Hacking temp sensor to skip ICE warm-up in negative temps?

The heat pump won't be able to make any heat when it's that cold. The minimum is around 12 degrees F. If you want to drive in colder weather in EV mode, you can drive with the heat off. You can use the seat heaters, rear defroster, and heated steering wheel. I've done it and I don't particularly recommend it. You might have to open the windows to keep the windshield from fogging up from the moisture in your breath.

Also note that if the battery temperature falls below freezing, the battery current in EV mode is severely limited and it will be very hard to keep the engine from starting. If the car is plugged in, the battery heaters will keep the battery warm enough for up to 3 days, and then the heating stops.

 

At 0F, just push the HV button to start the car in the HV mode. You are making the most out of the engine, both traction and heat. There is no point in trying to run PHEV as if there is no engine on the car. Besides, running HV mode on PP is probably more efficient and saves money than trying to run it EV mode at that temperature.

 

The battery cooler only runs when the battery temperature is over around 95F. It will ask when you park and you should always say yes. The setting in the menu is basically whether it will ask or disabled the cooling cycle all together, which is probably bad for the battery. The battery cooler basically runs the AC full blast for 30 minutes before charging.

The battery heater is automatic and runs in cold weather to keep the battery at around 34-40F (I can't remember the exact temperature range it regulates to). You should keep it on in the menu, otherwise charging in cold weather could be extremely slow, and if the battery is cold the car is more likely to start the engine instead of letting you use EV mode.

The battery heater only runs when plugged in. It will run before charging and it will keep the battery warm for up to 3 days afterwards while plugged in. In Canada and Alaska it might run for up to 30 days (the wording in the manual is vague or poorly translated). As far as I can tell, it only runs when plugged in, not while otherwise parked or driving.

When the battery is cold, the charge and discharge current is limited. I've gotten the battery a few degrees below 32F a couple times. In one case, the Hybrid Assistant app showed the maximum power the battery could provide was around 30kW. That means the power in EV mode is limited to around 40hp instead of the usual ~90hp. Toyota doesn't want you to have to worry about that, so the car is programmed to start the engine if you even get close to calling for that much power. You could slowly crawl around town, but if you try to accelerate quickly or get on the highway, the engine will start.

The charge current is also limited when the battery is cold. So long braking, such as going down a long hill, will cause the engine to start to provide engine braking so you don't overheat the brakes.

There are also other random conditions that will start the engine in cold weather, even with the battery above freezing. For example one is setting the cruise control with the battery >80% charged in cold weather. I'm pretty sure that's a software bug because it's been discussed here and no one has a good explanation for it.

 

bevs must do some serious battery conditioning to work in those temps

 

You can also just leave it in EV mode. It works but I'm not sure it's the best approach. I did about 60 miles of driving the other day in 0 degree F weather in EV mode. The car cycled the engine on and off to keep the coolant temperature between 120-140F. Eventually by about 50 miles into the trip it used up all the EV range. The engine never really fully warmed up until the car switched to HV mode since it kept cooling off during the EV portions of the drive. I made a few stops and left the car on idling for a while (it literally idled the engine to keep making heat), so I'm not sure what my overall gas mileage was. I suspect HV mode may be better because it will let the engine get warmer and it will run the engine at more efficient operating points instead of arbitrarily when the temperature drops.

If you want to take full advantage of regen braking, you need some "space" in the battery since the charging current is limited when it's fully charged. So in very cold weather it might be best not to charge the car. But then the battery heater won't run, so I wonder if there is a way to trick the car into heating the battery without charging?

 

Yap, that was my observation when I was driving daily commute in sub-zero temperature in the morning. It just wastes EV range and gasoline for no good reason. When it's that cold in the morning, I just hit the HV button to turn on the engine as soon as I start the car for my morning ~18 miles commute. I just saved the full EV range for my afternoon commute back home. By then the temperature gets up to the 20s making it possible to drive home 100% EV.

 

Might want to check the traction pack internal resistance after the pack is cold soaked down below 32 F.
DrPrius shows battery temps for 3 of the 5 stacks of the Primes traction pack. DrPrius also shows a general graph of the individual cell internal resistance.

It's only hear say about when the traction pack heaters actually are on, Professor John Kelly describes the traction pack heaters as on when the traction battery temps are 32 F or below.

I've noticed when using the Traction Pack to run the car the battery temps also rise. How much the Traction Pack temps rise depends on how many Amps are used while driving. Under 20 Amps will only raise battery temps a few degrees. While 50 amps continuous will raise those temps 20 or more degrees.

A driver, if patient enough, can learn how to get the car to run in EV only at temps below the 14 F the owners manual recommends as a cutoff temp for running in EV mode. BUT, it will not perform like a regular car in EV only mode at those very low ambient temps.

Maybe that is why they put EV Auto as a drive mode. Guessing to be most efficient to keep the coolant temps warm enough to run the cabin heat while still in EV mode and vice versa on really hot days too for optimal A/C.

 

The charging light comes on when the battery heater is running. I have yet to measure how much power or energy it uses.

It keeps the battery a couple degrees above 32 degrees F, and there is some hysteresis. I haven't checked enough to see what the temperature range is, but yesterday it was running and hybrid assistant indicated the battery was 42 degrees F at around 15F outside. That's the highest I've seen it go. I unplugged to add my kill-a-watt meter, but it didn't start heating when I plugged in again.

The ambient temperature is irrelevant to how the car performs in EV mode. Battery temperature is all that matters, and there's a steep transition around 32F. Above 32F everything is mostly normal. Power output and regen limits are only slightly reduced compared to warmer conditions. Below 32F, power is severely limited especially at low state of charge. The only caveat is, below 12 degrees ambient you can't use the heat otherwise the car will start the engine.

EV Auto works a little differently than how the car behaves in cold weather with the heat on in EV mode. But the behavior of EV Auto seems to depend more on temperature than EV mode and whether the engine is already warmed up. In my experience EV Auto will turn on the engine when there's higher demand for acceleration and keep it off otherwise. In EV mode in cold weather, the engine cycled on and off based on temperature regardless of whether I was accelerating or coasting, and that's probably not the most efficient way to run it.

 

NO. At those temps, it's efficiency is ZERO.
That is, it will make NO heat even if the electronics will let it try........which is doubtful.
Bad idea. Really BAD.

Bad things will happen that you don't normally think about if you have a human being inside a tin can with no heat when the outside temp is around 0F.
I know this from several years of owning original design VW vehicles.......with no heat.......in areas where it got REALLY cold. Ice forming on the inside of the windows was just one of the problems.

 

BAD

 

Note how @john1701a does his sub-zero commute in EV mode. The ICE starts and stops as needed to keep the coolant warm and provide cabin heat because the air is too cold for the heat pump to pump heat.

 

Bug heater boxes
Bad
you guys crack me up ... It must be the snow, ice, wind this month.
I'm glad my normal drive only has 3 or 4 50 amp hills at 20 mph.

I love when DrPrius voltage graph turns all red at 200 amps - - - -
Remembering the post about the Prime being a dog when tying to merge onto the
S Cali Freeways.....

Fire Bad - Tonto Tarzan and Frankenstien

 

Absolutely!

The only good fix was a gas heater; noisy as hell but better than freezing to death (had one when stationed in northern Maine..... brrr)!