Sunday, January 17, 2016

Mass market EV, hoping for Tesla

Previously, I griped about shortcomings of Chevy Bolt.

Chevy still has time to make it better. One would be to make the power 250 HP or more. This may be relatively easy as the 250 HP would only be used for brief burst in acceleration, not sustained.

The other is to make DCFC chargers and help expand the network. Although Chevy stated they won't do anything unless it benefits all their customers, which I read to mean gas cars, they can still change their mind. I mean, Bolt is capable of 150kW charging (3X larger battery than SparkEV), don't they want to test it? Then the test fixture can be made as commercial charger and sold or leased. Isn't there any engineering curiosity, like the guy who made world's fastest lighting BBQ?

Chevy chose to have large battery for range. At 60kWh, it would cost $12,000 (+ labor) to replace it if the pack costs $200/kWh (cell is supposed to be $145/kWh today). Even at $100/kWh, that works out to over $6000+labor when the battery dies after 10+ years. People aren't likely to spend that much money to repair an old car, especially one that performs poorer than comparable gas cars, such as Subaru WRX that comes with AWD and has more power. On top of that, Bolt takes close to an hour to get 80% (160 miles) range. Bolt is pretty much a disposable car.

Model killing Bolt

Then there's another coming: Tesla Model3. While 0-60 performance kicking butt of every car in its price range would be nice, there are two other important metric: range and DCFC time.

What's needed is smaller battery that achieves same range as Bolt and higher power charging. Tesla already has higher power charging: 120kW supercharger network. Assuming no taper to 80% like SparkEV, 15 minutes of charge would be 30 kWh. That needs to have range of 160 miles (80%). Then the total battery capacity would be 37.5 kWh, or round up to 40 kWh (or bit more for margin). 

At $200/kWh pack price, Model3 could be $4000 cheaper than Bolt. After 10+ years when battery prices have come down to $100/kWh, it would be $2000 cheaper. While $4000+labor isn't chump change, it is more palatable than $6000+labor for Bolt, especially if Tesla charges in 15 minutes.

The range on EV is primarily the freeway range. I will cover EV range in more detail in later post, but EV is fairly simple to characterize in that faster you go, the lower the range. What's needed is 160 miles at 65 MPH using 30 kWh (or 200 miles using 37.5 kWh), or 160/30 = 5.33 mi/kWh from battery to wheels.

Is it possible? Yes, of course! SparkEV does between 5 mi/kWh and 5.5 mi/kWh at 55 MPH with its enormous height and awful drag coefficient (0.326 vs Prius 0.28). If Tesla can keep the drag down, it can get away with smaller battery and achieve 15 minutes for 160 miles with their existing Supercharger network. Exact drag also depends on the frontal area, but if Tesla can achieve under 0.2 drag coefficient (EV1 was 0.19), it's probably enough.

On top of that, smaller battery would be lighter. Bolt battery is 960lb for 60kWh, and assuming same energy weight density, 40 kWh would be 320lb lighter at 640lb. That will reduce rolling resistance, further help in range.

Lighter battery would also help in acceleration. Tesla's old 40kWh Model S had 235 HP motor. Combined with lighter than Bolt by about 320lb (3200 lb) that would put it in league of comparable cost gas cars in lb/hp (13.6 lb/hp, better than Fiesta ST). But for 0-60 time, it should be far quicker than gas cars, probably under 6 seconds, maybe even reaching 5 seconds.

EV that does 0-60 close to 5 seconds, 15 minutes to charge 160 miles range, cost about $26K (or $24K in CA), extremely efficient aerodynamics (ie, less noise), now THAT is a kick ass mass market EV I'd be proud to own. Come on Tesla! Let's do this!

Edit Jan. 19, 2016

It seems some have doubts as to what I'm saying. Firstly, unless there's a revolution in battery technology (ie, not Lithium), price is expected to be about $100/kWh. It's also true that few people spend several thousand dollar upfront to fix an old car, especially poor people who tend to drive older cars. Then they'll continue to drive gas guzzling and high pollution cars. In case of pollution, we're not just talking about CO2, but immediately harmful stuff like sulfur oxides, CO, HC, NOx.

The aerodynamics I propose here is not a new idea. From Wikipedia, we can see several cars that have very low drag coefficient.

Limited cars like EV1 has 0.19 and VW XL1 of 2014 has 0.186. While some look comical, Prius has 0.25 (0.24 for 4th gen) while far better looking Tesla Model S has 0.24. Better aerodynamics doesn't have to mean weird.

By far the most impressive is Schlorwagen from 1939 that had drag coefficient of 0.15. If SparkEV has such drag coefficient while retaining current specs (frontal area, weight, battery, etc), it would get about 180 miles range, and charge 144 miles in 20 minutes using 50 kW charger (range using EPA figures) (WRONG! While there will be improvement, I ignored rolling resistance. See EV range blog post in the future for better data). This is only with sheet-metal work, nothing to do with battery.

From the looks of it, it's not conventional looking by the standard of its day. But it's something sort of, kind of looks like what could appear today or in near future. While conventional gas engine did not allow for much flexibility, battery technology can allow far more flexibility in shaping car's body for aerodynamics. For example, there is no poisonous gas coming from the engine that you need to seal off completely from passenger compartment.

When it comes to Chevy type of battery, they are cube blocks. But Tesla uses thousands of 18650 batteries that are typical of laptops. Arranging them in odd shape to have a body shape that minimize drag is far easier. Of course, this assumes Tesla will do such simple minded optimization. They could come up with entirely new way to do things.

But one thing that I hope Tesla doesn't do is brute force: stick ever larger large battery in EV approach. In short term, that'll cost more, weigh more, making it uncompetitive to comparable gas cars. In long term, that'll add to more waste at junk yard and not as many poorer people driving old, used EV, instead driving really hazardous polluting old gas guzzlers.

Edit Jan. 29, 2016

After doing "SparkEV range" post, there are few interesting findings using ecomodder web site.
  1. SparkEV EPA range roughly corresponds 55 MPH range.
  2. SparkEV EPA highway MPGe is roughly 65 MPH MPG.
  3. SparkEV uses 90% of battery capacity.
First point is quite important. I wanted 65 MPH for range, but the EPA seem to use 55 MPH for SparkEV. That's much easier than 65 MPH. Power is proportional to speed cubed. It's also true that Tesla S has 0.24 drag coefficient as tested by third party.

Tesla S60 model

I know SparkEV parameters, and actual experimental data (that I conducted), but I don't have first hand knowledge of Tesla with respect to power vs speed. Still, we can use ecomodder web site using known data to see what turns up. I start with SparkEV parameters, and change the weight to 4323 lb (Wikipedia S60), Cd to 0.24, frontal area to 25.8 (greencarreports).

Knowing that 65 MPH returns roughly the highway MPGe with SparkEV, we can tweak Tesla S parameters until 65 MPH is 97 MPG. Weight and drag and area are fixed to S60 specific data. I use 75% for motor and 90% for drive train and 1000 W for parasitic overhead (for giant 17 inch display?). Rest are kept the same. This results in 96.7 MPG at 65 MPH.^2&FuelWh=33557&IceEfficiency=.75&DrivetrainEfficiency=.9&ParasiticOverhead=1000&rho=1.225&FromToStep=5-200-5

Greencarreports states Tesla S would use 14 HP at 70 MPH, but the table shows that's only to overcome aerodynamics. Since the article was with respect to drag, I think the data I have is correct.

Guess for Model 3

Using the data from Model S, we reduce the weight to 3200 lb (320 lb lighter than Bolt by using 40 kWh battery instead of 60 kWh 960 lb battery). There's some speculation that Model 3 will be 80% the size of S. Given that S is pretty wide, I guesstimate about 90% width (frontal area) as 23.5 sq ft.

The number we are looking for is power at 55 MPH, the number that matches EPA range for SparkEV, and we'll use that number to validate if 40 kWh battery is big enough to achieve 200 miles range.^2&FuelWh=33557&IceEfficiency=.75&DrivetrainEfficiency=.9&ParasiticOverhead=1000&rho=1.225&FromToStep=5-200-5

65 MPH shows 113 MPG, even better than SparkEV highway MPGe (aka, efficiency). 55 MPH shows 9.3 kW. That's 15% better than SparkEV! Let's just make it 9.6 kW, what the heck. Why 9.6? If you have to ask, you won't like the answer!

Now let's see if 40 kWh will be enough to get 200 miles range. Recall that I wanted 30 kWh for 160 miles range (80% of 200 miles) so it can be charged in 15 minutes using 120 kW supercharger without taper. 3 hours at 55 MPH is 165 miles, close enough to 160 miles. Then the energy needed would be

9.6 kW * 3 hours = 29 kWh (oooohhh!)

This meets my requirement with margin to spare. Energy needed for 200 miles range would be

29 kWh / 165 miles * 200 miles = 35.15 kWh (aaahhh!!)

From various experiments, SparkEV is using 90% of battery capacity. From some forum posts, Leaf also uses 90% (22 kWh out of 24 kWh battery). Forum posts? Yeah, it could be garbage, but it sounds plausible from other users' findings as well. Then the battery should be

35.15 kWh / 0.9 = 39 kWh (WOW!!! I'm a genius!)

In effect, what this suggests is that Tesla could be building miniature Model S using 40 kWh battery and calling it mass market EV at lower cost than Bolt. What I wrote above could be a reality with very little design effort from Tesla. That's great news! Where do I buy one?

Keeping up with Teslas

Yeah, only if life is that simple. Tesla may not use 90% of battery capacity like SparkEV and Leaf. In fact, some forum posts say it could be as low as 70%. Let's check that with known numbers. SparkEV has EPA range of 82 miles. S60 has 3 times bigger battery. They have roughly similar power requirement at 55 MPH. Then the range for S60 should be 3 times that of SparkEV at 248 miles. Clearly, that isn't the case as EPA rates S60 at 208 miles.

But let's go the other way. S60 is 11 kW at 55 MPH with 60 kWh battery. To use all 60 kWh, the range would be

55 MPH * 60 kWh / 11 kW = 300 miles

70% of 300 miles is 210 miles. It seems Tesla is indeed using only 70% of full battery capacity. Let's see where it takes us with our Model 3 guess if 70% is used instead of 90%.

35.15 kWh / 0.7 = 50 kWh (ooops!!!!)

It's better than 60 kWh, but 50 kWh is still too high. Of course, most of these values are guesses. Tesla probably can make 40 kWh EV with 200 miles range as mini Model S, but the reliability (ie, range degradation) may not be up to their standard. But if mass market means bit more range degradation, they can relax their standard a bit. One might say "keeping up with Teslas, but not the high end"

Perhaps they can offer various battery options like they do with Model S. Then using 40 kWh battery while keeping 70% battery utilization would result in range of

40 kWh * 0.7 = 28 kWh
55 MPH / 9.6 kW *28 kWh = 160 miles

While it's not quite 200 miles range, 160 miles range is probably good enough for more than 2 hours at 65 MPH.

But what I'm after is 80% to be able to charge at full 120kW power without taper. 80% of 160 miles would be 128 miles. 128 miles is good for about 2 hours of driving in freeway, though it'll probably be 60 MPH. Combine that with less than 15 minutes to charge, that would still make for very compelling mass market EV, especially if the price is lower by thousands of dollars compared to 60 kWh Bolt.

But as I wrote above, I hope they don't just stick with large battery for range whatever they do. Brute force is not an elegant solution to engineering problems.

Edit Feb. 25, 2016

It seems ecomodder web site only uses motor efficiency for MPG figure, not the power in kW. Then the above analysis using power is wrong (oops!) One should use MPG column. Using 134.4 MPGe at 55 MPH,

134.4 MPG / 33.557 kWh/gal = 4.0 mi/kWh
4 mi/kWh * 60 kWh * 0.8 = 192 miles (60 * 0.8 / 120 = 0.4 hours, 24 minutes for 80% of 60 kWh)
4 mi/kWh * 40 kWh * 0.8 = 128 miles (40 * 0.8 / 120 = 0.27 hours, 16 minutes for 80% of 40 kWh)

Those would be roughly EPA's rated range. More usable is 65 MPH range. Using 113 MPGe at 65 MPH,

113 MPG / 33.557 kWh/gal = 3.37 mi/kWh
3.37 mi/kWh * 60 kWh * 0.8 = 162 miles (24 minutes charging)
3.37 mi/kWh * 40 kWh * 0.8 = 108 miles (16 minutes charging)

Huge oops!

However, that was assuming motor efficiency of 75%, something they may be able to improve upon for lower performance than Model S. At 90% efficiency, 65 MPH shows 136 MPG. Then 40 kWh range would be similar to 55 MPH case above. 128 miles using 80% of battery at 65 MPH is almost 2 hours of driving. That's quite satisfactory for almost all cases, especially if it allows 16 minutes to charge and cheaper by thousands of dollars than Bolt.

1 comment:

  1. The model 3 will be amazing since Tesla already knows how to make great electric. The model 3 will be 20% smaller and lighter. I'm placing an order on March 31st at my local Tesla store. The Super Chargers alone are a compelling case. I also own Tesla stock and it pays for the car and then some. I made a web page about the model 3