Big Battery Means Big Range, Cost for Tesla's Model S

[Doug_G]

Design News - Big Battery Means Big Range, Cost for Tesla's Model S

The recent introduction of Tesla Motors' Model S electric vehicle raises an important question: How is Tesla able to get a 300-mile range when other EVs are lingering around 100? The answer is only partially obvious. The Model S's well-known and oft-repeated 300-mile range applies only to the versions with a 85kWh battery.

Even in the world of electric cars, that's a massive battery. Consider that the series hybrid Chevy Volt uses a 16kWh battery and the all-electric Nissan Leaf, a 24kWh battery.

Still, Tesla is doing more than just dropping a huge battery into its Model S cars. The company is squeezing every bit of available juice out of its vehicles. While the 85kWh unit hits a 300-mile range (according to EPA measurement methodology), the smaller batteries do well, too. The Model S's 60kWh battery gets 230 miles of range, while the 40kWh version hits 160 miles. In terms of miles per kilowatt-hour, that's better than the Volt or the Leaf. It's also competitive with the Tesla Roadster -- a $109,000 two-seater built in 2008 that weighs 1,900 lb less than the Model S.

"Even though the Model S is a much larger and heavier car than [the] Roadster with ridiculously more cargo capacity, the total battery energy consumption on the highway is only about 10 percent more than the Roadster," wrote Tesla CEO Elon Musk and CTO JB Straubel in a recent blog. In their blog, Musk and Straubel point to a graph showing that the Model S, traveling at a constant speed of 20 mph, can get more than 450 miles on a charge. From 50 mph to 70 mph at constant speed, the vehicle gets between 240 and 335 miles of range.

Amidst the tumult of the recent Model S introduction, Tesla hasn't yet detailed how its engineers are able to do that. When we talked to Straubel in 2009, he identified a multitude of little features that when taken together, give the company more miles per kilowatt-hour than competitors. Straubel cited innovations in cooling, aerodynamics, and rolling resistance. He also described how specialized brake calipers and the changeover from a two-speed to a single-speed gearbox helped boost the Roadster's all-electric range.

[brianman]

My mental model has a hole / disconnect on a related topic, so maybe someone can help me out.

The Model S's 60kWh battery gets 230 miles of range, while the 40kWh version hits 160 miles. In terms of miles per kilowatt-hour, that's better than the Volt or the Leaf.

Yet the EPA MPGe rating is in the other direction. I'm aware the wall-to-battery efficiency comes into play, but I'm surprised it causes the position flip.


Carrying the train a thought a little further...
If the Volt or the Leaf replaced structural components (i.e. ignore safety considerations) and sacrificed passenger space (i.e. ignore convenience and utility of the vehicle) to switch to an 85 kWH Tesla battery, would they get longer range than the Model S?

[gocken2]

Carrying the train a thought a little further...
If the Volt or the Leaf replaced structural components (i.e. ignore safety considerations) and sacrificed passenger space (i.e. ignore convenience and utility of the vehicle) to switch to an 85 kWH Tesla battery, would they get longer range than the Model S?

I believe the low EPA MPGe on the Tesla is due to the car using power to condition the battery pack while charging. It's the charging efficiency where the Leaf comes out ahead not distance per KWh.

[ahimberg]

you're mixing numbers? to compare the 85kWh to Leaf's 24KWh, are you comparing the 300 range to 100 range of leaf, or the EPA numbers of 265 & 73? Leaf is still ahead but not by much.
as for the 60kWh, 40kWh, we don't know EPA numbers, if they scaled exactly we'd have 140 EPA for the 40kWh?

[brianman]

I believe the low EPA MPGe on the Tesla is due to the car using power to condition the battery pack while charging. It's the charging efficiency where the Leaf comes out ahead not distance per KWh.

Is the conditioning throughout the full 0 - 100% SOC transition or does it vary? If it varies, this suggests a crudeness in the EPA calculations.

For example, if charging from 20% - 30% has a lower "from wall" efficiency than 60-70% then one could argue that for "daily driver, limited miles" the efficiency calculation is way off. It sounds a lot like the difference between city and highway MPG differences in traditional (ICE) efficiency measurement.

Also, if the conditioning varies across SOC -- at least for the Tesla chemistry/chemistries -- there may also be missing aspect related to battery size. I would expect a "5 Wh" battery has far better "wall efficiency" than any "5 kWh", because you could simply choose a chemistry for the "5 Wh" where you are well below the actual charge that can be retained (and also not need any conditioning). Furthermore, you could "overfill" it with regen so that for the EPA test it has infinity MPGe (because it didn't lose charge below the 5 Wh threshold) -- as long as you don't run the test multiple times in a row.




I guess what I might be getting at is that either -a- Tesla is behind on wall efficiency or -b- the EPA measurement / analysis isn't sophisticated enough to tell the story. If the former, I'm curious to hear more about it.

I would argue a significant portion of the population will just compare the MPGe number somewhat blindly, like for MPG.


Anyway, the forum's relatively quiet so I figured I'd carve out some time to learn.

[jerry33]

In the Roadster it's been determined that the most efficient charge is done at 40 amps. Below 40 and there are higher line losses (as I understand it), above 40 and the cooling fans have to engage more. Until someone measures we won't know what the best charge rate is for the Model S. I'm not sure what rate the EPA charges at. If it's 30 amps (which would make a certain amount of sense because the L and V charge at those rates) and the Tesla behaves like the Roadster (an unknown right now) then the EPA charge rate is non-optimum.