Hybrid battery cooling fan causing Prius to drive erradically

sounds right.

 

It might be clogged, which would aggravate things. If you can peel off the rear carpetting you can check out the fan condition yourself. If it's similar to 3rd gen it will be a bit of a fight: there are a lot of fasteners holding the carpetted deck to the battery shell. They will pop out, but they are pretty tenacious.

(BTW, it's bad form to double post.)

 

There is a relationship between temperature of HV Battery and fan speed. The hotter the ambient (in car) the hotter the HV. Full use of AC, especially when first driving off may keep the HV cooler and the fan quieter. AC can then be rapidly lessened as comfort zone is reached. We have had a lot of these post this summer, probably not unlike any other summer tho.

 

It looks like you joined resently so Welcome.
The fan for the hybrid battery can also work harder if the filter is clogged. some pet owners have seen this happen. The vent is on the right rear passenger side between the seat and door. it can be removed and inspected. also some times the seat belt can cover it. I believe there is a thread on this if you search.