-battery, etc.) going on with the goal of giving today’s electric vehicles more range. I’ve read of a few people posting their opinions for the battery criteria required for BEV’s to go mainstream. Jumping into the fray here is my guess as what is required for mass adoption of BEVs:

  • BEVs must have a range of 250+ miles (Not saying 300+ miles because one of my ICE vehicles (a rather large truck) only has a range of 250 miles on a tank)
  • The battery in said BEV of range 250+ miles must be not much larger than the average tank of gas (note that the battery doesn’t have to have the same energy density of gas due to the electric motors higher efficiency)
  • There must be available charging stations to allow the 250+ miles to be replaced in approx 10 minutes or so

Easy as pie right? The Tesla Model S comes really close to all of those above with a luxury price tag. The sub $30k planned Tesla model should take the market by storm..