SUDDEN drop of about 10MPG

Sounds more like the engine is doing fine, and the computer is limiting electrical participation- so it's forcing the gas engine to do more of the work.

The computer will limit the hybrid electric contribution if the battery temperature is too high. Normally the battery is cooled by a fan that draws air from the passenger cabin. The battery favors the same range of temperatures that most people like, so for the climate you describe it would likely be air conditioned down to 70-odd degrees.

 

Had this happen to me for that very reason.

You can tell if it's this as the battery fan in the cabin will be audible. Normally it won't be.

Or, maybe the battery fan has failed/blocked, but I think that triggers a warning.

 

what services did they perform at 24k?

 

If it's like the Prime, the A/C can be set to front only. You might need to turn that feature off to get enough cool air back to the battery in that kind of heat. Also, take the A/C out of ECO mode. I agree with @Leadfoot J. McCoalroller that it's most likely a hot battery. If the car sat in the sun for a while, the battery might be hotter than a pistol before you even start the car.

 

As above. Firstly I'd check the for the fan under the back seat. That they're 1) not covered; and 2) clean. It's not hard to remove the vent cover of the corner vent - there's a filter inside - mine was clean, but others have reported it dirty (dog hair I think).

 

The other thing - at 100 degrees, the A/C will be working fairly hard to keep you cool - but I haven't noticed that it's affected it by that much.

Over how many tank-fulls has that 10MPG been measured?

 

Disconnect the HV battery fan ?

 

This is an example of one time when having AC on increases mpg.

 

It might be the 25k service which involves just adding fluids and checking brakes and stuff. Could be they screwed with tire pressure. They always screw that up for me whenever I take mine in.

 

I'd like to see an A:B test on that. I monitored battery temps for a while, and saw next to no battery temperature drop, before and after turning on AC. It sounds plausible, that keeping the cabin cool would also drop the battery temp, and it's become a mantra here, but I didn't see confirmation in my (admittedly limitted) testing.

 

I'd be going back to the dealer.

 

As fast as the car could go...

 

That is punishing conditions for the hybrid battery, the extreme.

 

Yeah, those conditions are quite extreme for the car. How do you park though? Do you leave the windows down a bit? Do you use a shade on the front windshield? Are the windows tinted?

 

So according to The Department of Earth & Climate Sciences at San Francisco State University, you can expect the temperature inside a typical car, left in direct sunlight, to rise by 34F in 30 minutes, and 43F in an hour, above the ambient temperature.

So if it was 108F (42C) outside, the cabin temperature was probably close to 151F (66C) after that hour. The battery would have been cooler, of course, as it's in shade, lower down, and has a much greater thermal inertia than the air in the car, but still hotter than ambient as it's vented to the cabin and been warming for an hour.

Most Lithium and NiMh based battery technology protection circuits kick in at around 113F (45C) to prevent damage by charging, and around 149F (65F) to prevent damage by discharging. So even if the battery was a few degrees hotter than ambient, the charge protection circuit was probably kicking in.

I don't know how the charge/discharge thermal protection logic works in a Prius, but I would imagine that it would want to prevent discharging via the electric motor at around 113F (45C), due to the huge reliance on the HV system for a number of functions (e.g. starting the ICE, air con, 12V charging, etc). It will want to preserve that charge as much as it can for critical systems, if it knows it can't charge the battery.

 

Yeah. When the cabin temperature is 140-150 as I would expect after parking in the sun, the battery is going to be pretty hot, too, and it'll run the fans to try to cool down. Problem is that running the fans just makes the battery even hotter till the cabin gets cooled down. I would fully expect the car to severely limit battery use till it gets to a safe temperature.

Even so, this is practically a new car, so I'd still want to have the dealer check it out. I'm guessing that this is not your first Arizona summer with this car, but it is the first summer you've experienced these issues.