Chevy Bolt EV Forum banner

1 - 20 of 59 Posts

·
Registered
Joined
·
314 Posts
Discussion Starter #1
Is it possible to understand exactly what MPGe means? It is 119 for the Bolt. This is based on 33.7 kWh of raw energy in a gallon. From this one would conclude that the range is 212 miles. But the EPA gives the range as 238 miles. A poster on another thread educated me that kWh might not mean kWh of battery. It might mean kWh of purchased electricity. That makes perfect sense. Perhaps one has to buy 1.12 kWh of electricity to get 1 kWh into the battery. But other users report that it costs only 1.04 kWh purchased to get 1. That is, the loss in charging is about 4%. And maybe they mean something else entirely. So I asked the EPA. I got a useless answer, but I did some more searching....it gets...weedy.

Most of what I found related to hybrids but one can pick out the pure electric part. A key formula is this. EAER = equivalent all-electric range. RCDA is the computed range to depleted battery. So we see here that they take the range they get and multiply it by a factor related to CO2 emissions. The two CO terms are the charge-sustaining CO2 emissions and the charge depleting CO2 emissions. Why these are relevant (and what they mean exactly) I do not know. But it appears to be quite different then just the cost of a purchased kWh of electricity.

EAER = RCDA*((CO2CS - CO2CD/CO2CS))

I am getting these details from ¶ii at https://www.law.cornell.edu/cfr/text/40/86.1866-12#b_2_ii

The bottom line seems to be that the exact computation of MPGe from the observed range is....messy, and not intuitive. Perhaps an expert here knows more about this?
 

·
Registered
Joined
·
1,044 Posts
The calculation for the MPGe doesn't depend on the size of the battery, it deals with something they call Wall to Wheel energy used. The calculation takes into consideration how much energy it takes to charge the battery. Now how the EPA tests that I have no idea. Are they using a standard outlet, and if so how much amperage? Or are they using a level 2 charger? The amount of energy used to charge your battery depends on how much amperage you are using to charge.

The calculation for MPGe is calculated by MPGe = energy content per gallon of gasoline / (wall-to-wheel electrical energy consumed per mile * energy unit conversion factor)
 

·
Registered
Joined
·
314 Posts
Discussion Starter #3
From the docs I saw I believe they assume a 220V or 240V charger, so I would say yes to LEVEL 2.

The wall-to-wheel phrase (a nice one) would indicate that they are taking into account the loss from the wall. That makes perfect sense. But I gather from one user here (and I think others who measure consumption say the same) that that is under 5%, maybe just 4%, not the 12% that is the diff. between 119 and 134 (which arises from (238/60) 33.7). Maybe wall-to-wheel someone means something other than the loss in the line? Can you tell me what the "energy unit conversion factor" is in your note?
 

·
Registered
Joined
·
112 Posts
They have a highway range to depletion and a city range to depletion.

Est range = 0.7 ( .55*highway + .45*city) = 238
 

·
Registered
Joined
·
112 Posts
MPGe is calculated as follows:

0.7 * Charge Depleting Range * 33.7 / Recharge Event Energy

Two MPGe, city and highway are computed

Blended MPGe is .55 * highway + .45 * city
 

·
Registered
Joined
·
112 Posts
I believe they fully charge the battery, run it to depletion and then recharge and measure power supplied to the vehicle after charging to 100% (when vehicle stops charging). I don't know what those "bag phases" are, suspect they are for measuring emissions on hybrid vehicles. I am guessing here

https://www.law.cornell.edu/cfr/text/40/600.116-12

You only see data in the document, not the MPGe or estimated range calculations
 

·
Registered
Joined
·
112 Posts
This confirms what I believed to be true (above):

Electric Vehicle - City Test Procedure Summary -
Following SAE J1634 Recommended Practice, the
battery is fully charged, the vehi
cle is parked over night, and then t
he following day the vehicle driven
over successive city cycles until t
he battery becomes discharged (and the vehicle can no longer follow the
city driving cycle). After running the successive city
cycles, the battery is recharged from a normal AC
source and the energy consumption of the vehicle is
determined (in kW-hr/mile or kW-hr/100 miles) by
dividing the kilowatt-hours of energy to recharge the ba
ttery by the miles traveled by the vehicle. To
calculate the energy consumption in units of mpg
e
(miles/gallon equivalent) we use a conversion factor of
33.705 kilowatt-hours of electricity per gallon of gasoline
(which is basically a measure of the energy in
gasoline (in BTUs) converted to electricity). The city
driving range is determined from the number of miles
driven over the city cycle unt
il the vehicle can no longer follow the driving cycle
 

·
Registered
Joined
·
314 Posts
Discussion Starter #11
Excellent. Thanks for the explanation. So they are blending city and hwy. Makes sense. And the 0.7 is a factor (that was perhaps discussed elsewhere at this site) that is a major adjustment related perhaps to how they do the tests (maybe on "treadmills" so no wind resistance) or also in consultation with the manufacturer?

A quick calculation using your formula got me the mpgE=120, close enough to the EPA's 119.

And your post above: (364.4 .55 + 310.63 .45) .7 = 238. Perfect!

To summarize from the doc you pointed me to:

Range = (364.4 .55 + 310.63 .45) .7 = 238.142
MPGe = ((364.4 .55)/67.4206+(310.63 .45)/66.508 ).7 33.705 = 119.724 (which rounds to 120, when EPA value is 119, so small discrepancy here).

And the two %ages are applies only to the miles, so they are doing that properly.

I still ask: What is the exact definition of "recharge event energy", which is about 67 kWh in the two trials. It certainly sounds like it is wall energy (someone else mentioned "wall to wheel") and a quick search on the term indicates that it might well mean that -- the amount of purchased energy.

Can you say something about that 0.7: where it comes from? What it is accomplishing?
 

·
Registered
Joined
·
112 Posts
Sorry for the formatting, they fully charge it, let it sit over night, run repeated city (or highway) test cycles until the car can't go any further (this MAY involve it not being able to maintain the speed in the cycle) and total distance is measured. Car is fully charged and power into the vehicle measured (which as you correctly point out is not battery capacity).

The 0.7 correction factor is to account for people not driving in the real world like the test cycle. Call it a way to keep people from being disappointed that their range isn't as good as the sticker range,
 

·
Registered
Joined
·
112 Posts
They account for wind resistance in the test cycle (which is run stationary) by adjusting the resistance of the machine in runs on. They use the frontal area and Cd of the vehicle to make that adjustment.

The Recharge Event Energy is a measure of energy supplied to the vehicle to fully recharge it. So essentially power supplied from the wall (would have to read the SAE doc to know if they measure at the wall or at the car plug, should be a very minor diff).
 

·
Registered
Joined
·
314 Posts
Discussion Starter #14 (Edited)
Right: I missed some of your posts in this flurry. So I think this does it! They used 66 or 67 kWh purchased to get the full charge. This is more than the 4% extra over 60 that some people are posting, but their chargers might be better. Your earlier post said "normal AC source" which might be normal 110V. The level 2 chargers are better one imagines.

And it might not be putting exactly 60 kWh into the battery, as you mentioned.

So I think I have a handle on all this now. Excellent. I feel smarter (or at least more knowledgeable).

EDIT: And as I noted in my Hill thread, when I drove about 4 miles dead-flat at a steady 30 mph the rate was much higher than I expected. This is not how one drives in the real-world. So it confirms that steady driving at modest city speed can yield very high rates. Indeed, the EPA test had 364 miles predicted for the City test on 60 kWh battery. That is a rate of 6.07. I can see that such a thing is possible in an ideal setting.
 

·
Registered
Joined
·
112 Posts

·
Registered
Joined
·
979 Posts
Can’t say I know much about mpg-e as I fall asleep every time I attempt to read about it.

Pure opinion: Although I don’t like the Washington DC guy who spends most of his time in Florida & who has been bragging lately about eliminating regulations. Cheetos.

But AFAIC this is one that should hit the skids. There’s so much variability around the inputs to those estimates that the result is virtually meaningless. Aside from the fact that the public probably doesn’t know what it means and won’t take time to find out. Gas is gas for goodness sake.

Could have achieved just as much accuracy by telling customers on the window sticker something like “this EV will use somewhere between 15% - 30% of the energy that a comparable gasoline powered vehicle does”. Saving the manufacturers a lot of coin in coming up with a formulated point-guesstimate that’s merely a dot on an wide array of possible outcomes.

Don’t get me wrong I really like the EPA. Especially because us Cdns can just jump on the train and ride for almost nothing. We have an overpaid genius in Ottawa Canada with a big pension who’s in charge of converting 238 miles to kilometres. Each model year he works it out to 383 kms most of the time. That’s our C-EPA, pretty cheap in total.:eek: Allows more funds to go towards universal Cdn health care. Not bad eh.


EDIT: Thinking more about it. The conversion from 238 miles to 383 kilometres is probably done by our Cdn subsidiary GM of Canada Co, then the 383 km is supplied to the overpaid genius in Ottawa who is in charge of collecting the kilometre numbers from all the auto co's Cdn subsidiaries & he then puts it correctly on a spreadsheet most of the time.
 

·
Registered
Joined
·
314 Posts
Discussion Starter #18
Yes, learning more of the intricacies of MPGe (and the 238 EPA range) shows me that the method of computing them is not as crystal clear as one might think. But: Clearly a range number is needed to market EVs, and the EPA does a decent job of getting a fair comparison across manufacturers. An MPGe number is debatable.... After all, if cost of elec somewhere is very high, then the MPGe number is not as useful in terms of what it is trying to convey.

It is nevertheless remarkable is that the 238 range (or 4 mi/kwh) does seem like a perfectly good lower bound on mileage absent heat and conditioning.

As I mentioned elsewhere, there is also a not-much-used MPGghg figure which compares total greenhouse gases and changes from state to state depending on the method used to generate electricity. No one looks at this, but I pleased to know about it, since many say things like: "Oh, total greenhouse gases for an EV is not better than for ICE". While that is false, it is actually close to being true, or is even true, in places where elec. is almost entirely from coal, and such places, while in the minority, do exist in the USA (Wyoming).

Cehjun: Certainly at least once you can charge your car to full from, say, below half, and tell us how many kWh were needed to get there, as a %age over the kWh added to the battery. You said your 4% was too low. But does it jump to....6%? or much more. I realize that if you charged from close to 0 that would be a better measure. Oh WAIT: The EPA test though does not work with an empty battery. In fact, I think they start with a full battery. Run a relatively short test (7 miles or so) and then charge to full again. So just writing this makes me realize that what they are doing to measure the energy input from the wall is working with a near full battery, and that is more demanding, perhaps a lot more demanding, then typical charging. So perhaps, Cehjun, what you should do (just once only!) is charge from something like 95% down or 90% to full and see what the %age loss is. For that range it might be quite high, tho I still imagine the use of a Level 2 charger will be better than household current at 110 in terms of the loss. What the EPA does is that it figures out how much wall energy is used for this charge to full, but then it linearly extrapolates that up to a full tank -- at least for the MPGe work; for the range work this point is not relevant I think. That seems probably quite wrong (though I suppose it is arguably a reasonable thing to do -- all they really care about if consistency across models).

Sparky: The doc you pointed me to has for the city test: 7.413 miles using 1.2751 drop in State of Charge. From this, the calculated mileage to depletion is given as 364.4 miles.

Where is that 364.4 from :

7.413 /1.2751 60 is only 348

If I pretend there are 62.68 kWh in the tank then I get 364.400 exactly. How they would deduce there was 62.68 kWh in the tank I do not know, tho I suppose it is in the realm of possibility. Any comment on the arithmetic that leads from 7.413 miles and 1.2751 kwH to depleting-mile count of 364.4? (the fudge factor of 0.7 only comes later, so is not relevant here).
 

·
Registered
Joined
·
3,822 Posts
There’s so much variability around the inputs to those estimates that the result is virtually meaningless. Aside from the fact that the public probably doesn’t know what it means and won’t take time to find out. Gas is gas for goodness sake.

Could have achieved just as much accuracy by telling customers on the window sticker something like “this EV will use somewhere between 15% - 30% of the energy that a comparable gasoline powered vehicle does”. Saving the manufacturers a lot of coin in coming up with a formulated point-guesstimate that’s merely a dot on an wide array of possible outcomes.
MPG is a poor measurement of efficiency for petrol vehicles too. For example, many people might mistakenly assume that improving the efficiency of a truck by 1 MPG is the same as improving the efficiency of a Prius by 1 MPG. Going from 10 MPG to 11 MPG is a 10% improvement, whereas going from 50 MPG to 51 MPG is a 2% improvement. Most other countries use L/100kM, the number of liters to travel 100 kilometers. Lower is better. Is there an EV conversion to this method of fuel efficiency too?

The conversion from 238 miles to 383 kilometres is probably done by our Cdn subsidiary GM of Canada Co, then the 383 km is supplied to the overpaid genius in Ottawa who is in charge of collecting the kilometre numbers from all the auto co's Cdn subsidiaries & he then puts it correctly on a spreadsheet most of the time.
That guy probably has a secretary put the numbers into a spreadsheet. His job is to be a people person.

 

·
Registered
Joined
·
979 Posts
So perhaps, Cehjun, what you should do (just once only!) is charge from something like 95% down or 90%to full and see what the %age loss is.
Okay I can see myself doing that. Not because of mpg-e exploratory research. But because it’s potentially useful in how one L2 charges one’s Bolt on a day to day basis.

And with that in mind: comment on the design of the experiment: Wouldn’t it be better to do 2 charge-ups:
1.) A. record the Bolt’s kWh’s used from my previous fill to hilltop, then B. charge it back to hilltop ~88% SOC, record EVSE kWh’s used (and compare to Bolt’s), then
2.) shortly thereafter charge it from hilltop 88% to full 100%, record EVSE kWh’s
?


This may take a little while to accomplish:
  • doing it between 7pm and 7am, cheapest hydro rate because hey this’ll be relatively expensive. and BTW regardless of what GM says I don’t like stressing my battery bygoing up to 100% full when I don’t have to,
  • haven’t been using the Bolt much due to yucky roads wherein my wife’s car is used as the “beater” vehicle,
  • don’t ask me to pull out the 110v trickle cord, I haven’t looked down there since I picked up the vehicle new 10 months ago. besides I can’t measure EVSE usage with the trickler
 
1 - 20 of 59 Posts
Top