How to Check the 12-Volt Battery in a 2004-2009 Toyota Prius

Updated on January 9, 2018
Toyota Prius 2004-2009 body style.
Toyota Prius 2004-2009 body style. | Source

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

The Prius is an amazingly well-engineered vehicle, but like any other car, the battery can be its Achille's heel. Luckily, the battery isn't actually used to start the engine like in a traditional vehicle (using a starter motor that we can hear cranking an engine to life). Instead, the 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 high voltage battery pack is the one that actually starts the engine). 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.

If you have no braking issues, these indicators generally indicate that the battery is weak, and the low voltage is affecting the computers that control your ABS and VSC.
If you have no braking issues, these indicators generally indicate that the battery is weak, and the low voltage is affecting the computers that control your ABS and VSC. | Source

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 putting the Prius in "Ready" mode.
  • Loss of radio presets when turning on your Prius.
  • Slow-running coolant reservoir pump that speeds up after the Prius is in "Ready" mode.
  • 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 brake."

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

The Prius' battery lives here, hidden under a panel in the rear, passenger-side corner of the hatch.
The Prius' battery lives here, hidden under a panel in the rear, passenger-side corner of the hatch. | Source

How to Test the Battery

When I became curious about the health of my Prius' battery and wanted to get it checked out, I drove down to my trusted local AutoZone store, having had batteries in my previous vehicle tested there 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, but I decided to forego advising him that he was suffering from ignorance and went home to 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 any 2004-2009 Toyota Prius. The best time to test the 12-volt battery in your Prius is after it has been sitting for several hours or overnight. This way, you can check the resting voltage of the battery. I've included pictures to assist you with this procedure.

Step 1: Power your Prius on by pressing the power button once.

MFD screen on. Make sure the car isn't in "Ready" or "Accessory" mode—don't press the brake to power into "Ready" mode.
MFD screen on. Make sure the car isn't in "Ready" or "Accessory" mode—don't press the brake to power into "Ready" mode. | Source

Step 2: Press the "Display" button once.

Once the MFD has booted up, press the "Display" button (red arrow) once.
Once the MFD has booted up, press the "Display" button (red arrow) once. | Source

Step 3: Press the two indicated spots six times in alternating sequence.

Press these indicated spots (green arrows) six times in the order indicated on the arrows (i.e. top first, then bottom, then top, and so on).
Press these indicated spots (green arrows) six times in the order indicated on the arrows (i.e. top first, then bottom, then top, and so on). | Source

Step 4: Press "Menu" on the top right of the screen.

Pressing "Menu" will take you to the "Diagnosis Menu".
Pressing "Menu" will take you to the "Diagnosis Menu". | Source

Step 5: Select the "Display Check" option.

Press "Display Check" (red arrow).
Press "Display Check" (red arrow). | Source

Step 6: Select the "Vehicle Signal Check" option.

This will display some information about your Prius, in particular, the current battery voltage.
This will display some information about your Prius, in particular, the current battery voltage. | Source

Step 7: Take note of the voltage.

The voltage of the battery is displayed in the top left of the screen (red circle). The Prius is still not in the Ready or Accessory modes.
The voltage of the battery is displayed in the top left of the screen (red circle). The Prius is still not in the Ready or Accessory modes. | Source

Step 8: Press the "Power" button once.

Pressing the "Power" button will put your Prius in "Accessory" mode, placing a small load/drain on the battery. Again, note the voltage.
Pressing the "Power" button will put your Prius in "Accessory" mode, placing a small load/drain on the battery. Again, note the voltage. | Source

Step 9: To exit, press and hold the "Display" button.

Alternatively, you can just power off your Prius.

Instructions for Entering the Hidden Service Menu in Your Prius

Determining if Your 12-Volt Battery Is Dead

Remember the two 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, when these photos were taken, 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. This 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 since it will only get worse with time, however, with a good battery, topping off the voltage with a float charger every now and then can help extend its useful lifetime by a few years.

Where to Get a Replacement 12-Volt Battery for Your Prius

If you need a replacement battery, you have a couple of 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, like the Optima Yellow Top version for the Prius.

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 expensive tools or taking your Prius to the dealership? 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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      • jesimpki profile image
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        jesimpki 6 months ago from Radford, VA

        Thanks! Yes, this one is mine and I still drive it daily. Averaging about 54-55 MPG during the summer, a bit lower in the winter at around 47-48.

      • WheelScene profile image

        WheelScene 6 months ago from U.S.A.

        Hey jesimpki,

        This is a really good post. You go into great detail and the pictures really help. Is this your Prius? Do you still have the car?

        Do you get great gas mileage with the car?

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