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How To Check Toyota Prius 12 Volt Battery 2004-2009 Models

Updated on September 19, 2014

Toyota Prius 2004-2009 Body Style

Source
These indicators themselves when you have no braking issues generally indicate the battery is weak and the weak voltage of the 12 volt battery is affecting the computers that control your ABS and VSC.
These indicators themselves when you have no braking issues generally indicate the battery is weak and the weak voltage of the 12 volt battery is affecting the computers that control your ABS and VSC. | Source
The 12 Volt battery lives here, hidden under a panel in the rear passenger corner of the hatch.
The 12 Volt battery lives here, hidden under a panel in the rear passenger corner of the hatch. | Source

Toyota Prius 12 Volt Battery Testing

If you're in a hurry, watch the short video below how to check the voltage of your Prius' 12 volt battery with the hidden service menu!

The Prius is an amazingly well engineered vehicle but like any other car, the 12 volt battery can be its Achille's Heel. Luckily, the 12 volt battery isn't used to actually start the engine like in a traditional vehicle (using a starter motor that we can hear cranking an engine to life). Instead, the 12 volt battery in a Toyota Prius is used to boot up the electronics that manage the engine and hybrid components and activate a relay to connect the high voltage battery pack to the electrical system (this battery pack is the one that actually starts the engine in a Prius). It can still be used to power accessory devices like cell phone chargers and the radio when the car isn't "running" or in READY mode.

Signs Of A Weak 12 Volt Battery In A Prius

  • Failure to start up or no response from the power button when attempting to start.
  • Dim interior and exterior lights before starting that then brighten after making the Prius READY.
  • Loss of radio presets when turning on your Prius.
  • Slow running coolant reservoir pump that speeds up after the Prius is made READY.
  • Failure to boot into READY mode with the gear selections flashing on the Odometer Display.
  • Error message displaying on the Multi Function Display (MFD) that states "There is a problem with the transmission "p" lock mechanism.Park your car on a flat surface and fully apply the parking break."

This is not meant to be an exhaustive list as there are other weird issues that can arise when the 12 volt battery gets low.

Helpful Pictures for Testing Your Prius' 12 Volt Battery

MFD screen on, Prius not in READY or Accessory Mode
MFD screen on, Prius not in READY or Accessory Mode | Source
Press the DISPLAY button once.
Press the DISPLAY button once. | Source
Press these indicated spots 6 times in the order indicated by the arrows.
Press these indicated spots 6 times in the order indicated by the arrows. | Source
Press MENU
Press MENU | Source
Press DISPLAY CHECK
Press DISPLAY CHECK | Source
Press "Vehicle Signal Check"
Press "Vehicle Signal Check" | Source
The voltage of the battery is displayed in the upper left portion as indicated, Prius still not in READY or Accessory modes.
The voltage of the battery is displayed in the upper left portion as indicated, Prius still not in READY or Accessory modes. | Source
Prius in Accessory Mode now placing a small load/drain on the battery.
Prius in Accessory Mode now placing a small load/drain on the battery. | Source

Steps to Test Your Prius' 12 Volt Battery

When I became curious about the health of my 12 volt battery and wanted to get it checked out, I drove down to my local Autozone store, being familiar with having had batteries in my previous vehicle tested there before in years prior. When I showed the store manager what kind of vehicle I had, he blatantly stated that they weren't allowed to touch ANYTHING on hybrid vehicles. Now he may have misunderstood and thought I wanted him to get cozy with the high voltage battery and decided to forgoe advising him that he was suffering from ignorance and decided to go home and look for another way to test my battery.

Luckily, Toyota left a hidden service menu in the software for all Toyota Prii, the method of access below will work on 2004-2009 Model Year Toyota Prii. The BEST time to test the 12 volt battery in your Prius is after it has been sitting for several hours or overnight, that way you can check the "resting" voltage of the battery. Use the pictures to the right to assist you through the following steps.

Step 1 : Power your Prius on (without pressing the brake to power into READY mode) by pressing the power button once.

Step 2 : Once the Multi-Function Display or MFD has booted up, press the DISPLAY button once.

Step 3 : While the display options are showing on the top and bottom of the MFD, press the 2 spots indicated in the picture to the right 6 times, starting with the top postion, then the second, then the first...and so on and the service menu will display.

Step 4 : Press the MENU button on the screen in the upper right corner, this will take you to the Diagnosis Menu.

Step 5 : Select the Display Check option.

Step 6: Select the Vehicle Signal Check option. This will display some information about the Prius, notable the present voltage of the 12 volt battery. Take note of the voltage displayed at this point

Step 7 : Press the power button once to put your Prius into accessory mode, note the voltage displayed at this point, it will be lower than what we saw in Step 6 because we now are placing a load or drain on the battery.

Step 8 : To exit out of the service menus, press and hold down the DISPLAY button until the MFD reboots or simply power your Prius off.

Video Instructions On Entering The Hidden Service Menu On Your Prius!

Determining If Your 12 Volt Battery Is Dead

Remember the voltages we found previously? For the resting voltage, generally anything around 12.4 Volts and above is considered acceptable. For the voltage under load (when your Prius is in Accessory Mode), anything below 12.0 Volts is unacceptable, At the time of testing for the photos displayed here, I had just finished a long drive in my Prius so the battery should have been at a higher voltage than what I found above. Which indicates my battery probably needs replacing soon.

You may be able to buy more time on your current battery by using a top off or float charger designed for 12 volt lead acid batteries. This isn't going to cure the problem as it will only get worse with time, however on a good battery, a top off from a float charger every now and then can help extend its useful lifetime by a few years.

Was this hub helpful to you in determining the status of your 12 volt battery?

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Where to Get a Replacement 12 Volt Battery For Your Prius

If you need a replacement 12 volt battery for your Toyota Prius, you have a couple options:

  • Get it replaced at your stealership, I mean, dealership.
  • Purchase the OEM battery through your dealership's service/parts department and install it yourself
  • Purchase an aftermarket replacement battery such as the Optima Yellow Top version for the Prius from eLearnAid.

Easy DIY Tasks For The Toyota Prius

Are you looking for other hands-on, do-it-yourself type things you can handle that don't require taking your Prius to the dealership or expensive tools? Why not give a go at changing your own oil, which can be very simple! Of course, remember to reset your maintenance required indicator when you're done.

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