So I was just out for a quick run to McD's for a milk shake in the Prius. I hear the sound of air moving around behind and to my right and I realize the fan for the HV battery had come on. A knot immediately formed in my gut. Instead of going into the drive thru, I instead park and turn off the car and inspect the vent. Nothing is blocking it. Get back in, and get going again, and it takes about a minute for the fan to come back on. Instead of proceeding to get my shake, I instead keep driving. The fan was on, but low. After a couple of miles, the fan turns off. I keep an eye on the battery SoC and it appears to be normal. I put the MFD into the diagnostic display mode, and the 12v charge is 14.1. I return to McD's and get my milkshake and check the 12v SoC. 14.3 while driving. Fan hasn't come back on. In about 30mins, I'm going out to check on the 12v SoC to see what level it is. Is this the normal mode of operation for the HV fan? To switch off and on? I have never noticed it doing that before, and I have owned the Prius for almost 9mos now.
Ok, so I just took her out for a test drive to listen and can hear nothing from the fan. EV operates as normal. HV behaves as normal. Upon further research the fan does come on and off according to the temperature of the HV battery and the fact that it came on indicates that the blower might need a simple cleaning. *knock on wood* I need a new pair of underwear.
It's totally normal for the fan to come on at low speed in warm weather. From memory the fan starts at it's lowest speed when the battery temperature is somewhere around 39C (102F). Given that the daytime temperatures in your region have been up around 100F, then it's not at all surprising that your fan is coming on. If however you're hearing it running louder and at higher speeds, then definitely inspect it to make sure it's not full of dust. It's also worth noting that if you do a lot of hills, particularly if you're frequently regenerating to near full SOC, then that is definitely going to add heat to the battery.
The only time you can hear the fan while driving is if the fan is at its highest speed which means the battery is near overheating. If you put the car in park, go the backseat and put your ear to the vent you can hear it on lower speeds. I would clean the fan to make sure if was operating 100%.
Our 3rd gen fan behaves a lot like that, for sure. I'm watching fan speed with a ScanGauge, have seen it get up to 4 (out of 6 I believe) but have yet to hear it.
