Follow Up to the EV Article

I wrote two articles last week. One was mostly ignored and the other broke the internet. So of course I’m most interested in updating the one that got ignored.

I’ve wanted to write an article about EVs for a long time, but worried it might get in the way of all the kvetching about CAD that I need to do. I’m no longer worried about that, and given the way that people ignored that article in droves, well, what else is there to say? I even tried to write about EVs in a non-politically threatening way, you know, spread it around, make fun of everybody.

One happy thing that happened as a result of that EV article was that I got a message from a guy who I’ve known for, I don’t know, maybe 45 years, but we have been out of touch for a long time. You’d think I’d be a better friend, I mean I was in his wedding after all. His name is Greg Gartland, and he left a couple comments to the EV article based on his ownership of a Tesla Model 3. Greg might be a little obsessive about somethings, we have a lot of things in common actually, but he has collected a bunch of data on the use of his Tesla. I don’t know if this is something you can get natively from the car or if he went out of his way to collect it. (One of the cool things about EVs for engineers is all of the different kinds of data you can get from them.)

Something I’ve noticed from my own EV usage is that the charge capacity of the batteries is very temperature dependent. This information wasn’t directly available from Greg’s Tesla data, but we can derive some other info that tells the same story. When the temp is in the 60-70F range, you get a great charge, or in the case of the Tesla data, great efficiency. When it’s in the 20F range, you get about half of that range/efficiency. I haven’t compiled a chart of that from my own usage, I’m just going from observation. Greg’s Tesla data shows the cost per mile plotted vs temperature. So even without being able to read the numbers, you can see that when the average temperature is around 20F, the cost per mile (of the electricity) is about $0.025, and at about 65F, the cost per mile is around $0.016.

(For comparison, a car that gets 25 mpg and pays $3.50 for a gallon of gas costs about $0.14 per mile.)

I also plotted temperature against mile per gallon equivalent (MPGe). MPGe for window stickers on new cars is calculated by comparing the total amount of energy available in a gallon of gas, and comparing it against the amount of electrical energy it takes to go some distance. It’s a little bit of a round-a-bout calculation, but it’s hourly government work. In Greg’s data, MPGe is calculated as (price per gallon) x (cost to charge) / (miles driven). The Tesla’s computed MPGe from the provided data averages around 230, while the EPA value is 134. So the correlation isn’t direct, but I’m showing my work here.

The upshot of the chart is that you get better efficiency when the temperature is about the the usual range of temperatures in – Fremont California of all places. Which just happens to be where Teslas are built. Coincidence? Maybe.

And my final chart was one where I plotted the distance between charges vs MPGe. This shows some surprising things, and you might have to know Greg to be able to interpret this graph. First, it shows me that both Greg and his wife drive the car around town. You can see that there were a lot of short trips and some got good efficiency and some didn’t. One of my fondest memories of Greg was him drag racing his Toyota Corolla against everything and anything else on the road. So all of those low efficiency numbers are I’m sure from Greg hot rodding around town in his Model 3. I can totally see that. Maybe we can get another chart showing speeding tickets as a function of miles driven. ;op

And then as long as we’re on the topic of Tesla batteries vs cold weather, it might be a long time until I get another blog where this example illustrates my point so well, but… there was this guy in Finland (and for those of you who are recent products of the American educational system, we have made finding Finland easy for you – it’s in the red circle in the upper right)…

…who blew up his Tesla Model S with 60 pounds of dynamite. Replacing the battery would cost about $22k, so he just blew it up instead. The used market for electric cars in Finland must be brutal.

He could have just moved to a warmer climate like Moscow perhaps to save his batteries. I’m sure there are a lot of charging stations in Moscow. Or maybe further south. Crimea is nice this time of year.

Anyway, the $22k moral to that story is that batteries have a hard time in cold weather.

Have you ever tried to start a diesel engine at -40F? Doesn’t work nearly as well as the batteries. My parents owned a diesel VW Rabbit when I was a kid, and we often had to plug it in – to heat the diesel, not to charge the batteries.

Maybe someone from Texas or Arizona could send me some data to look at battery performance in the heat.

When we throw a lot of money and engineering attention at a problem, things have a way of changing fast. Look at 3D print. For years that process languished because it was getting no development attention. The materials were all fragile and limited. The machines were expensive. Once development started going in that direction, we have made a huge leap with printable materials, and anybody who wants a printer can have one.

I think we are going to see the same kind of development for charge storage. As the volume of electric vehicles goes up and the stakes are bigger than just powering cell phones, we are going to see a breakthrough in battery technology. New batteries will be more compact, less toxic, and store charges that make electrification more and more practical.