measuring the FE (?) of electric cars

[Erdrick]

With electric cars looking to make a comeback, I have thought of a couple things that need to be addressed. Many companies are working on the range of their cars, and trying to use that as a selling point. While 100 miles on a single charge may work for the daily commute, any long-distance trips would be out of the question. Ranges of about 300 miles seem to be what many companies are shooting for -- as they should. This is (about) what the average person is used to getting out of a tank of gas.

Which brings me to my question. Calculating the FE on a tank of gas is easy. Tanks don't shrink or expand over their lifetime, and gas doesn't change THAT much with temperatures. Batteries on the other hand, DO lose capacity over their lifetime. A car having a 300 mile range today will have 270 next year, and 253 the next year, and so on down the line. This is assuming that the car companies don't put in measures to drastically reduce batteries' tendency to lose capacity over time.

And now we truly reach the question of this post. How are we going to calculate the efficiency/economy of our trips in electric cars? Will it be done with kwh? Will you hook up your car at night, have a meter on it, and figure out how far you drove for said amount of kwh? With batteries taking more power to charge to 100% than to 80%, the numbers might end up getting skewed, especially when you consider the degredation of the battery.

Sure this won't be a problem for the average person. As long as they can achieve the range that they need, they should be happy. However, we as hypermilers are faced with a new dilemma. We most likely will lose the control over our numbers that we all love so much. I for one have no immediate plans to get an electric vehicle, but with oil running out they probably will eventually grab a decent chunk of the market place.

Could this spell the end of hypermiling? Well, the number side of it anyway...

[JanGeo]

ARG lost the post!

Battery energy is measures in watt-hours which is derived from the nominal voltage multiplied by the amp hours of the battery or cell. A typical laptop Lithium cell is 2 amphour and 3.7 volts nominal or about 7.4 watt-hours.
You measure the efficiency of an electric vehicle by the watt-hours per mile. In the case of my electric scooter I use about 28 watt-hours per mile so if I wanted 100 miles range it would be about 378 lithium cells or about 2.8KWH battery. Right now I am using about a 0.7kwh battery pack (25ah 36 volts SLA). A well designed electric car uses about 6x more energy per mile than my scooter.

The T-Zero runs on 6800 cells 50KWH battery and uses 165 watt hours per mile. About 6x what my scooter uses because they don't go 20 miles an hour so there are more losses at higher speeds but pretty close to my calculations. Fortunately at $0.25 per KWH electric rates you are looking at $12.50 to charge it up to go 300 miles which is pretty cheep at the high Newport RI Electric rates. Other parts of the country have rates in the $0.05 - 0.10 per KWH.
You still have to practice your hypermiling with an electric because heavy throttle and braking increases motor losses because of the I squared R (Current squared times motor Resistance).

[omgwtfbyobbq]

Originally Posted by Erdrick

And now we truly reach the question of this post. How are we going to calculate the efficiency/economy of our trips in electric cars? Will it be done with kwh? Will you hook up your car at night, have a meter on it, and figure out how far you drove for said amount of kwh? With batteries taking more power to charge to 100% than to 80%, the numbers might end up getting skewed, especially when you consider the degradation of the battery.

Batteries tend to complicated, but, for a given range of conditions, figuring efficiency usually isn't too bad. For instance, a typical charger will have some efficiency given some temperature and initial battery charge, just like battery discharge efficiency tends to be fairly consistent depending on temperature/current/etc. Essentially, w/ an EV, there isn't as much change in motor efficiency compared to load, so one of the most effective efficiency oriented driving techniques, P&G, won't do much for an electric motor since unlike an ICE, it's efficiency is more or less the same regardless of load.

This means driving an EV efficiently pretty much comes down to
-Not driving too fast and incurring a large penalty from aero drag
-Minimizing brake use
Everything else is pretty much based on the initial design of the EV.

For instance, different batteries have different charging/discharging efficiencies. NiMH are at about 66% charging/discharging efficiency, Li-whatever are at nearly 100% charging/discharging efficiency, and Lead Acid can be anywhere from ~50-90% charing/discharging efficiency. Motor/controller efficiency tends not vary quite as much, but still does... And then there's the actual power different vehicles require. Otoh, does charging efficiency matter that much if the owner gets all their power from on-site renewable generation? It's definitely a YMMV situation, but given a setup I think we could gauge efficiency fairly well.

[StorminMatt]

You also have to remember that HVAC can greatly reduce the range of an electric vehicle. Not only does A/C reduce the range. But so does heating. ICE powered cars produce TONS of heat by design. So heating takes NOTHING from the range of a normal car, and we don't even think about the energy required to heat the inside of a car. But on an EV, heating will reduce range GREATLY. Add this to the fact that batteries put out less in cold weather, and you can see why EVs are not looked at as such a hot idea in colder climates.

Then again, there is always the possibility of using a propane heater of some sort. Indeed, this may be looked at as the best solution to the problem of heating the interior of an EV.

[DarbyWalters]

Need to come up with a KwH to MPG conversion.

[StorminMatt]

Originally Posted by DarbyWalters

Need to come up with a KwH to MPG conversion.

Then again, this is not going to be so easy. After all, electricity can be generated in many different ways. And the efficiency of the equipment used can vary widely. Also complicating this is the fact that some power plants don't burn fossil fuels (hydroelectric, nuclear, wind, etc). The best that could be done would be to come up with an average figure.

[Mike T]

More to the point, how long will it take to regain a full charge after driving 300 miles with no electrical accessories or lights whatsoever being used?

Freeway rest stops get pretty boring after the 3rd hour

[2TonJellyBean]

You could convert it all to BTUs to compare efficiency... and then do price per BTU to compare the cost per km/mile.

A kWh has 3413 BTUs. A gallon of unleaded has 115,000 BTUs approx, with winter grades it varies.

Instead of BTUs joules would actually be better, but I like BTUs. Then I can easily compare the car to the BBQ to the house for consumption... at 27 MPG on the highway, my vehicle uses 10 times as much energy as it takes to keep the house warm when it is -10°C/14°F outside.

[Erdrick]

I do like 2TonJellyBean's idea. Using a common denominator sounds like a decent fix. BTUs are a very commonly used energy counter, so they should work. That still doesn't take into account HOW the electricity was made though, as StorminMatt said. Charging your car on a coal-power-fed grid, and using a wind turbine to charge your car are two totally different things.

Which brings up another question... are car companies being dishonest about the new electric vehicles? Cross that out... they ARE being dishonest when they claim that their cars are zero emissions vehicles. While the cars may breathe clean, they sure eat enough fossil fuels to make that possible.

Electric cars won't really impress me until we have sustainable ways to charge them. I can only imagine this happening through the owners of said cars using wind/solar power to charge the batteries.

[GasSavers_Ryland]

Originally Posted by Mike T

More to the point, how long will it take to regain a full charge after driving 300 miles with no electrical accessories or lights whatsoever being used?

I'm hoping that the A123 lithuim batteries start getting used more for electric cars, and that they take advantage of those cells being able to handle being charged to 80% of full in 5 minutes, even if you play it safe and make it 50% in 10 minutes, that would give a vehicle with a 300 mile range another 150 miles while you wash the windows and check the tire presure.