Yes, the conditions you describe are the trigger; afternoon errands seems to be the cause. I've seen it 3 or 4 times. Nate
I had this happen to me again today. My car was parked outside for about 2 hours. It was only 95 degrees out, but like a billion on the inside, and I started the car, rolled down the windows and sat for a min while I connected my iPod. Then I heard the battery fan really start to blow, pretty loud, and so I put it in drive and started to drive. It was like NO battery power, lots of delay time between pushing the pedal and moving. The gas engine never shut off either. The drive was only about 5 mins. On the screen it never showed power to or from the battery. Re-gen braking didn't occur either. When I lifted my foot off the gas, it was as I was still giving it gas. I parked it in the garage when I got home, so It was cooler and let the car run until the fan turned off, which wasn't long after.
Interesting. It gets hot and humid here in Florida. With my car parked outside in the sun all day, I am sure it is quite warm in the car, often unbearable for the moment until I can get the AC going. Car runs fine.
I've seen MPG drops on hot days, but I've never noticed what I'd call sluggishness. My scale of reference may be lower, though...
I experienced this once myself - fan was on full speed and it seemed that the battery didn't want to give for about 10-15 seconds - then things seemed to return to normal. I had just started out after parking. I usually crack my windows about 1in (2.5cm) and use the HeatShield as well, though sometimes I do forget. If it happens again, I may report it as well.
i noticed sluggishness one time and that was on a friends's Prius. it happened when we were sitting at Seatac last year waiting for his luggage and we were sitting with air on for about 10 minutes. i noticed that the charge went down to pink before the engine kicked on. it was just before the 2nd kickon when we pulled out of there and the car was sluggish but only lasted about 20 seconds maybe. hard to say since we were at an airport and the speed was low enough that you had to be familiar with the car to notice anything. by the time we hit decent crusing speed, car was completely recovered. now, why the battery will go that low on occasion is beyond me. but the loss of power is fairly well documented. remember a car mag (car and driver i think) tested the Prius at 0-60 with discharged battery and noted a gain of something like 2-3 full seconds on the elapsed time.