The Prius is an incredibly well-designed vehicle, but like any other car, the battery can be its Achilles heel. Fortunately, the battery is not actually used to start the engine like in a traditional vehicle (using a starter that we can hear turn an engine to life). Instead, the battery in a Toyota Prius is used to turn on the electronic components that manage the engine and the 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 is not running or in “Ready” mode.
If you are not experiencing braking issues, these indicators usually indicate that the battery is low and that the low voltage is affecting the computers that control your ABS and VSC.
Signs of a weak 12-volt battery in a Prius
- Does not start or there is no response from the power button when trying to start.
- Dim the interior and exterior lights before starting and then illuminate 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 Prius is in “Ready” mode.
- Do not start in “Ready” mode with gear selections flashing on the odometer display.
- An error message appears on the multifunction display (MFD) that says: “There is a problem with the transmission lock mechanism” p “. Park your car on a level surface and fully apply the parking brake.
Note: This is not meant to be an exhaustive list, as there are other strange problems that can occur when the 12-volt battery dies.
The Prius’s battery lives here, hidden under a panel in the rear corner of the passenger side of the hatch.
How to test the battery
When I was curious about the condition of the battery in my Prius and wanted to have it checked, I drove to my local trusted AutoZone store, having tested the batteries in my previous vehicle in previous years. When I showed the store manager what kind of vehicle I had, he blatantly said that they are not allowed to touch anything on hybrid vehicles. Now he may have misunderstood and thought he wanted him to be comfortable with the high voltage battery, but I decided to stop warning him that I was suffering from ignorance and went home to find another way to test my battery.
Fortunately, Toyota left a hidden service menu in the software for all Toyota Prius. The access method below will work on any 2004-2009 Toyota Prius. The Best time to test your Prius 12-volt battery is after it has been idle for several hours or overnight. In this way, you can check the resting battery voltage. I have included pictures to help you with this procedure.
Step 1: Turn on your Prius by pressing the power button once.
MFD display on. Make sure the car is not in “Ready” or “Accessory” mode; do not press the brake to enter “Ready” mode.
Step 2: Press the “Display” button once.
Once the MFD has started, press the “Display” button (red arrow) once.
Step 3: Press the indicated colon six times in alternating sequence.
Press these indicated points (green arrows) six times in the order indicated by the arrows (that is, up first, then down, then up, and so on).
Step 4: Press “Menu” at the top right of the screen.
If you press “Menu”, you will access the “Diagnostic Menu”.
Step 5: Select the “Screen Verification” option.
Press “Screen Verification” (red arrow).
Step 6: select the option “Vehicle signal check”.
This will display information about your Prius, in particular the current battery voltage.
Step 7: take note of the voltage.
The battery voltage is shown in the upper left of the screen (red circle). The Prius is not in Ready or Accessory modes yet.
Step 8: Press the “Power” button once.
Pressing the “Power” button will put your Prius into “Accessory” mode, causing the battery to run down or run down a bit. Again, look at the voltage.
Step 9: To exit, press and hold the “Display” button.
Alternatively, you can simply turn off your Prius.
Instructions for entering the hidden service menu on your Prius
Determine if your 12-volt battery is dead
Remember the two voltages we found earlier? For quiescent voltage, generally, anything around 12.4 volts or more is considered acceptable. For 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 ride in my Prius, so the battery should have been at a higher voltage than what I found above. This indicates that my battery probably needs to be replaced soon.
You may be able to save more time on your current battery by using a floating or better charger designed for 12-volt lead-acid batteries. This is not going to fix the problem, as it will only get worse over time; however, with a good battery, topping up the voltage with a floating charger every now and then can help extend its life by a few years.
Where to get a 12-volt replacement battery for your Prius
If you need a replacement battery, you have a couple of options:
- Get it replaced at your dealer, I mean, dealer.
- Purchase the OEM battery through your dealer’s parts / service department and install it yourself
- Buy a replacement aftermarket battery, such as the Optima Yellow Top version for the Prius.
Simple DIY tasks for the Toyota Prius
Looking for other practical, DIY-type things you can handle that don’t require expensive tools or take your Prius to the dealership? Why not try changing your own oil, which can be very easy? Of course, remember to reset your “maintenance required” indicator when you’re done.