Yesterday, I had to drive into the Sierras to celebrate a friend’s birthday. Due to a confluence of events (which included putting two new colonies of bees into hives), but mostly my lack of attention to the actual distances and the amount of climbing involved, I found myself in the predicament of not knowing whether I would make it to the charging station, BEFORE running out of juice.
It started at the Folsom Supercharger – my journey actually started in the Bay Area, but the important part started in Folsom. As I was headed up to Nevada City, some 50 miles away, I had to do a quick mental calculation as to how much energy I was going to need. I was already late for the party so I didn’t have the luxury to just “top it off”. I just needed enough to get me there and then back to my next destination, the Supercharger in Vacaville - another 90 miles. I put enough energy into the car so that it indicated a range of 160 miles. I knew my margin wasn’t that big, but I always had the same Folsom Supercharger to fall back to, if I needed to.
Having never been up to Nevada City, and also having never driven any electric vehicle up a mountain range, I wasn’t aware how much energy was going to be used just to overcome the climb to Nevada city. When I arrived at my friend’s, the car indicated that it had 80 miles of range left, while the distance I had left to travel later was 91 miles.
Seeing that I had done a lot of climbing to get there, I assumed I could gain some range back as I went downhill, both from regenerative braking, but mostly from just using less energy when going downhill. I didn’t worry myself – and I didn’t want to bother the host for an extension cord and outlet to charge at a measly 3-4 miles per hour.
When I left the party, at 11pm, some of my expectations turned out to be correct: From Nevada City to Auburn, and from Auburn and on to about North Highlands, north of Sacramento, I lost less range than I gained in distance travelled.
- At Aubrun: 60 miles of range left with 64 miles to go
- At Rocklin: 52 miles of range with 52 miles to go
- At North Highlands: I had 45 miles of range and 42 miles to go
I was quickly approaching a point at which it would not be possible for me to turn back towards Folsom’s Supercharger, as my backup to Vacaville. I continued to monitor the displayed numbers, now also monitoring my 5, 15 and 30 mile usage averages to make sure I was still gaining on range vs distance left. This is what that display looks like, although this image isn’t reflective of my trip:
Energy Display (Click for more info)
After Sacramento, as the predominantly downhill freeway transitioned to more consisted flats, I noticed that my range was dropping faster than my distance travelled. One thing that has been an unknown to me with this car is its efficiency at different speeds. I know that when my wife drives it, which is the norm, she gets much more distance out of each overnight charge than I do. I like the powerful acceleration in this car – it’s way better than my 300 horsepower V8 – and utilize it, so I typically get 30-40 miles less per charge. What this car doesn’t do, and it should, is to give you some indication of “burn rate” comparisons:
- at current speed, you’re using x kWh (and your range would be m miles)
- at 65mph you’d be using y kWh (and your range would be n miles)
- at 60mph you’d be using z kWh (and your range would be o miles)
- at 55mph you’d be using w kWh (and your range would be p miles)
It’s hard to figure those numbers out by observation since they are impacted by the incline of the road, the wind, and even the car ahead of you.
Over the next 40 miles, I reduced my speed to 65, then 60 and then 55 in order to keep the range estimate within a mile of the distance left to go. But what helped even more, was drafting behind trucks and buses. I first tried it with a Safeway truck, but I found that the driver’s speed and desire to pass cars, had me accelerating and decelerating too much so I gave up. Next I drafted a large bus, which was fine, except that the bus driver obviously didn’t like me so close behind, and kept trying to change lanes to let me pass, tap on the brakes to see whether I was going to get out from behind and eventually I gave up, and let the bus pull away and leave me behind going at 60mph (it’s a bit surreal how fast other cars pass you when you’re doing 60mph). A double tanker provided me with 10 minutes or so of drafting, at a nice constant speed and that helped tremendously.
I finally made it to Vacaville with 1 mile of range left, and by the time I was parked at the charger, I was at 0. Here’s what that looks like, since I assume no sane person would ever go so low to witness it for themselves:
This is my energy usage profile for the last 30 miles. Not bad?
If you’re wondering if this was a very stressful situation, I have to say that it was, only until I told myself that getting a flat-bed tow-truck around Vacaville won’t be the end of the world and won’t cost that much – and I do have tow insurance. I might have to sleep in the car while wait for the tow :-) With that decision made, the stress level dropped and this whole thing became more of an experiment.
By the way, for this of you who haven’t witnessed this, it is incredible how quickly the Supercharger charges the car - at a rate of 110kWh! 361volts at 300+ amps! At one point the car was literally humming at a roar (I believe from the fans cooling the battery cells) – and you can also hear the hum from the big transformers nearby that power the charging units.
(This is image is from the Tesla iPhone app).
One thing I was hoping to learn from this experience, and maybe I should be thankful I didn’t, is how the car behaves once it is completely out of juice. I suppose that when it says 0 miles of range left, it doesn’t actually mean that the engine will shut off and the car will coast to a stop. I sort of expected a message to the effect of “You are now running on reserve power. Please find a safe spot to pull over and park the car. Here are phone numbers for tow operators and mobile chargers closest to your location…”.
If you’ve experienced a complete shutdown due to battery depletion, I would love to hear about the car’s behavior leading up to it, and after. (I’m assuming there’ll be some power left for locking and unlocking the car doors, etc.)