This Article Is Reprinted Courtesy of Doug_G From the Tesla Motors Club. Thanks Doug!
I’m currently driving my Model S through its third Canadian winter. I thought it might be helpful to discuss some of my “lessons learned” about driving the Model S in the cold.
Feeling a little cold soaked…
Model S Performance in the Cold
You can walk up to your totally cold soaked car, hop in, and drive away. You don’t need to warm it up. It won’t chug and cough like a gas car; it’ll just glide away. The cabin heat will even come up pretty quickly. No fuss, no muss.
That said, there are a few differences in the way the car drives after it’s been sitting in a frozen wasteland (aka parking lot) all day.
When the battery pack is below freezing the car will not permit charging, as this will damage the cells. One side-effect of this is that you also lose your regenerative brakes. This is shown by the yellow dotted line on the speedometer. If the car has cold soaked below freezing you may have no regenerative brakes whatsoever. Pay attention, because the car will really really coast – this can be very surprising the first time!!! You’ll have to use the conventional brakes until the car warms up.
The dreaded yellow line
If the battery pack is extremely cold, you will also have a power limit. This isn’t usually much of an issue because if it’s that cold, it’s hard to go full throttle without spinning the tires. Cold rubber on frozen pavement doesn’t make for great traction. So this is really more of a curiosity than anything.
The battery pack has a heater. It’s necessary so the car can charge in the cold. When you start driving a cold car the pack heater turns on, and it can draw up to 6 kW. So can the cabin heater. Simply sitting at a stop sign you’re drawing enough power to go over 40 mph. Fortunately it does get better as the car warms up!
Cold versus Range
When the batteries are extremely cold, there will be a little bit of range loss, which is shown by a blue segment on the charge bar. As the batteries warm up that “lost” range will come back, so it’s not really lost! The car has to be very cold for this to happen.
Obviously energy needed for heating reduces your range. If the car is cold soaked this can have a big effect on your total range. Starting out with a warmer car can make a big difference to how far you can drive. We’ll talk about preheating in a moment.
You will also experience reduced range due to increased aerodynamic drag. Drag moving through the air is the dominant impact on the car’s range at any temperature – and it gets worse in the cold. Cooler, denser air is harder to push out of the way. There is also a small effect due to the tires having worse rolling resistance, but aerodynamic effects are much larger.
(Tip: Safely drafting a large truck has an even more dramatic impact on range in the winter. Do not tailgate; keep a safe following distance and you will still see improvement. Note however that strong crosswinds will adversely affect the effectiveness of drafting.)
The Model S cabin heater has two (hidden) modes. If the drive train is cold, all heat comes via resistive heaters, which can draw up to about 6 kW. That’s a lot of power.
As you drive the car, the drive train will naturally heat up. Once that happens, Model S uses the drive train coolant to help heat the cabin. Essentially it takes waste heat from the motor and inverter and uses that to heat the cabin. This makes a huge difference to the power consumption – a fully warmed-up car will only need 1-2 kW to keep the cabin warm even in extreme cold conditions. In comparison, the original Tesla Roadster needs 4 kW to keep its tiny cabin not-terribly-warm using only resistive heaters. This is a big advantage of Model S engineering that Tesla never talks about!
The upshot here is that cabin heater power consumption gets better quite dramatically after you’ve been driving for a while.
Seat heaters are essentially free. They use miniscule amounts power compared to the cabin heater. Use them liberally. Crank up the seat heaters, and lower the cabin temperature a little. You’ll save a bunch of energy for driving.
Shelter your Car
I have seen huge improvement in driving range after parking the car in an unheated garage instead of outdoors. That’s because even unheated garages are often significantly warmer than outdoors. Also being sheltered from the wind will help during preheating, reducing heat loss and increasing the effectiveness of preheating.
There are multiple reasons why a warm car is better: warmer cabin, warmer battery pack, and warmer drive train. As I’ve mentioned, a warm drive train means less cabin heater power usage. Starting with a warmer car always helps!
Range mode reduces the amount of power used by the heaters. This is most useful if you’re heading out on a long road trip and your car is cold soaked.
Once the car is fully warmed up Range Mode doesn’t really do anything – the power required to maintain battery and cabin temperature is modest. In fact if you preheat the car before you leave Range Mode doesn’t do much of anything.
Unfortunately Range Mode also operates when you’re preheating the car. As we’ll see below, preheating while plugged in prior to a long road trip will significantly improve your range. You want the battery pack and cabin nice and toasty. Unfortunately Range Mode gets in the way! It reduces heater power even though you’re plugged in. The result is less preheat and more range consumed!
Therefore my recommendation is that Range Mode normally be turned OFF. Use it if you have to start driving a cold-soaked car – in that situation it might just save your bacon. All all other times turn it off; otherwise you’ll do what I do and forget it’s on. Then you’ll slap your forehead when you jump in your car and discover it’s not as well preheated as you expected!
Preheating for Long Trips
If you preheat the car using the remote phone App, it will turn on both the pack heater and the cabin heater. The pack heater only switches on if you prewarm via the phone App, not if you turn the cabin heat on from inside the car.
If your car is plugged in, it will use electricity from the plug to do this preheating. Note however that you need a fairly powerful charging station; it could need up to 12kW and a 110V plug will only provide a tenth of that. The rest will come from your battery pack! You definitely want to use a 240V source.
The first advantage you’ll notice is that the cabin is toasty, and the windows have probably have melted out of ice and snow. The second thing you’ll notice is that you have regenerative braking at about half power instead of zero. That’s because your battery pack is a lot warmer.
Most importantly, because your battery pack is closer to normal operating temperature, the pack heater won’t have to work as hard. After driving on the highway for a while, simply drawing power from the battery pack will help keep it warm. That’s free heat!
The upshot is, preheating your car will hugely improve your initial power draw, and that means more range!
One caveat – and this is a biggie – if you can’t plug in, don’t preheat!!! You’ll just burn up power that would be better spent moving the car. In that case, hop in the car, turn on Range Mode, and drive!
Preheating for Short Trips
Short trips are a totally different animal. This is mostly about comfort and convenience. Preheating means a nice warm cabin, clear windows, and some regen right away.
If you’re plugged in, go ahead and do a full half hour preheat if you can. If you only have 5 or 10 minutes to preheat, go ahead and preheat. It will definitely take the edge off.
If you’re not plugged in then it’s a different story. You could end up using quite a bit of battery power preheating. This is not just running your battery down; it’s also using up a little bit of your battery’s lifetime. My usual compromise is to prewarm the car for 10 minutes to make the cabin reasonably comfy, then off I go.
I encountered the worst-case scenario for Model S range last winter by going Christmas shopping in extreme cold (below -20C). Drive 20 minutes, cold soak two hours, repeat all day. I used most of an 85 kWh battery pack in relatively few miles. The thing is, it’s still pretty hard to run the pack down to zero this way; the stores would close before that happened. But it is one situation where the 60 kWh pack might be a disadvantage.
Firmware updates to the HVAC controls (and the improved dashboard vents) have fixed most of the issues I’ve had with window fogging in the past. The Auto mode handles perhaps 95% of conditions very well. For the other 5%, some simple adjustments will help.
If your windows are totally frozen up, hit the defrost button twice so it turns red. This will melt out the windows. Don’t leave it on full power any longer than you have to, because the cabin will get way too hot. Then the normal settings won’t give you any heat until the cabin cools down, and your windows will fog up again!
For normal defrost, hit the button once so it turns blue. This will direct air up to the windshield. If you start feeling cold, use the manual setting to put air on both your feet and the dashboard.
Once the cabin is warmed up the fan speed automatically drops back, and in extreme cold and/or humid conditions this may not provide enough air flow over the windows. The solution is simple – manually increase the fan speed.
(Sometimes I’m lazy and just up the cabin temperature a bit, because the control is easier to access. But don’t do that if you’re pushing the car’s range limits, the fan is cheap; heat is expensive.)
In very extreme icing conditions, it may be nearly impossible to keep the window clear of freezing rain. This isn’t a problem specific to the Model S; I’ve had it happen in other cars (Hwy 401 in Ontario is notorious for this). The simple solution is to slow down a bit; this will reduce the cold wind blowing over the windshield, allowing the heater to raise its temperature enough to melt the ice. (Thanks Peter_M for the tip!)
Finally, if your windows are fogging don’t use the recirculate mode. Sure it will help keep the cabin warm, but it also means you’re circulating moist air over the windows.
One Last Thing
Snow tires. Just do it.
The Model S has excellent traction control and stability control – it responds very quickly and positively when the wheels start to slide. But you need to have good grip on the road, and that means good tires. It’s amazing how confidence-inspiring the Model S handling is on snow and ice – but only if you put on a good set of winter tires.
The Pirelli Sotozeros that Tesla sells aren’t very good. Actually they’re crap. They’re “performance winter tires”; in my experience this use of the word “winter” really means “late fall/early spring”. These tires are so bad in extremely slippery conditions that the car may not move at all. In fact the car doesn’t even try to move… the traction control doesn’t allow the wheels to turn and it just sits there. It’s something of a basic requirement for driving that the wheels turn! In this situation just turn off TC and go very easy on the accelerator. But the better solution is to avoid the Pirellis altogether!
I’m currently using Michelin XIce3 and they’re a huge improvement. Many people on the forum swear by Nokian Hakkapeliitta R2, which is a great hard-core snow tire.
Have Fun and Stay Safe!
And don’t forget to read my road tripping tips: The Rules of Model S Road Tripping – Blogs