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Why Heat Kills MPG In Toyota Prius And What To Do About It

Heatwave got your MPG down in your Toyota Prius? Take a look at why that happens and what you can do about it.


Heat is an excellent tool for efficiency until it becomes too hot. The right amount of heat can help combustion engines run at peak operating economy, and when it comes to power, a little heat goes a long way.

What happens when you get too much heat, say in your hybrid battery? What are the symptoms and consequences? This article will examine how hot summer temps in your Toyota Prius or other hybrid car are detrimental to that MPG life.

Why Do Batteries Hate Heat?
Heat is a battery killer because it makes the batteries work harder. Batteries exposed to excessive heat can stop working. Bulging or bubbling can happen in NiMH, which leads to electrolyte evaporation.

Toyota Prius Hybrid Fan

When the electrolyte evaporates, the battery loses the ability to recharge properly. In short, heat significantly reduces the service life of your battery. Batteries that see hot temps a lot do not live as long as they could in more moderate climates.

Batteries do not like heat because heat does not like batteries. It is just science.

What Can Be Done To Help Your Battery?
Heat causes the batteries to be less efficient, and the less energy you can store in your battery, the lower your fuel economy. It is as simple as that.

To help with your battery, you first need to know what keeps it cool. Toyota Prius comes standard with air cooling. The fan that blows air over and across the battery gathers it from inside the cabin.

If your internal cabin temps are hot, your battery will be too. Keeping the A/C in good working condition and operating on hot days will help keep those battery temps down.

Good temperature regulation (as stated before) will help the battery retain more capacity and keep it from failing too soon. We know that keeping the battery cooler does the trick but what are some best practices for hybrid owners in hotter regions like Arizona?

Nano Ceramic window tint is a massive help. The tint helps reduce cabin temps. When parking, try to find shade and crack the windows slightly. This will help keep the heat from building up too much inside the cabin.

I use a front windshield screen to help keep heat out, along with tinted windows and shaded parking (where available). These practices keep the heat to a minimum and my battery in stable condition.

I have seen some Prius owners run a tube from an air vent to the inlet for the fan. I think this is a bit excessive, but there are benefits. Keeping your battery cooler could (and maybe I should do this) keep it in the perfect zone for optimal efficiency.

Excessive Heat and hybrids do not mix well. If your battery has seen its time, the heat could put it over the edge. Keep your fan clean, park in the shade, tint your windows, and yes, washing your car will help too.

Thank you all for reading. Remember Today's Adventure is Tomorrow's Story. I look forward to seeing you all in the following article.

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Peter Neilson is an automotive consultant specializing in electric cars and hybrid battery technologies. He holds a Bachelor of Science in Automotive Service Technology from Weber State University. Peter can be reached on Linkedin and you can tweet him at The_hybrid_guy on Twitter. Find his page on Facebook at Certified Auto Consulting. Read more of Peter's stories at Toyota news coverage on Torque News. Search Toyota Prius Torque News for more in depth Prius coverage from our reporter.


jg (not verified)    June 26, 2021 - 7:52PM

Something else the heat can do is melt your transmission. Back when I had a 2001 Prius, we drove across the lower half of the country and going through Arizona the temperature was at least 117 degrees for a long time... We maintained around 75 mph for most of the day and when we stopped at a motel, all sorts of lights came on when the car was shut off. It would start, but the engine would sputter and die when trying to drive (interesting caution light show as well). Long story short, the motor had some melted components and Toyota said it wasn't covered (it was still within the powertrain warranty). The dealer fought for us and got Toyota to pay for a brand new transmission, and I switched to a high-quality synthetic transmission fluid -- never had any other problems in 300,000 miles. Even had the original brake pads when I finally traded it in after it stopped maintaining highway speeds.

Michael Giambrone (not verified)    June 29, 2021 - 12:52PM

How can I tell which battery modules need to be replaced in my battery pack for my 2012 Prius 3rd gen ?