Dead Prius Battery Explained

Prius c 03 2012 DCA 3646
mariordo59/Flickr/CC BY-SA 2.0

Walking out to your car to discover a dead battery is never the beginning of a good time. Since headlights were added to the front of cars and trucks, the hurried and the forgetful have been leaving them on, resulting in a dead battery and a much longer day than they may have hoped for. Battery technology has come a long way, and most modern vehicles can go all night with the radio on or the dome light illuminated and still have enough juice to start the engine in the morning. But the headlights, when left on, can still be a killer.

Are Prius Batteries Different?

You would think with a Prius having such a massive bank of batteries there would always be enough juice to get things going, right? Sadly, this is not the case. Your Prius uses the same type of 12-volt battery that other gas hogs use to start the engine. That big bank of batteries your thinking of (the ones that make your Prius a hybrid) are commonly referred to as “traction batteries” because they serve the sole purpose of powering or being charged from your wheels.

The Prius electrical system has some characteristics outside the realm of usual car logic that can lead to a dead battery. While most modern cars electronically turn off your headlights to avoid battery drain, in the right circumstance some Prii (that’s plural for Prius) will actually turn them on. The bad news is the driver, or an unsuspecting passenger never knows that the lights came on and didn't do anything about it. This results in the dreaded dead battery and jump start.

Prius Battery Dead

Here are some examples of how to kill the battery in a Prius:

Scenario 1: The Chatting Error

  1. Stop the car and park with the lights on.
  2. Open your door to exit the vehicle.
  3. With the door open, turn off the car completely.
  4. Exit the vehicle, not knowing that your lights came on.

Scenario 2: The “let me just grab that CD for you.”

  1. Park your car and walk away as usual.
  2. Remember your friend’s CD, and open the passenger door.
  3. Turn the car on to eject the CD.
  4. Shut off the car and exit the passenger side door, lights stay on!

Why does this happen? Why did your Prius headlights come on, or stay on, for what seems to you like no logical reason? The answer is in the headlight controls. If like most drivers these days you use the automatic light feature, you are relying on the logic of your Prius to know when to turn them on and when to shut them off. Surely, this isn’t too much to ask, is it? Apparently, it is, based on the number of people who have suffered a similar dead battery fate due to headlight activity.

How to Stop the Battery From Dying

Don’t use the automatic headlight feature. Turn them off manually each time you get out of the car, and then turn them back on when you return to drive. Some people call this old-fashioned and aren’t happy with that solution. If you would rather use the auto lights just remember that an improper sequence of events at home or in the parking lot could result in your lights staying on until you return to your vehicle, hopefully not with a dead battery.

Most people aren’t even aware of what they did exactly that led to the lights coming on, and the ensuing death of their battery. They may even repeat the error. But once you realize that the process of turning off a Prius and getting out can turn your routine into havoc, you’ll pay more attention. Note also that Toyota recommends replacing your battery every 3 years or so. Doing things like cleaning your battery terminals can help keep your battery fully charged and even make it last longer. A battery that has been drained fully and then recharged is far more likely to fail soon than a battery that has been well maintained with a peak charge.