July 9, 2013

I've been very interested in the ins and outs of rapid charging at a Tesla Supercharger station ever since they first announced the build-out of the Supercharger network. I've become even more interested of late, because I'm considering taking the 2013 Tesla Model S on my next bi-annual trip to the remote Oregon Coast to visit my parents.

The Supercharger network will loom large during this journey, and I need to know exactly what I can expect in terms of downtime at each stop.

Tesla's claims for charging speed are not absolute. Weasel-words like "up to" and "as fast as" pepper the discussion. One says a Supercharger will "provide half a charge in about 20 minutes." At the Supercharger unveiling they said they could "provide 3 hours of driving in as little as 30 minutes."

These claims are astounding, even if they prove to be half true in less-than-perfect conditions. But they didn't give me the specificity I needed to plan my trip.

So I headed to the nearby Hawthorne supercharger with our 2013 Tesla Model S, arriving with just 29 miles in the tank. My plan was to take screen grabs from the Tesla iPhone app at intervals as it charged, then put everything in Excel and graph them when I got home.

The route to Tesla's Hawthorne Supercharger shares a driveway with sister company Space X, and as my wife and I wheeled into the drive we could see the business end of a Falcon 9 booster rocket on the factory floor through an open bay door. The street address: 1 Rocket Road.

Two other Model Ss were parked and charging when we arrived at the station. I knew from Tesla owner message boards that the power feeding each 90kW charger is split between two adjacent spots; park next to someone on the same circuit and the two of you will be sharing that 90 kW. These folks apparently knew this, because there were open spaces between them.

But the spots they had chosen left us no way to get on a dedicated circuit of our own. We'd be sharing with one or the other of them, so we'd be dealing with reduced power until one of them left.

True to my luck at supermarket checkout lanes, I chose wrong. The car I hadn't parked next to finished and left the scene not 10 minutes after I plugged in. I quickly unplugged and moved to his now-vacant spot to have a full 90 kW to myself.

It paid off. The screen shots on my iPhone app showed a charging rate improvement from 184 to 277 mph. Over the next couple of minutes it rose to 292 mph. In case it isn't obvious, this usage of "mph" refers to miles of driving range added to the battery for a given hour spent on the plug. A Tesla Model S battery doesn't hold that much, so this is an at-the-moment charging rate.

After about 25 minutes of this, with the battery range standing at 159 miles, I decided to switch to extended range mode, figuring that I would need maximum range on the northerly legs of my trip to Oregon. I'm not sure if it was coincidence or not, but the switch to Max range came with a drop in the apparent charging rate to 230 mph. When I got home and studied the screen grabs more carefully I discovered the amp draw of the charging system had plummeted from 240 amps to 150 amps about the same time.

And then, about 15 minutes later, another Model S crept into the lot. In keeping with my supermarket check-out luck, he parked next to me, taking away my sole-status on the 90kW circuit in the process. My battery's charging rate sank to around 200 mph for the duration.

It's hard to know it the fall-off in charging rate was due to my selection of Max range status, the presence of another car or the fact that my battery was edging close to the full mark. Battery charge rates do tend to tail off near the end, but there were too many variables in play this time to sort it out properly.

In the end my battery reached 230 miles of range after 63 minutes, at which point I called it quits and pulled the plug. I'd added 201 miles of range during that time, a rate that works out to 190 mph. Slower than the best-case claims of Tesla, but this was far from a best-case situation. Besides, this is still lightning fast compared to other EVs, which charge at about 20 mph on a good day.

Interestingly, my actual observed overall charge rate was slower than every corresponding in-process value I'd captured on screen grabs from the iPhone app, even though it has access to the very same range and time data. I cannot explain why this is so. Perhaps it has to do with charging losses, the difference between the inflow through the plug and the actual power amount captured by the battery.

Whatever the case, here's the graph I made using the two measured quantities I was really interested in: range added versus clock time.

Though the charging rates fall far short of what the app reported at every step of the process, the ebb and flow reflected in the changing slope on the graph matches the sequence of events.

The shared charging for the first 10 minutes occurred at 160 mph. All-alone charging in normal range proceeded at 260 mph until I switched to Max range at 34 minutes, at which point it sagged to 180 mph. Finally, when the "new guy" showed up for the last 13 minutes (when, it must be said, the battery was nearing full) the charge rate dropped to 107 mph.

And maybe that's the point. The comings and goings of other cars is a normal situation. I'll never be able to control who parks where while our Tesla is plugged in. Power-sharing with another car parked next door seems inevitable, something I should account for when I estimate my idle time.

Tesla, for its part, has announced they'll soon be upgrading the Supercharger network, upping the rating of the chargers from 90 kW to 120 kW. The fine print on their Web site graph reveals that's where their "200 miles in 30 minutes" claim comes from, alone and unshared with other cars, of course.

However long I sit at the various Superchargers, I still need to plan on a three day trip if I try for Oregon this summer. The northernmost Supercharger is at Folsom, CA, and this experience suggests I need to plan for as much as 90 minutes once there to get the maximum 265 miles of range into the battery before I head "into the wild" off Tesla's grid.

That's important for me because from there I need to make it as far as possible before I spend the first of two nights charging off-network on conventional Level 2 charge stations at hotels or NEMA 14-50 outlets at campgrounds. Highway 101 doesn't have a lot of conveniently-located choices in places you'd care to stay with your family, so pickings are slim. And one of the best options is likely to be stoner-central for a massive reggae festival on the week we're considering.

On second thought, this exercise (and the reggae festival) has me thinking I should delay the effort until my December Oregon trip. By then Tesla's growing Supercharger network is set to bridge the gap to the Oregon border, and the new stations will most likely feature the new 120 kW equipment. I should be able to make the run at or near my usual gasoline pace, with just one overnight stop.

Dan Edmunds, Director of Vehicle Testing @ 5,976 miles