How Much Range?

[This fragment is available in an audio version.]

When someone wants to talk to me about my car, invariably the first question is some variation on “How far can you go on a charge?” The next is “How long does it take to charge?” Ladies, gentlemen, and other flavors, please take note. These questions are wrong. I’m here today to explain why, and suggest what the right ones are.

[This piece provoked by my recent Trans-Canada driving experiment.]

“How far can you go on a charge?” · For almost everyone, 95% of their driving is commuting and shopping and going to the gym or whatever. Every contemporary electric car you can buy has more than enough range. Most EV drivers I know charge less than once a week.

Therefore, the question is really only relevant if you need to drive long-haul. I’m going to define “long haul” as “more than 250km” (about 150 miles). That number 250 may be controversial but I think it’s reasonable, because as of mid-2021, it’s becoming easy to buy an EV with that kind of range, with the price creeping further into mass affordability every quarter.

Now, when you’re long-hauling, you’re never going to use all of your range. To start with, when you’re using one of the fast-chargers on the highway, the process slows down when your battery hits 80% full, by a factor of as much as three. So if you arrived at 20% full, it’d take you the same time to get from 20% to 80% as from 80% to 100%. Since you want to get back on the road, and you don’t want to hog the charger unduly, you usually take off when you hit 80%. So to answer the long-haul range query, start by subtracting 20%.

Not only do long-haulers not start out full, they don’t run the battery down to zero. These days, there’s always the danger that when you get to the charger, it’s broken or busy or you just can’t find it. So you need to leave some reserve. People who plan ahead generally look for a charger where there’s another nearby to serve as a Plan B. What do we mean by “nearby”? Well, if there’s a Plan B charger a couple of blocks from your target, you’ll be willing to run pretty far down. If the chargers are say 50km apart, you’re going to want more reserve.

So the correct arithmetic isn’t “max-range - 20%”, it’s “(max-range - 20%) - Plan-B-safety-margin”.

Of course, there’s a special case when you’re starting from home, or ending up there. Where by “home” I mean somewhere that there’s a reliable low-tech level-2 charger where you leave your car plugged in all night and it’s back at 100% in the morning. So when you’re starting from home you don’t need to compensate for that 20%, and when you’re ending up at home you don’t need the safety margin.

But it’s more complicated than that. Because your range depends on how fast you’re going, how often you’re stopping, whether you’re going up and down hills, how hot or cold it is, and how hard it’s raining. For example, the worst-case scenario I can think of is the eastbound BC Highway 5 (“The Coquihalla”) which is 500+ km long, mostly uphill, and has approximately 0km of flat sections. Also, it has a speed limit of 120km/h. Also, it’s in Canada, which means that the local climate includes rain, snow, and extreme temperatures.

Among all these variables, there’s one you can partly control: your speed. I’ve been told that the formula for air resistance includes at least one quantity containing the square of your speed. So when a long-hauler is calculating the next leg of their journey, they’ll need to take that into account.

So the correct question is something like: On the rare occasions I’m driving cross-country, how far can I go in one hop, after you take off 20% for charging efficiency (unless you’re starting at home), allow for Plan B at the destination (unless you’re ending at home), and compensate for speed, weather, temperature, and hills?”

So someone who asks me that question is apt to get a long answer. Or in the (unlikely) event that I don’t want to explain, or the (common) event that I don’t think they have the patience, I say “Max 400km best-case, but I can always get 300.”

The right question · I suggest “Can you go 250km between chargers on a cross-country trip?” I confess that I’m influenced by the design of Petro-Canada’s Electric Highway project, which aims to have chargers no more than 250km apart. I think that’s about right.

Depending on how the ecosystem of EVs grows, we might end up using either a larger or smaller number. Of course, the more charging networks are out there, the easier Plan B gets, so the minimum viable long-haul-leg range gets smaller.

“How long does it take to recharge?” · If you own an EV, your life will be much easier if you have reliable access to a “Level 2” charger. This can cost less than a thousand bucks if you’re lucky enough to have a garage that already has decent electrical service. But it’ll be more for most people. For those who park on the street or in their apartment’s basement, it can be a real problem.

With that Level 2, then for basically every electrical car on the market, if you adopt a discipline of “Plug it in overnight whenever it gets down below half charged”, you’ll never have to think about it.

So once again, this only matters when you’re long-hauling. But then it matters a lot, because it’ll have a major influence on how fast you get there.

Once again, the answer is complicated. This time I’ll cook the factors down into a list:

1. How far do you have to go? If the next leg is much less than your range (after all the corrections and adjustments listed above) then just charge up that much, plus enough for Plan B.

2. How fast can your car charge? Some of the older and cheaper electrics can barely soak up 50kW. Mainstream high-quality cars these days can use 100kW (up until 80% full, that is). The Porsche Taycan and Hyundai Ioniq 5, however, can both use more than 200kW and this is what I’d expect from the whole next generation of electrics.

3. How fast can the charger pump electrons? In my recent Trans-Canada trip, I encountered “fast” chargers at 50, 100, 200, and 350kW.

The take-away · In areas of the world with a decent charging network, pretty well any reasonably recent EV will long-haul. Probably the most important quality-of-life factor is your charging speed.

The areas of the world without a decent network are shrinking and will shrink lots more, quickly.

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