Charge Anxiety

Driving Out of my Comfort Zone

Nearly every EV owner I meet has experienced charge anxiety. It’s that dread that one feels as you stare at the dashboard and watch your battery charge trickle towards zero.  It appears that your chance of feeling charge anxiety increases in direct proportion to the following four metrics: the smaller your battery, the older your car, the busier you are, and the greater the distance between chargers.

I’m pretty lucky. My car has a pretty big battery. Supposedly it can hold 77 kWh and take me over 300 miles on a single charge.  Plus, since the car is basically brand new. the battery has yet to significantly degrade and should still hold as much charge as when it left the factory.  I’m reasonably busy, but I usually have a couple hour-long windows each week where I can get a good charge. And rarely have I been more than 20 miles from a charger. Since I’ve owned the car, I really haven’t been seriously worried that I might get stranded somewhere with empty batteries…

until this past weekend.

On Saturday morning I had my first real charge anxiety experience. My brain woke me up early.  It was crunching numbers in an attempt to figure out if we could actually make the trip we had planned to Prescott, Arizona. Looking back I was feeling a bit overdramatic, but in the wee hours of Saturday morning I felt like I was a NASA astronaut planning an Apollo mission.

In my head I knew four things. 1. I had 70% charge in the batteries. 2. There is a charging station in Prescott a little over 100 miles away. 3. The cell service along major sections of the route is pretty spotty. And 4. If my calculations were wrong and we didn’t actually make it to the charger before we hit empty, the trip would get ugly quickly. It would be like we were on the dark side of the moon and there’d be no way to contact Mission Control for help.

OK, so it’s not as if we’d fly off into space and never be seen again. But when I woke up that morning I was desperate to increase my confidence that we’d make the trip smoothly. And I’ll admit that the thought of leaving the EV behind and taking our old SUV was very, very tempting.

Why was I so concerned about a 100 mile drive if my car was supposed to be good for 300 miles? As I lay in bed sweating that morning I knew there were two variables that I couldn’t completely pin down. First, the bulk of the drive would be at highway speeds. As I mentioned in my last post, our miles per kWh goes down when we drive at high speeds. How much? I was not exactly sure as we hadn’t done long highway drives yet. Second, Prescott, Arizona is 4200 feet higher in elevation than Phoenix.  I have noticed that even the elevation change between our house and school has impacted the efficiency of the car, perhaps by as much as 15%. The drive we had planned would basically be up a mountain, with a number of ups and downs along the way. 

I did some math in my head.  I knew that around town, if I was driving conservatively, I could average about 4.3 MPkWh.  With the combination of elevation change and highway driving, what if, as a worst-case scenario, I only get 3 MPkWh?  That would allow me to drive 116 miles before I hit 20% on the batteries, the point at which the car dashboard starts blinking red warning lights.  That should be just about right… if that was indeed the worst-case scenario. 

Before I really forced myself to decide whether or not to take that plunge of faith, with the likelihood that I would give up hope and fire up the gasoline powered car, I had one more thought. It occurred to me that about a third of the way to Prescott, there was an outlet mall. If I was deciding where to put public EV chargers, an outlet mall was an ideal location. So I pulled out my charger app and sure enough, my hunch was right. The Walmart parking lot in the town of Anthem, Arizona, is home to four chargers.  That was the safety net I needed to relieve my anxiety.

My wife later confided that she had reasons to be nervous that I hadn’t even considered.  Her direct reference points for battery reliability are her mobile phone and laptop computer and both have been acting very strangely lately.  Her previous phone would sometimes read 60% one minute and then only 20% a half hour later.  And while the battery life on her laptop used to be maybe as much as six or seven hours, it now sometimes only retains enough charge to stay unplugged for 30 minutes.  She knows that these behaviors mean something is wrong. But she reasoned that if the batteries she relied on daily could behave so erratically, our EV could very likely encounter similar problems. That uncertainty would indeed up my anxiety levels as well!

So, how did it go?  Thankfully, we did not get lost on the dark side of the moon, nor did we get stranded on the side of a desert highway.  But we did find that we our fears were entirely legitimate.

The entire drive from home to Prescott (including a stop in Anthem to top up the batteries) spanned 117 miles, just a bit longer than I thought it would be. As for the efficiency… it looks like the “worst-case scenario” I calculated that morning was slightly better than what we actually experienced.  When we arrived at our destination, the car calculated that we averaged 2.96 MPkWh. 

At times, however, it was significantly worse.  As we wound our way up Black Canyon, the car was calculating a trip average of 2.5 MPkWh and under.  If we hadn’t topped up our charge in Anthem, my “charge anxiety” would have been through the roof.  At 2.5 MPkWh, I think we would have ultimately made it to Prescott, but we would have gotten to the charger with maybe 5% left in the batteries.   

We found our twin at the Prescott charging station.

The drive home was significantly more relaxed. We charged to 80% at the start of our trip and then got a significant gravity boost as we coasted down the valley to Phoenix. For the 104-mile return trip from the charger, we averaged 5.0 MPkWh, leaving us with plenty of charge when we got home.

I figured that elevation change would affect efficiency, but I had no idea that the impact would be so large.  According to the size of my batteries, heading south on the route I took I can go 385 miles on a full charge. But heading north I can only go about 228 miles on a full charge. That’s a full 40% less when I’m driving up the type of elevation changed I experienced.

Gasoline powered cars also use more energy to go uphill. But it’s never seemed significant enough for me to consider that factor when I thought about how much gas I needed to completely a specific trip.  With the EV, however, I’ll definitely be thinking about not only distance, but elevation change as well.  It clearly has a huge impact.

The trip has also convinced me to pretty much ignore the “miles to empty” reading that my car proudly displays on the dashboard.  When I’ve watched it on past commutes, I’ve often thought that the estimate drops maybe 40% faster than the miles actually covered. For this particular trip, I sat down and carefully did the calculations. Over the course of the 78 mile drive from Anthem to Prescott, my car’s “miles to empty” meter dropped from 284 to 140. That’s a change of 144 miles, which means my car was nearly twice as optimistic as it should have been.  While I like to think of myself as an optimist, I do believe that “blind optimism” can be a very dangerous thing.

Driving home from Prescott, the car’s range estimate was perhaps the most accurate it has ever been. I covered 104 miles while the “miles to empty” reading changed by 100 miles.  As near as I can tell, my car bases its range estimate on about 5.0 MPkWh, an efficiency I would say I’ve reached on maybe 10% of the drives I’ve taken in the car. 

So thanks to our weekend getaway I have learned a very important thing about my electric vehicle. It’s “miles to empty” meter works very well… but only when driving down a mountain.