Ironically, most people identify cold weather with vehicle battery failures. In all reality, most batteries are damaged in the hot weather, but since batteries are more efficient when warm than when cold and vehicles are easier to start in warm weather than cold weather, the damage doesn't show up until the following Winter.
Hybrid Batteries are no exception, but a lot more sensitive to the heat. A conventional lead-acid battery, used since the 1900's is located under the hood, next to a gasoline engine where the exhaust manifolds approach 1400 degrees F on the highway in the summer. Nickel Metal Hydride Hybrid batteries, on the other hand, are located in the passenger compartment and start to deteriorate at 110 degrees F. Some , like Ford products, have their own little heater/A-C system to maintain the temperature. On the other extreme are early model Hondas that had no cooling or ventilation system and suffered higher failure rates. Toyota uses an electric fan to draw passenger compartment air, through the battery to keep it cool. That works pretty well aside from the fact that most Toyota Hybrid owners were never instructed on how that system works.
I bring this up because it has been pretty hot in Chicago lately and every time I see a Prius owner driving around with their windows rolled down I have to resist yelling over "you may not want to do that!". Here is why. Not only do we have heat and humidity in Chicago, we also have Cottonwood Fuzz and dog hair. The air intake for the Hybrid Battery is located near the rear seat door jamb and has no filter. Air intake for the cabin is filtered through a cabin filter, but not if you are driving along enjoying the spring day with the windows down and Rover in the back seat with his head out the window.
The hybrid battery is pulling cottonwood fuzz, dog hair and ambient temperature air through the battery and creating an insulating wooly blanket between the battery modules to keep the heat that is normally generated in the battery from being removed with 95 degree air that is blowing on the battery, and that battery is going to over heat.
The moral of the story; if you own a Hybrid, get the cabin air filter replaced every 15,000 miles, make sure your Air Conditioner is serviced and functioning properly, keep Fido in the "other " car, don't drive around with the windows down in hot weather.
Thanks for reading this,