How to Check the 12v Auxiliary Battery Health
2004-2009 Toyota Prius


Before doing anything read my disclaimer & safety info.



Unlike traditional cars where starting is dependant on the 12v battery being able to crank the engine, the Prius only uses the 12v battery to power on computers to "start" the car. The much larger traction battery actually starts up the engine when needed. With a traditional car you can normally tell the 12v battery needs replacement by sluggish starts or no starting at all. With a Prius you may experience strange things, dimming dash lights, a drop in MPG's and if the 12v is completely dead obviously no "starting." The good news is that you can test the battery without any tools and this simple procedure. In this tutorial I just want to expand/compile info from various sources to make this super easy.

Preface: This procedure should be done when the car hasn't been driven for at least 30 minutes. You can also power on the headlights for about 30 seconds to drain off any surface charge the battery may have before doing this as well. Also make sure no electrical accessories are on before starting....headlights, defroster, fan, radio, etc.

1) There are a couple of ways to get into System Check Mode. With your foot off the brake hit the power button once to enter ACC mode. Wait for the MFD to boot. Either method will get you to step 2, I've used both....

MFD method: Push the "Display" button and your MFD should display similar to the photo above. Note that the green and red circles are hidden targets you'll need to push, you will not see these. Do the following sequence: Hit the GREEN target then the RED target while in Display mode 3 times (6 touchscreen hits total: g, r, g, r, g, r), System Check Mode should now load, see next photo.

Light method: Hold down the "Info" button and while holding this down turn the parking lights on and off 3 times, System Check Mode should now load, see next photo.

2) Hit the "Menu" button, circled in red.

3) Hit the "Display Check" button, circled in red.

4) Hit the Vehicle Signal Check button, circled in red.

5) Test 1: In Vehicle Signal Check mode you get a reading of the 12v battery. I've read various ranges of what this should be, the above referenced PriusChat thread indicates 12.4-12.8v is normal range in ACC mode (1 power button push w/no brake pedal applied). Elearnaid indicates 11.9-12.6v as normal. The lower this reading the worse off your 12v battery is. So, I took this photo after leaving my car sit overnight and got 11.9v, on the low end and definitely way below what PriusChat is saying. When I replaced the battery with a new Optima Yellow Top it read 12.3v and that was out of the box.

Test 2: Again, without your foot on the brake, hit the power button again. You just put a current load on the battery and it should be about 12v. Depending on your reading you may not want to proceed to put more load on the battery. If you turn on the headlights, rear window defroster and heater fan you'll start seeing the voltage drop. Elearnaid says about 11.3v is where you should be at with a new battery and 10.2v and less the battery should be replaced. I was in the mid 10v range so I decided to just replace it for a Yellow Top!

Test 3: Apply the brake pedal and push the power button once. Now you're in ready mode and you should see the voltage jump to 13.6-14.4v range. This is the charging voltage and it will be higher in the winter.

6) To exit Vehicle Signal Check Mode power off the car OR hold down the "Display" button for about 5 seconds.





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Last revised: Sunday, 29-Oct-2023 16:36:58 EDT