The Charge Time Metric
Thursday, May 26, 2011 at 03:11PMMaybe there is room for another way of comparing electric cars.
We liked the format of the X-Prize competition because competitions are inherently fair: cars ran in the same conditions and energy consumption was measured by a clear metric, MPGe, so there was no cherry-picking numbers or conditions. For example, the efficiency events were run on a closed track without altitude gain or loss, and the energy used was very carefully measured by competent and impartial judges. Under MPGe the energy consumed by vehicles with diesel, ethanol, electric or hybrid drives is referenced to the energy contained in a gallon of gasoline.
But the problem with MPGe is that you have to explain it all the time. For all the X Prize Foundation’s efforts, and the EPA’s, and the Department of Energy’s (they all use and promote the MPGe metric) it’s just not that easy to understand.
But the other day Edison2’s engineering staff were asked a sensible question about our new electric model: how long does it take to charge? Our short answer was, “quickly enough to be acceptable and viable for most people” and, as you would expect, there was a longer answer as well. Real-life charging time depends on how far you go and how fast you go. Of course that’s true of all cars but it’s energy not spent on pushing a heavy car with ordinary aerodynamics that really counts. So, as we like to do, we ran the numbers.
Some of our early blog posts explained our coastdown figures, which are derived using a recognized SAE standard and measure total resistance to motion caused by aerodynamic drag, rolling resistance and mechanical losses. The VLC’s coastdown numbers are the best ever for a 4-seat car and therefore our energy consumption, regardless of energy source, is also the lowest ever.
That’s a bold statement but the problem lies in getting its significance out in an easy-to-understand way. How about how long to recharge after driving a certain distance and speed?
Our performance projections for our electric VLC model take into account a little higher rolling resistance than the X-Prize competition cars (because the batteries make it heavier) and allow for 84% total efficiency through the charger, batteries and electric motor, a figure we think is realistic. On that basis, the electric VLC will take slightly under 4 hours 30 minutes to recharge from a standard 110 Volt 15 Amp socket after a 100 mile run at 70 mph.
To put that in perspective, neither the Nissan Leaf nor the Chevy Volt can actually go 100 miles at 70 mph before their batteries go flat. In fact, the EPA rates the Leaf at a 73 mile range and Nissan concedes it takes 20 hours to recharge from a 110 V socket. Also according to the EPA, the Chevy Volt will go 35 miles on its battery and Chevy says it will then take 10 hours to recharge.
So, 100 miles for 4 ½ hours of charging, 73 miles for 20 hours or 35 miles for 10 hours. You make the call.
Reader Comments (14)
Miles per Charge-hour? divide total range by time to fully charge.
write it: M+h
so
E2: 22.2 M+h
Leaf: 3.6 M+h
Volt: 3.5 M+h
and the proof that nobody's trying anymore:
EV1: 20 M+h
assuming the official numbers of 8hr charge and 160mile range are correct.
It doesn't cross fuel sources very well, but makes for good comparison between electric battery cars.
The EV1 number really makes the new products standout as half-assed. I was going to add the Aptera, as in many ways it's the closest thing to the Edison going, but the numbers are all over the place, and the project keeps changing. Wikipedia only has numbers that were projected during the earliest phase.
To the skeptical lay-person, 4.5 hours to charge is completely unacceptable. 4.5 hours? It takes about 10 MINUTES to gas up my car no matter how fast I drive. No, this still won't be attractive enough to make people want to buy an electric car.
How about charging from a "dryer outlet", 220V/30A? That at least theoretically would reduce recharge time down to closer to an hour. Still much too long, but closer.
It is unfortunate that range is such a big part of the electric car discussion, but it is. And charge time will always be compared to refilling time.
Therefore, a long charge time (>~20 minutes?) will be acceptable only if the range is 300 or 400 miles, or conversely a short range of <100 miles must have a very short charge time.
Don't get me started on the EV1. What an appalling waste of engineering effort, time, and money. All thrown away, literally. The wonderful cars were all crushed. Sigh.
I think Edison2 would be best served by continuing to promote and explain coastdown.
Rather than:
"coastdown figures, [...] are derived using a recognized SAE standard and measure total resistance to motion caused by aerodynamic drag, rolling resistance, and mechanical losses"
How about something simpler:
"How far the car rolls in neutral from 60mph".
Having that posted on the window sticker would make it much easier to see how "green" the car is, no matter the power source.
This is excellent news! What is your Wh/mile at say 55 or 60 MPH? It sounds like you have even lower consumption per mile that I had guessed (which was ~102Wh/mile).
@Pauls, the main difference with an electric car, is that because you can charge it every night, each and every day you can drive it 100 miles, and so the charge time doesn't really matter if you don't exceed that range in a normal day. If you drive 25 miles to an from work, plus the occasional errand, you might average 60 miles on most days -- actually half of all drivers only travel ~40 miles or less in a day. So you charge it overnight, when the car would be just sitting there anyway.
And, the MPGe of this is phenomenal! It must be well over 300MPGe, and it probably accelerates more quickly and it a lot quieter than the ICE powered versions. I'd put a battery that can go 200 miles in it. If you could get hold of the DBM Energy Kolibri battery, which weighs about 3.6kg/kWh, a 24kWh pack (equivalent to the Leaf) would weigh just ~85kg or ~188 pounds, and the VLC electric might go 250 miles! The DOD on the Kolibri is 97% vs the 95% for the Leaf.
Please keep us informed! Oh, and how did the wind tunnel testing go?
Neil
I like more technical numbers, it lets me make a more informed consumer decision. They debated doing color grades but that would give me zero useful information.
Edition2 is the best served by continued to expalin and promote coastdown
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Another voice looking for wind tunnel results. :) But really what I'd like is a peek at the overall design winner (wind tunnel plus all the other factors you're taking into account). I'm sure we're still a ways off from seeing that.
My friend leased a Volt and I drove it yesterday. I have to say, it's the first car I've seen since January 2004 (when I bought my Gen3 Prius) that I'd trade in the Prius for... if I had a garage to charge it in.
I'm amazed that GM actually made a car I want. It handles beautifully and feels like a European car... very solid. (By the way, note to Edison2... find a trick to make very light doors feel solid as they close... maybe a little mechanism like they use in kitchen drawers that can't be slammed-- they ease shut, so you can't tell if the drawers are light or heavy. People don't like it when they shut their car door and it doesn't feel solid...)
As a Leaf owner, I agree completely with Dominic that the key metric is "How many miles of range do I get for an hour of charging?" We are very rarely charging from empty to full. It is more about waiting impatiently for enough charge to go somewhere in particular.
The level 1 charger for the Leaf sucks down 1.4 kW according to my meter. The time averaged efficiency number on my Leaf (as reported on the dashboard) is 4.2 miles per kWh, for a mixed commute that involves a 400 meter hill, 10 miles of highway, 8 miles of local roads. I drive normally with no weird tricks to boost fuel economy. Throw in charging losses, and a reasonable figure for Leaf charging is 4 miles of range for every hour on the L1 charger.
But that's not the right number to pay attention to. Almost all Leaf owners are using L2 charging. That currently feeds the car battery 3.3 kW, and draw about 3.8 kW of power. But within two years, all these cars (including the Ford Focus EV) will be slurping up 6.6 kW from the L2 charger. That gets us roughly 25 miles per hour of charging, which is competitive with the figures mentioned for the VLC.
Yes, it is nifty that the VLC can get away with L1 charging only. But considering that most folks will charge at home, it is not *that* nifty. Edision2 should consider using a J1772 port, which could give a ridiculously awesome answer to "How many miles can I get from an hour of charging?". I have no idea how much weight it would cost, but remember that electric car people are already choosing to pay a substantial weight penalty for batteries anyway. We just love stuff like this.
So, assuming a 16amp circuit load at 120vac, $0.10 per Kwh, and a pessimistic 5 hour charge time...
It costs $0.96 to go 100 miles. A 25mpg vehicle consuming fuel at $3.00/gallon, costs $12.00 to go 100 miles.
Oops, I wasn,t a careful reader. Instead of a 20 amp outlet you're using a 15 amp outlet. At 80% loading for continous loads as per NEC (code), I should be safe in assuming that the charger does not draw more than 12 amps.
120 volts x 12 amps = 1440 watts or 1.44kw
1.44kw x 4.5 hrs = 6.48kwh
I currently pay about 8 cents per kilowatt hour
.08 x 6.48 = 52 cents to go 100 miles at 70 mph
MIT just announced a new flow battery that can be recharged by simply changing the battery fluid. They call the black fluid Cambridge crude. Battery fluid changes would be about as quick as pumping gas into a car, eliminating the problem of long recharge times interrupting trips. The flow battery can also be recharged conventionally from electrical outlets.. The scientists involved started the battery company A123 so they really know how to bring serious battery products to market. They think the flow battery will be about half the weight and cost of present EV batteries. The battery would essentially be made new every time the fluid is replaced so maintenance and battery fade should be eliminated. It sounds like a winner to power the electric VLC.
I like your choice of 120 volt 15 amp charging because it means virtually any electrical outlet can be a recharge point. With the efficient VLC you get at least as much range per hour of 120 volt recharge as the heavier EVs get tethered to rare and very expensive hi amp recharge stations. There should not be much range limitation with a 120 volt recharge because 120 volt outlets exist almost everywhere.
In Japan after the earthquake they found the electric cars saved the day and were very reliable, Gas was unavailable but grid power was quickly restored and thats all EVs needed to keep going; making vital deliveries of food, medicine, and emergency workers. The big windmills in Japan were not damaged by the quake and kept on producing power.
Japan may not have any nuclear power plants operating by next April because the public has the legal right to block restarts after required yearly maintenance of the reactors. However anyone can produce their own power with solar cells or windmills, and never again worry about the middle east or being overcharged by BP, Exxon and the Koch brothers.
Assuming the estimate of 1.44kw x 4.5 hrs = 6.48kwh per 100 miles is corect, then the electric VLC will get 563 mpge, figuring 36.6 kWh/US gal.
563 MPGE!
My bet is that hub wheel motors would be even more efficient, and these motors would eliminate gear whine that is noticeable in some EVs. Solar racer hub motors are up to 98% efficient. Four small hub wheel motors on a VLC would make all sorts of handling tricks possible, (including zero turning radius by putting wheels on one side in forward and the other side in reverse).
I think at this weight class you'd be forced to maintain too high a spring rate and ride comfort would suffer if you started running hub motors. Having the motors in the wheels makes huge difference in unsprung weight.