Tesla’s Battery Guru Describes A New Cell With Massive Lifetime
By Brad Templeton
Sep 9 2019
A recent paper on new battery chemistries published by a team including Jeff Dahn, the physics professor from Halifax who is often called Tesla’s battery guru. The claimed results suggest a cell with a lifetime of 6,000 cycles. This would translate in a large pack car to the “million-mile battery” which Elon Musk has predicted for Tesla.
Professor Dahn knows very well the difference between an interesting battery result in the lab and much more involved tests, and the further difference between the involved tests and a battery in high volume production with high quality and good prices. The latter has not been shown and is hard to predict. The paper forecasts such batteries are good for robotaxis. In addition, these batteries do well even when run on a 0-100% discharge cycle. Today’s batteries do best on a 20-80% discharge cycle, so you get 66% more range from the same pack if you can safely do 0-100%.
First of course, such a battery would be good for regular electric cars. While no car today has a million mile lifetime (Elon Musk has claimed Teslas do) more battery lifetime is always good. Even if the car falls apart after half a million miles, you could still move that pack into a second car. It also reduces the embedded energy in making the batteries and the cost of recycling them. I doubt any car interior is ready for a million miles, nor is the exterior ready in some climates. Frames and electric motors might very well last.
The important difference with the robotaxi is that its wear happens fast. Put into heavy use, it can be doing 200 miles/day or more — even more if it does a lot of highway travel. The average New York taxi is driven around 180 miles/day — average urban speeds during core driving hours are just 12 to 20 mph including stops — while the average private car does only 40.
At 200 miles/day, a car would rack up a million miles in just over 13 years. A taxi with a more normal 250,000 mile life (which is how long New York taxis last) is worn out in 4 to 5 years. If you can make the vehicle last that long, you seriously reduce the depreciation part of the robotaxi economics model and thus reduce the cost of a ride, since today depreciation is the largest cost factor in operating a car. Depreciation of the battery has been one of the largest costs of operating (and fast charging) an electric car. In fact, you can reduce depreciation so far that it becomes secondary to other costs like energy, risk, maintenance and logistics.
Robotaxis will do a lot of their work during the three weekday peaks, which are morning and evening rush hour, plus the lesser peak of lunch. You can either equip the car with enough battery for the whole day, or you can put less battery in and try to get some charging done during the day. That saves you some weight and some space, but since a larger pack lasts longer, it doesn’t save you much other money. Daytime charging can be a plus for cars that will do long highway travel, and of course night travel.
Today, night charging is the whole game. Vehicles are idle at night, and that’s when power is the cheapest. Even if you thought a 2pm recharge would let you have a smaller battery, you would not do it because electricity can cost 2x to 4x as much. Results from Tesla suggest that having the 75kwh battery only loses about 5% of efficiency from that extra weight over the 50kwh battery. You don’t save a lot.
If your choice is to put in a 200 mile battery into the taxi, and recharge only when power is cheapest, or put in a 100 mile battery (which costs half as much but lasts half as long) and charge mid-day when power is expensive, the math points to the 200 mile battery, except on the question of space.
Once private cars become robocars, they get the ability to scoot off to charging stations when not driving their owner. This actually lets the private car get away with less battery for city driving, because it can boost up mid-day while the owner works or shops — but not if the daytime price is 4x as high.