39C (102F) today. But it's true that heat can hurt battery life. So can leaving it with a low charge in cold weather. The key point is that Prius battery replacements are rare. Consumer Reports estimated a 4% rate at 11 years old and a 5% rate at 12 years old. Only 6% of 2004 US Prius trade-ins had any kind of power train issue, vs 19% for Jetta and 12% for Golf.