Gen5 PHEV AC intermittently blowing warm during battery cooling

Car: 2025 Prius PHEV XSE with 8000 miles on it, purchased 3 months ago at 5000 miles.

Problem: When the battery needs cooling (such as half an hour into a highway drive in EV mode), AC alternates blowing cold and warmish at 20-30 second intervals. This seems to happen when the scan tool indicates the battery is at or above 104 degrees Fahrenheit. Today that took me about 25 minutes of 65-60 mph driving, at an ambient temp of 78-80 degrees F.

Additional info:
- This isn't during extreme cooling load -- happened today when it was only 78 degrees F.
- Per the scan tool, the compressor is cycling on and off, maxing out at about 1 kW draw. So, it doesn't seem to be at its capacity limit at all: I've seen it draw 2-3 kW when cooling down a heat-soaked car on a hot day.
- I'm aware this model has a refrigerant-cooled battery -- but I'm very surprised cooling the battery causes such a hit to interior comfort, especially in such mild conditions! Such a system should presumably be able to cool the cabin evaporator and the battery at the same time?
- It cools fantastically well under all other conditions. Long HV mode drives have been fine (battery presumably doesn't get as hot.) EV mode drives at lower speeds / stop and go have also been fine (so it's not like it just happens when I've been driving for more than half an hour.) It cools down a hot car in 95 degree weather excellently.
- My scan tool doesn't show any codes. It's possible it can't talk to whichever module is responsible for the AC, though.

Does anyone know if this is normal behavior for this car? If so, that's unfortunate! If not, I probably have to convince my dealership I'm not delusional. Also, what battery temps do other gen5 PHEV drivers tend to see?



Thank you!

 

Neither the HVAC nor the car are in eco mode
Normal drive mode, normal HVAC mode.
Mild weather outside. And yet, something starts cycling the HVAC air supply temperature when the battery gets hot.

Attaching a recording of my drive. You can see I start out on local roads, with the windows open and no AC (enjoying the nice weather!), get on the highway, and as the battery temp gets up to 104 F (40 C), the AC compressor starts ramping up, and then goes into that weird cycling mode. That cycling continues after I run out of charge and go into HV mode.

Weather outside on this day was between 78 and 80 degrees F (26 C ish)

Unfortunately my scan tool doesn't seem to be able to pull any of the interesting data from the heat pump system. No EXV positions, temperature readings, or anything.

 

Hey, I got a PHEV for a reason, and that reason wasn't to run on gas when I didn't have to

I totally understand this approach if you're saving electricity for local driving at the other end of your highway drive. But if you're just driving on the highway, it seems to make sense to want to use up the battery, especially if you can charge at your destination.

I'm not so worried about it being hard on the engine when it switches over to HV, because I've observed that it still lets it warm up before wringing it out, even when you run out of EV range, as long as you're not doing any crazy acceleration.

 

I'm curious if anyone else with this same vehicle has noticed similar? @LRO, looks like you also have a 5th gen Prime, so I'd be curious whether you notice anything like this. Even better if you have a scan tool

 

You'd be surprised, with the air blowing right on you, even if the overall cabin air temperature doesn't change, it can be very noticeable if the supply air temperature itself changes!

I agree with you that it would make no sense to obsess about it if it weren't affecting comfort -- just trust the car to do the right thing. But in this case, it's doing something that feels not quite right to me.

 

That's a good idea, would be interesting to see if that changes its behavior.

I do think in general cars try to maintain a fairly constant evaporator temp. Certainly even on cars with engine-driven compressors and clutches that cycle on and off, the evaporator has enough thermal mass to keep the air fairly constant. So I'm surprised to see this behave differently, and only in this specific operating condition. One reason is that as soon as the evaporator starts warming up, you introduce quite a lot of humidity into the cabin from the condensate collected on the fins.

For what it's worth, I don't find "I'm sure it has quite a juggling act to do" to be a particularly compelling excuse for poor cabin comfort in a vehicle that initially came out in MY2023 -- EVs have had battery coolant chillers for ages (though rarely direct refrigerant cooling!), so getting this right feels like table stakes for building an electrified vehicle. It's also got individual refrigerant valves for the cabin evaporator and battery cooler, so I would expect it to be independently controlled (similar to a multi-zone mini-split system in a home.)

 

Thank you for looking anyway, though!

Home systems like this usually have a continuous valve for each unit, which both controls the amount of cooling _and_ acts as the expansion device in the refrigeration circuit.

I believe our Prii also have several continuous stepper-motor valves, but I wonder whether there's some on/off reconfiguration needed under some circumstances (it also has several solenoid valves as well.) Maybe that's playing a role.

I have a lot of the service manuals sourced from... somewhere... but not the New Car Features. Might be worth the $30 at least to satisfy my curiosity. The service manuals I have are mostly around individual sections of the system and how to troubleshoot, but are lacking an overview. There's a section for troubleshooting the air not being cold, and it doesn't mention "the battery might be hot" at all, which makes me think this behavior is unusual. I'm at work and the computer with all the good stuff is at home, so I'll check it later.

TCCN has videos on the gen4 Prime and the pre-2026 Rav4, which are both ever so slightly different than our gen5 PHEVs, so it's hard to know quite how they differ.

• gen4 Prius doesn't have the battery refrigeration, but does have the fancy multi-mode heat pump
• Rav4 has battery refrigeration but doesn't have a cabin condenser, uses the heat pump to heat the coolant instead.
• Not certain whether gen5 is just like gen4 + the battery refrigeration from the Rav4, or if it's totally different.

Weber Auto has a battery teardown, which shows the stepper-motor valves, but doesn't get into the rest of the refrigerant circuit.

 

It went into HV at the end due to running the SOC down, so that part’s normal at least.