Learning, Battery,

How to Tell If Prius 12v Battery is Bad (5 Signs)

The Prius is one of the best Hybrid cars around. Taking proper care of its battery goes a long way in maintaining the performance of your Prius engine.

In General, you can tell whether your Prius 12-volt battery is bad by seeing these signs.

  1. Fails to Hold a Charge
  2. Erratic State of Charge
  3. Frequently Switching to the Combustion Engine
  4. Malfunctioning Car Electrical Systems
  5. Strange Engine Noises

Immediately catching the signs of a bad battery is the best way to prevent damage to your Prius. Let’s go into further detail about what exact symptoms of a faulty battery you should look for.

Signs of a Faulty 12-volt Prius Battery

prius toyota car
Video | Doug DeMuro

Fails to Hold a Charge

Bad batteries can’t effectively hold their charge and will continuously deplete their stored energy, regardless of whether or not the car is in use.

Let’s say you charged your batteries overnight. You’d expect it to be fully charged by morning, but the actual state of charge is at 75% or less. Your battery failed to hold its charge and steadily lost power overnight.

However, failing to hold a charge may also point to issues with the electrical plug and cable connected to the battery. Checking with an expert before investing in a new battery is best.

Erratic State of Charge

State of charge is the amount of remaining charge in your batteries.

Newer Prius models indicate the state of charge on the digital display on the dashboard or center console. You can use the display to access information on the battery, like the state of charge percentage. For older models, a battery LED indicator will appear on the dashboard if your battery is low.

energy monitoring system of a toyota prius car
Video | Evanstal

The state of charge will gradually go down as you drive around.

A 12-volt hybrid Prius battery lasts around 100,000 to 150,000 miles or 8 to 10 years of use. It takes a while for your battery’s state of charge to go down, even if you’re using the car daily.

Your 12-volt Prius battery is faulty if the state of charge erratically changes, such as depleting faster than expected or suddenly increasing.

Frequently Switching to the Combustion Engine

The Prius is a hybrid car that prioritizes using its electric motor. Typically, it only switches to the combustion engine if there are battery or electric motor issues.

Switching to the combustion engine more than usual is a clear sign of a faulty battery.

Your Prius needs to draw enough power from the battery to keep its electric motor running. Otherwise, it will immediately switch to the combustion engine to prevent the car from shutting down.

It’s not uncommon for the Prius to suddenly switch engines. It could be due to a loose wire connection or a component suddenly disconnecting. But if it becomes common and isn’t caused by component issues, consider replacing the battery.

Malfunctioning Car Electrical Systems

interior dashboard of a toyota prius
Video | Doug DeMuro

If your car lights seem dimmer than they used to or the doors don’t unlock immediately, then it might be a sign of a bad battery.

Your car’s electrical systems rely on the battery to work properly. These systems include headlights, window rolls, and electronic locks. This is a major issue for the Prius since hybrid cars depend heavily on batteries. Some users even report their engine failing to turn over due to a bad battery.

Get your battery checked out immediately if you notice any issues with your car’s electrical systems.

Strange Engine Noises

The Prius uses a quiet hybrid engine, so strange noises like grinding or clicking are definitely a cause for alarm.

A bad engine can’t supply enough power to the car. The electric engine will attempt to work double time to draw more power from the battery before switching to the combustion engine. However, this causes the battery to heat up and affect the surrounding components.

engine of a prius toyota car
Video | Doug DeMuro

The car will try to control the temperature by increasing the speed of its internal fans. As a result, you’ll hear loud whirling, clicking, or grinding sounds from the engine.

Replace the battery as soon as possible since repeated exposure to heat will damage the surrounding components.


Toyota. https://www.toyota.com/electrified-vehicles/warranty/

National Institute of Deafness and Other Communication Disorders. https://www.noisyplanet.nidcd.nih.gov/have-you-heard/are-hybrid-cars-too-quiet

Video References

Doug DeMuro


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About Alex Robertson

AvatarCertifications: B.M.E.
Education: University Of Denver - Mechanical Engineering
Lives In: Denver Colorado

Hi, I’m Alex! I’m a co-founder, content strategist, and writer and a close friend of our co-owner, Sam Orlovsky. I received my Bachelor of Mechanical Engineering (B.M.E.) degree from Denver, where we studied together. My passion for technical and creative writing has led me to help Sam with this project.

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