Before I create a rumor that isn’t true, let’s just all be clear up front that the power module in the Tesla Roadster is air-cooled – it’s the battery pack that is liquid cooled. In the three months or so I’ve been driving the Roadster, the difference was never particularly clear or apparent. Yesterday during the drive down to Orland, CA and the Berry Patch Restaurant I learned several things about the Roadster. Heat management is the topic of this blog. (Quick shout out to the fine folks at the Berry Patch – the only thing missing from fine southern hospitality was the sweet tea; thanks for providing the charger and being such gracious hosts during such a long stop!),
The first heat management lesson of the day was received as an aside from a fellow driver right before departure. He commented that he used to drive a Roadster, and one of the tricks to use during charging is to park so that the PEM is shaded, and leave the trunk open to help it breathe. The PEM (Power Electronics Module) by the way, is the part of the car that converts AC to DC and DC to AC, and other power management related tasks. For more details, you need an engineer instead of a user.
The thinking is that this would have helped me the previous day when preparing for yesterday’s stage. The charge on a WCEH charger kept running, but stopped putting power into the battery, during the heat of the day in Grants Pass. The lesson continued yesterday’s stage from Grants Pass to Santa Rosa.
I applied that new knowledge when charging at Yreka – the open trunk shaded the PEM nicely, and charging went fine. It was also about 11am with the worst of the heat to come.
The second heat management lesson of the day was along the drive to Orland, CA. It was hot, I had the AC on max. A situation on the road necessitated a quick burst of speed to steer clear of things. Everything seemed to work out fine. About a minute later down the road, the AC stopped with the cold, and started blasting hot outside air at me. It seems that temperatures were high enough that everything was near the upper end of tolerance. The burst of speed generated excess heat in the battery pack, and the car decided I could sweat – all the AC was taken for cooling the pack back down. I’ve read about that (the car will prioritize the pack over the cabin for heating and cooling if it can’t do both), and thought about it in the abstract. I also think it’s the right design, but it’s all much more real now that I’ve actually experienced it for myself. It was so hot, I think there were three of those episodes on the run down to Orland.
The real heat management lesson of the day though came in Orland. One of the Model S participants was there ahead of me charging. They needed about an hour to finish up their charge and get back on the road. They seemed to be charging fine, though maybe a bit slow (my perception). In retrospect, I suspect the Model S was using more power than normal cooling all the bits and bobs that it cools. The important observation though, is that Model S DOES actively cool all of that stuff. It might charge slower in the +100 degree heat with full sun exposure we had yesterday, but it does charge. That’s important.
When they left and I got the Roadster charging, it took me about an hour and a half or 2 hours to realize that the charging wasn’t happening the way I expected. I was expecting around 50 miles of range per hour of charging, but was probably getting closer to 20 or 30 (or less). The battery itself was fine, but the power distribution module (my new understanding / belief) was so hot that power couldn’t get to the battery efficiently. I had the trunk open, the PEM shaded; still was going slow. I did some math, and realized my 2h charging stop was on its way to being 9h (180 miles @ 20 miles/hour), and maybe worse. I wanted to be in Santa Rosa that night.
Here’s what I did:
Using a combination of “cleaning” the PEM with a damp cloth, to finally leaving water standing on the PEM and wet cloths draped on it, I kept the top of the PEM as wet as I could as a means of drawing heat out. The good news is that I even had this option – I could put my effort into cooling the PEM, and it made a difference in the charging. The bad news is that this is hardly a user-friendly approach to cooling – especially because I had to be out in the +100 weather to get this done. Does this put me into the innovator end of the adoption curve? I think of myself as an Early Adopter at most. Clearly this isn’t a cooling approach that will be usable in mass adoption!
I believe this was one of the key things that Tesla learned by building and supporting the Roadster in the wild, and suspect that this is why Model S uses liquid cooling for a larger chunk of the energy system.
Here’s a closer view of the left side of the PEM – you can see the water is evaporating on the close side of the PEM. Between the ambient temperature, plus the heat from charging, standing water was pretty much gone in 5 minutes.
On one hand, I’ve never read about anybody doing anything like this on the TMC Roadster forums. On the other hand, I feel like I’ve discovered something that veteran Roadster drivers have known for years, and now I’ve joined a club or something 🙂 Any Roadster owners who didn’t know about this, you can materially change the temperature at the top of the PEM with effort and water.
The end result was my 2-3h charging stop was more like 5 or 6 hours (and not the 9+ I think I was lined up for at one point), and I just barely beat midnight getting to my room in Santa Rosa. Then again – I did make Grants Pass to Santa Rosa through some brutal heat today. After what I’ve learned today, I don’t see how that trip in that heat, in an EV, would be possible without active cooling of both battery and power/charging systems.
I am looking forward to cooler temperatures closer to the coast. Santa Rosa feels more like 80 today – downright mild!