anyway to run front/rear windshield defrosters in EV mode w/out engine starting?

The rear windshield heater should run without activating the engine. It is all electric. Hence why your rear window has those black electric grid lines running around it. The rear windshield heater button also activates your side mirror heating elements. Look for them on the icon.

Unfortunately, the front windshield uses heat from either the engine or the HVAC system to heat. If there is not enough time for the HVAC system to heat up, the system will activate the engine to assist.

Similarly if you turn up the regular heater in your vehicle, it will activate the engine if the HVAC system has not heated up yet. I think past 72F the engine will activate. Depending on the heat and fan speed load required.

In winter, the engine will be the quickest way to heat up the cabin of your Prime. Or you can select pre condition in your charge schedule to have the vehicle use your last temperature settings before you turned the vehicle off.

This way your Prime will precondition your interior 10 minutes before your selected departure time. It will use the EV/Plug to do this. And the engine will not turn on. But only when plugged into a charger and there is a set departure time.

 

I had this issue too. I was determined to keep the EV mode on and keep the windows defrosted without using the ICE. I found I could do this by turning on the A/C for about a minute right when I noticed the window fogging in a few spots. I pointed the vents all the way up so the cold air wouldn't blow on me. This dehumidified air and defogged the mirror before it got bad. Make sure not to set it on recirculate. I don't know how well this would work in really cold weather. I live in southern CA and it was probably in the high 40's to low 50's. If your windows are really fogged-up and you can't see well, it is probably safer to just turn on the defroster and allow the ice to run, so you can see soon as possible.

I got tired of this process though and bought some RainX interior antifog spray. I applied it to my interior front windshield 11/11/22. Ever since, my windshields have been fog free without the need for the defroster, AC or fan. It is a little tricky to avoid streaks and smudges when applying. Make sure the windshield is clean first and follow the instructions on the bottle. I still got a little bit of smudges/streaks, but not enough to be distracting for me or affect my visibility. Before applying this to my car, I applied it to my bathroom mirror and it prevented fog from appearing on the mirror after a hot shower with the door completely closed.

 

I completely avoid the front defrost button unless it's an emergency. If I need defrost, or if it's below 12F outside and I don't want to use the engine, here's what I do:

0. Leave the climate in OFF state beforehand (if parked outdoors and it's below 12F, it would start the engine immediately if heat is requested)
1. On the touchscreen climate settings, select the defrost / feet combo setting
2. Manually set the fan speed (typically 1 or 2 works fine for me)
3. Turn OFF the A/C button
4. Turn OFF the Recirculate option
5. Set temperature to LO
6. Crank up the seat warmers and steering wheel (or wear winter coat and gloves)
7. (optional) Put on mask reduce humidity from my breath
8. (optional) Close driver and passenger vents manually by pushing them to the side, to avoid frigid air on your face

I totally feel like a crazy person but it's an obsession at this point to avoid starting the engine on short winter drives. (I really should switch to a full EV!)

 

Nope

 

You are not crazy. I did exactly the same when I owned my PP. My routine was very similar.

 

I live in Minnesota and almost never use defrost for the reasons given. I often open the front windows an inch to let humid air out and dry air in. Usually, a couple of minutes is adequate.

 

This annoys me as well, but I think there's a sound, logical reason the car does this.

In any car with AC, windshield defrost/defog turns on heat and AC simultaneously. The AC dehumidifies the air, which is then heated and passed over the windshield.

But in the Prime the heat pump IS the AC. You can get AC out of it, or you can get heat, but not both at the same time.

So to supply dehumidified heated air to the windshield,it needs to use some other heat source. I'm guessing that the car cannot effectively perform a standard dehumidified-air defrost without running the ICE.

It can, however, blow regular-old non-dehumidified hot air over the windshield, as one of the setting on heat. Without running the ICE.

When I first got the car, I wouldn't turn the heat on, here in Virginia. And then the windshield would start to fog up, and you know the rest.

Now, I've gotten into the habit of running the heat, as soon as I start the car, using the setting that blows heated air over the windshield,and down to your feet.

If I do that, up front, and take the mileage hit, the windshield stays defogged, and the ICE does not run.

Also, as stated above, you should be able to run rear defrost without ICE start. Not stated, the heated mirrors are on the rear defroster circuit, so if you need to defog your outside mirrors, hit rear defrost.

 

Sometimes, here in the north, the air inside is dry but there is literal ice building up on the outside of the window - while driving. AC won’t help that, but you really do need to get the ice off in a hurry…. and I’ve always assumed that’s why the gas engine and front defrost are linked. As mentioned above, cracking the window a bit to let humid air out and dry air in usually does the job for me in cases where, say, I get into the car after a workout.

 

Actually, the way the A/C+Heat pump is designed in the Prime, you can get heat and cooling at the same time. The compressor always feeds refrigerant first to a hot condenser coil in the cabin. But a damper controls whether any air blows across that coil. In Heat mode, that damper is opened; with cooling, it is closed. The refrigerant then passes through the outside coil in the engine compartment, but an expansion valve makes that coil function as either a (hot) condenser for cooling , or a (cold) evaporator for heating. There is also a (cold) evaporator in the cabin which is used for cooling, which is the last step before the refrigerant returns to the compresser. For dehumidification without cooling, the outside coil is used in cooling mode so the inside evaporator is cold and provides dehumidification, but air is also directed over the interior condenser to heat it back up. So for milder defogging, the gasoline engine is not needed, but if you want to defrost, then you need some serious heat, and the gasoline engine is fired up.There is also a coil for the engine coolant, and dampers control whether cabin air flows over it.

In traditional heat-pumps, the refrigerant flow is reversed between heat and cooling function, but the flow is always the same way in the Prime. They just play games with expansion valves and dampers to switch between heat/cool/dehumidify.