Cold weather driving sucks up the battery

[Braumin]

As I said, Tesla did some neat things to help the HP work when the temperature gets very low, but given some comments, when it is very cold the heat from the motors does not appear to be up to the task. Interestingly, if the heat from the motors was in fact sufficient why would a HP be needed at all? The HP adds complexity and cost while simple using heat from the motors is free heat - just like the free heat from an ICE car engine.

So, good question!

BTW, I am referring only to the heat generation side of the HP; the HP also serves the very important role providing AC.

It doesn't use the motors to generate heat. The heat from the motors can and does go into the battery, which the heat pump will scavenge if there is enough temperature in the battery.

The heat pump will scavenge heat from the outside down to -10°C. After that, (and likely with some % before) the compressor will be run inefficiently to generate heat on its own. This is the same compressor used for cooling. In this mode it will put the refrigerant through the evaporator and the condenser inside the cabin, so the net heat output is equal to the power used by the compressor only.

There are also some low voltage (12V) heaters as well but they would not be able to heat a cabin at -30. They are just to assist.

 

[EastCoast85D]

One of the things I had to learn fast was driving in cold weather when I first bought my 85D. A couple things I do is pump the air pressure up to 50psi (45psi should be good on a M3) to eek out that little extra efficiency that can be used for heating instead of overcoming rolling resistance. I have also coated my interior windows in anti-fog spray and run recirc for the heat. I also keep a blanket in the car for passengers in case I have to run the heat lower then the 72 I normally have it set. I have a couple other ideas to help with winter driving but haven’t implemented them yet.

Also air gets denser the colder it gets so you will be fighting aero drag more in the winter. I drive the speed limit in winter and drive 5-10 over in the summer in comparison.

I was able to get 300wh/mi in my car which is rated range on my car in sub 20 degree F temps the other week during that cold spell.

 

[ATPMSD]

@Braumin

At what outside temperature do you find the car’s HP based system to become ineffective?

 

[Zacster]

When I pre-heated the cabin it was out on the street on both ends of my trip and there was no charging available, that was a problem. I was heating the cabin before we left on these short trips as it was 3F with winds at 30-50mph, so pretty damn cold. For that matter the door handles would stick too from the blowing snow. It was just a total mess. The blue snowflake was there and never went away, an indication that it was just too cold. It didn't surprise me that the car didn't do well as I know I never gave it a chance to heat up. If I were home I'd have plugged it in and let it heat up off the house power. I guess I could have asked my brother-in-law to let me put it in the garage and plug in off the 115v but that would've meant he needed to clear out the space.

Anyway, I'm home now, the temps are in the 50s/60s which is really warm for NYC, and all is well with the car. Well almost, I got in a fender bender before this little trip and that is going to cost $$$ for the insurance company.

Oh, another thing that was just posted above, my tires, which were at 42 when I left before it got cold, came down to 38psi and the tire indicator came on from the cold too. I put them back up to 42 and they must be above that now in the warmer temps.

 

[Braumin]

@Braumin

At what outside temperature do you find the car’s HP based system to become ineffective?

Never is the easy answer. I have driven on the highway at -30°C. Lots of heat. More than most gas cars that struggle to have any cabin temp when it's really cold.

Even when it's really cold the car warms up pretty quickly. It DOES use a lot of power once the heat pump is running in heat generation mode, but that's to be expected. It is no more efficient than the pre-heat pump cars in that mode since it is just wasting power to make heat, just like a resistive heater would.

I do wish they had left the high voltage resistive heaters for supplemental heat though. When the compressor is being worked hard, it is very loud and I do question the longevity of the compressor when it is working that hard. If it still had the resistive heaters it could use them too, although that adds a lot of complexity and extra power cables for what is really not needed most of the time. It can get -30 to -40 here for a month stretch, but most places are not so unfortunate.

 

[Braumin]

Also wanted to point out that the heater uses less of the extra power than I thought. When they added the new energy graph it now splits the power for HVAC away and lets you know what is used for motion. When it is really cold, as much or more of the extra power used is just because the air is denser and harder to drive though, the tires may not be rolling as easily (especially when there is snow on the road) and bearings will be harder to turn, etc. When you look at the extra power used (compared to "rated power consumption of course) the extra power used for driving is usually higher than the extra power used for HVAC.

 

[AAKEE]

Teslas patent says ”lossy mode” on the heatpump From -10C and below.

It is not easy check without knowing the battery temp*.

If the battery temp is above 12C the heat pump use this energy until the battery temp is 12C (or slightly below) To heat the cabin. If you charge the car (inside The garage) just before the drive or did supercharge the consumption will be very low (not much above summer consumption) when the car uses the excess battery heat to heat the cabin. This, even if the ambient temp is-20C.

The car use the excess heat energy until the battery is about 12C (I have seen 11.8C quite a few times), then the var stops using the battery heat and either use outside heat with the heat pump(if ambient is about -10C or higher) or use the lossy mode (if the ambient is below about -10C).

When the heat pump stops stealing energy from the battery the battery heats again, from internal losses and from heat losses in the motor(-s) supplied to the battery.

Driving with a warm battery that cools of so the heat pump starts in lossy mode will cause a noticable difference in consumption. The difference can be average 200Wh/km with warm battery to 250Wh/km with lossy mode, it could be even more in really cold weather.
I have some data logged about this.

In ambient above -10C the heat pump seems to work fine and the consumption is clearly lower even when the battery reach 12C and the heat pump use ambient outside air to heat the cabin.

A home charging session with batt temp of +25C will be enough for supplying the heat pump at about 45 minutes at -20C ambient if the car hase been in a heated garage.

*) I have Scan My Tesla and a screen in front of the steering wheel showing, among other things, the battery temp.

 

[AAKEE]

.

The heat pump will scavenge heat from the outside down to -10°C. After that, (and likely with some % before) the compressor will be run inefficiently to generate heat on its own. This is the same compressor used for cooling. In this mode it will put the refrigerant through the evaporator and the condenser inside the cabin, so the net heat output is equal to the power used by the compressor only.

See my answer above about the heat pump using the battery heat. As long as the battery is warm enough ( +12C) the heat pump will use the battery excess heat before going into lossy mode.
Charging late/just before a drive is a smart thing in winter time, as the heat losses from the charging can be used instead of battery SOC to heat the car. (But not if parking outside with a very slow charger, like 2-3kW as a big part of the charging energy will be battery heat. In that case charging asap on arrival could be the smartest, to start charging when the battery still has some heat left.)

There are also some low voltage (12V) heaters as well but they would not be able to heat a cabin at -30. They are just to assist.

The main reason seem to be making it possible to differ the temperature on the driver and right side front passenger.

I lost my heat pump function in the beginning of a -11C 250km drive. It seems like these did not supply any noticable heat. Putting the hand at any outlet was not very pleasant.

 

[STS-134]

Never is the easy answer. I have driven on the highway at -30°C. Lots of heat. More than most gas cars that struggle to have any cabin temp when it's really cold.

Even when it's really cold the car warms up pretty quickly. It DOES use a lot of power once the heat pump is running in heat generation mode, but that's to be expected. It is no more efficient than the pre-heat pump cars in that mode since it is just wasting power to make heat, just like a resistive heater would.

I do wish they had left the high voltage resistive heaters for supplemental heat though. When the compressor is being worked hard, it is very loud and I do question the longevity of the compressor when it is working that hard. If it still had the resistive heaters it could use them too, although that adds a lot of complexity and extra power cables for what is really not needed most of the time. It can get -30 to -40 here for a month stretch, but most places are not so unfortunate.

I've not ever heard the compressor run full blast for more than 2-3 minutes after starting the car, although I haven't drive in < -10°C weather. But I would think that the system could avoid using the compressor as a mechanical heater after a few minutes by stalling the motors and using them to generate heat, then scavenging that heat to dump into the cabin or batteries.

 

[AAKEE]

As I said, Tesla did some neat things to help the HP work when the temperature gets very low, but given some comments, when it is very cold the heat from the motors does not appear to be up to the task. Interestingly, if the heat from the motors was in fact sufficient why would a HP be needed at all?

There is two factors, even if the heat would be enough if we calculate the energy/power it might be a problem that on an average drive in winter you never get the battery to reach 30C. I would say at least 30C batt temp would be needed to be able to deliver 22-25C air into the cabin to reach 20-22C cabin temp (as there is temperature deltas at any heat exchanger).

So without a heat pump that can steal heat at down to -10C and deliver it at a pleasant temprerature, we can not use battery heat except after a Supercharging session and thern only for a brief moment.

The HP adds complexity and cost while simple using heat from the motors is free heat - just like the free heat from an ICE car engine.

So, good question!

BTW, I am referring only to the heat generation side of the HP; the HP also serves the very important role providing AC.

Without the heat pump we could not use the free heat generated in motors and the battery.

 

[AAKEE]

I've not ever heard the compressor run full blast for more than 2-3 minutes after starting the car, although I haven't drive in < -10°C weather. But I would think that the system could avoid using the compressor as a mechanical heater after a few minutes by stalling the motors and using them to generate heat, then scavenging that heat to dump into the cabin or batteries.

At -30C the heat pump sure run in lossy mode for long when the battery is cold. Its easy to feel the vibrations in the steering wheel.

 

[Despized]

In my experience, in town in low temps will cause the vehicle to crank up very often to warm up the battery/cabin from trip to trip if short enough. During these periods I see my 2023 Model 3 RWD hit up to 600Wh per mile when warming up and operating around 300-450Wh per mile. Though if on consistent highway traveling it is much better in comparison.
In the example screenshots, I did 102 miles from 1 supercharger to another and maintained 80mph in -6F with wind you can see along with various factors. This was on 12/21/2022 in South Dakota. The best I have done in -21F was 142 miles with 1.9% left, but I maintained 65mph with -35F Windchill at the worst point, but did get warmer over time to -10F. This is because currently there is no supercharger between Fargo, ND and Watertown, SD.

This was near the start of the trip.

This was at the end of the trip. I also maintained 72F climate control on auto the entire time.

 

[Braumin]

That's a great pic @Despized

Shows exactly what I had meant in my previous comment. The cold weather causes more issues for the drivetrain than the HVAC. I was surprised about that prior to the new energy graph. All I knew before was that when it was cold, my power usage went up and I attributed it to my heater. Turns out the heater isn't as big of a part as I thought.

 

[AAKEE]

That's a great pic @Despized

Shows exactly what I had meant in my previous comment. The cold weather causes more issues for the drivetrain than the HVAC. I was surprised about that prior to the new energy graph. All I knew before was that when it was cold, my power usage went up and I attributed it to my heater. Turns out the heater isn't as big of a part as I thought.

80 mph is ”quite fast”, hence +39% consumption (nothing wrong woth that, just a note ). This offset the percentage that heating use downwards, when looking at heating compared to driving energy.

As i wrote before, supercharging with battery conditioning before set the battery temp very high.
This gives the heatpump a lot of surplus heat energy to use and in these conditions the car will only use slightly more energy / km and at summer.

In really cold weather, Id say I have seen the consumption be 50% more with a cold battery than with a recently supercharged battery.

So the graphs ate fine but not at all representative to a car with a cold battery.

 

[AAKEE]

Here's on example on winter driving with warm battery.

While -5 or -7C is not very cold, still averaging 161Wh/km at 79km/h average speed ( 50% was 110km/h ( 70 mph) highway and 50% was a 80km/h (50mph) country road. My car is rated 159Wh/km. With a cold battery the consumption had been

Cell temp was ~53C after the short charge and at home ~32C.
The consumption would not be much higher in very cold WX for this trip, as the heat pump could use battery excess heat.
Of course, cold air causes higher air resistance so we would see slightly higher conspumption due to that.

 

[T061]

That's a great pic @Despized

Shows exactly what I had meant in my previous comment. The cold weather causes more issues for the drivetrain than the HVAC. I was surprised about that prior to the new energy graph. All I knew before was that when it was cold, my power usage went up and I attributed it to my heater. Turns out the heater isn't as big of a part as I thought.

Perhaps the graph is a bit misleading. The graph tells us an extra 22.5% is used due to cold weather, but the breakdown is a bit misleading because it says only 7.8% was used by Climate, suggesting more losses by the air drag and tire drag. In reality, I think more was used for “heat” than what’s reported under “Climate” and here’s why:

Driving in the cold will cause access heat loss from the entire car, including the passenger compartment as well as the battery pack.

It's the same HVAC system used to heat the battery and the passenger compartment. You have the two motors that generate heat and the Heat Pump that moves the heat around + generates heat. In -6F, the Heat Pump can’t scavenge heat from the ambient air outside, so 100% of the heat needed to keep the battery+passengers warm is heat loss, again creted by compressor + two motors.

Certain amount of heat generated by the hvac system is reserved for maintaining the appropriate battery temp. This is likely reported under "Driving" segment and not under "Climate" or "Battery Conditioning"; the former shows "cabin heat energy" and the latter shows access energy used raising the battery temp for super charging.

What the graph does not show is how much extra energy is used to maintain optimal battery temp for driving. This is likely reported under "Driving" part of the energy graph which also accounts for things like extra air/road drag, drivetrain losses, etc.

Again, we know that the sum total of all additional losses amounted to 22.5%. We don’t get a breakdown of each component (this would likely be impossible or just an estimate).

I think in addition to 7.8% Climate losses, a large portion (maybe another 7-8%?) of the extra "Driving" energy is due to raising/maintaing battery heat (again, not captured by "Climate" tab), which is why I think the graph is misleading; it causes to people to think that only extra 7.8% energy is used for “heat” during the cold climate, when in reality it’s probably double that (if you consider the car as a whole).

Tesla could give us another tab to separate this extra energy used to heat the battery from "Driving," but that might again confuse people, so I’m not necessarily advicating for that (although I personally would like to see it).

Additionally, It may also be tricky to figure out exactly how much of the total heat generated is used to maintain the cabin heat and how much for battery heat, since both are part of the same "HVAC System." The part currently reported under "Climate" is a fraction of that total energy, and is probably just an estimate.

 

[Regaj]

... What the graph does not show is how much extra energy is used to maintain optimal battery temp for driving. ...

The car will actively heat or actively cool the battery once certain - fairly extreme - thresholds are crossed. But the notion that the car maintains some kind of optimal battery temp simply isn't true.

Tracking cell temps across seasons and across various driving situations shows the "normal" pack temperature range - which is to say, a temperature at which the car is neither actively heating nor actively cooling the battery - is quite broad.

 

[AAKEE]

The car will actively heat or actively cool the battery once certain - fairly extreme - thresholds are crossed. But the notion that the car maintains some kind of optimal battery temp simply isn't true.

Tracking cell temps across seasons and across various driving situations shows the "normal" pack temperature range - which is to say, a temperature at which the car is neither actively heating nor actively cooling the battery - is quite broad.

Yep.

Adding to your post:

- When parked down to -20 over night not connected
-When parked for three days at -20C to -15C connected to the charger:
There was not battery heating.

When driving in cold weather, if the battery is below about 0C / freezing, the car heats the battery but only briefly to get above (or about) freezing.
If the battery is above 12C the heat pump (on octovalve/heat pump cars) will use the heat down to 12C to heat the cabin.
After the battery us down to 12C the car let the battery heat from battery heat losses and engines heat losses until the battery is at about 17-18C, then the heat pump use the heat until the battery is at 12C again.

 

[STS-134]

When driving in cold weather, if the battery is below about 0C / freezing, the car heats the battery but only briefly to get above (or about) freezing.
If the battery is above 12C the heat pump (on octovalve/heat pump cars) will use the heat down to 12C to heat the cabin.
After the battery us down to 12C the car let the battery heat from battery heat losses and engines heat losses until the battery is at about 17-18C, then the heat pump use the heat until the battery is at 12C again.

What if it's > 12C outside? I would think it makes more sense to take heat from the atmosphere than from the battery.

 

[AAKEE]

What if it's > 12C outside?

Like in the middle of the summer?

I would think it makes more sense to take heat from the atmosphere than from the battery.

I havent really checked but if supercharged with a warm battery and 12C outside it seems to use the battery heat.
Probably it will use the warmer source, as it will increase the COP of the heatpump.