Replacing the Prius 12V Battery

I went and bought a new 12V battery from Toyota today since my check showed that the voltage was low. I picked it up and it was in a great cardboard package that made it easy to carry. Too often I get car batteries with no packaging and nothing to hold.

I got it home and stuck it in the garage until I had some time after work. Once I did, I grabbed these tools to get started.

• 10mm socket
• 12mm socket
• socket wrench and 2” extension
• utility knife
• screwdriver

I read someone else’s instructions that mentioned these two sizes. I eventually grabbed a 6” extension as well for the sockets.

The first step is to clean out the trunk.

Then open up the flip floor where the spare is and remove the box from the car. There also is a panel on the right that is above the battery. It has a tab under the left bottom (front) and there are two behind the plastic panel in the rear. I broke the front tab trying to loosen it. No help here, just work with it.

This will expose the battery and the 12V regulator assembly. In the image below, I’ve marked the regulator in front with a red arrow and a green arrow shows where the battery is behind.

There are a mix of 12m and 10mm bolts. I didn’t remove things in this order, but this is how I’d recommend it.

Unhook white plastic connectors from assembly on top of battery and to the voltage regulator. They are all easy clips that have a part that pushes in easily with you fingers and they slide out. There are 3.

• Unscrew the 10mm screw from the negative side of the battery that goes into the rear of the car.
• Undo the 10mm on top of the voltage regulator
• Unscrew the two 12mm bolts holding down the voltage regulator and remove it.
• There’s a 10mm and a 12mm, or 2 12mm bolts holding on the vent in. It’s below the green arrow above. Once you remove the bolt, wiggle it out
• A 12mm bolt holds down the inside of the battery
• A 10mm bolt holds down the outside of the battery item
• Pull out the battery and then you can unscrew the two terminals.
• On positive terminal, I had to use my screwdriver to widen the connector to get it off.

Here’s just before removing the battery.

When I had the battery out of the car, I transferred the positive and negative connectors to the new battery. This took a minute, and reinstallation, the reverse of pulling it out, took less than 5 minutes.

I connected everything back up. One warning, when I went to screw in the negative terminal and the bolt touched the metal, it sparked. I dropped the wrench when that happened. Be careful, but expect the spark.

I have always worked on car batteries as negative terminal off first, and on last.

Once everything was screwed down, before I reinstalled any trim or boxes, I started the car to see what happened. It started right up. I went into maintenance mode and checked the voltage

14V. Success!

I then put the trim in and the box and closed up the car. I checked tools to be sure that I hadn’t left anything in. I then drove down the road, and everything seemed to work.

Except the auto-driver’s window.

I checked online, and it’s easy to fix. With the windows working, take it down halfway. Then pull the switch so that it rolls up and continue holding up the switch for a couple seconds. It will work again.

I’ve seen a bunch of “keep your settings” instructions. If you are electrically inclined, you can hook up another power source to keep things working. I didn’t think it was worth it.

One other thing. This car is just low enough, and the bolts deep enough that you will be bending over. I’m 6’ and my back was aching a bit halfway through this. I had a stool to work on the battery outside, but it was useless in the car. Be prepared for that.