So yesterday I decided to check the cooling fan in the back hatch. Disassembling was easy and took less than a half hour. Once every thing was removed I took the upper most duct work off, and then I was able to see the fan. Mine was very clean with only a small about of dust on the blades. I removed the dust using the vacuum cleaner. My Prius is a 2005 with 116,000 on it. A very clean car that I purchased a few weeks ago. After the fan inspection I feel more confident the battery pack has lived a good life so far. I expect to get many more miles and years out of the pack under normal driving conditions.