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


Before doing anything read my disclaimer & safety info.



If you haven't already, check the health of your 12v battery. As I said there, unless your car doesn't start at all it's tough to know if you need a new 12v battery in a Prius. After testing my 12v battery and finding it to be weak I decided to replace it because I don't want to be stranded somewhere. Yes, it can be jump started but I've done that before with other cars and would rather preemptively replace this. And just to be clear, this is the small battery in the Prius, NOT the traction battery used to power the electric motor!

Tools Needed:

Parts/Supplies Needed:


Preface: If you read some of the reviews on Amazon you'll see that the stock battery isn't great. With a high performance direct fit Optima Yellow Top available I cannot see buying the OEM battery. If you follow this guide you'll probably even save some $$$ and have a better battery. Plan on ~1 hour or less for the job. If you're doing this as a planned replacement you will want a backup 12v source hooked up so you don't lose settings and have to initialize systems. If your battery is completely dead the repair manual indicates systems will need to be initialized (RM IN-34). I'm not including that procedure as this tutorial assumes preemptive replacement.

Normally I don't plug an individual website but am making an exception here. I know it sounds weird to buy an auto battery from a website titled but they've been selling Optima Yellow Tops for the Prius for years; I bought 2 of them from this company (current price, no discounts or anything for writing this). They have the old version too but why buy that when they have a direct fit now (link above). Not only did ElearnAid process & ship my order crazy fast they packaged the batteries with care and sent replacement instructions. Since they packaged this so well I had to take a few pics on the mind-numbing subject of packaging! Packaging 1 Packaging 2 gotta love that flat-rate box for a dense object like a battery.

1) Push on the area indicated by the blue arrow and remove the fuse box cover.

2) Flip up the positive cover on the fuse block, circled in red is the 12v positive access location for jumping. Circled in white is the strut bolt which can be used for the negative ground point.

3) Hook up the aux 12v power source so you don't have to initialize systems and loose all your settings (major thanks to Hobbit for this tip!). Hook the jumper cables up in the order shown in the pic. I used the second Optima I bought since it was easy but if you have another car you could hook up to that battery (car off) as if you were going to jump the Prius. A portable jump starter would be even easier. You could even use an AC to DC adapter if you have the correct clips. When you've got the new battery swapped out remove the cables in reverse order: 4, 3, 2, 1.

4) Follow all steps except 3 & 5 to remove the rear covers.

5) Optional step: Remove the two 12mm bolts circled in red on the floor and the one 10mm bolt circled in red on the top of the brake control power assy. Next push on the release tab on the connector circled in blue, disconnect the connector then remove the brake control power assy. Torque on all 3 bolts = 14 ft-lb (19Nm or 194 kg-cm). I removed this for easier access to the battery and to get better photos for this tutorial but the repair manual does not list this as a step; it can be done without removing this.

6) Use a 10mm box wrench to loosen the negative battery post bolt. Just loosen it and then wiggle the clamp off the battery post, circled in red. Once the negative lead is removed cover the metal in some way so that you don't accidentally short anything out. You can tape it up, wrap a rag over it a few times, whatever works for you. As you can see the bolt on the negative battery faces away from you so you can't get a socket in there. When putting this back on and tightening just make sure it doesn't wiggle on the post, don't over tighten. If you do actually torque the positive terminal you can use your box wrench on that to see how tight it is and try to match tightness on the negative bolt.

7) Use a small flat screwdriver to remove the 3 claws and slide the positive cover off. If you start with the claw closest to you the others kind of wiggle off with a little help from your screwdriver.

8) Use a small flat screwdriver to push down the tab on each connector circled in red and then disconnect them. You can now pull the entire wire harness out of the way of the black vent duct.

9) Remove the two 10mm bolts and slide the vent duct out. Torque = 35 in-lb (4.0 Nm or 41 kg-cm). Note when reinstalling, I just tightened the silver bolt by hand as it goes into a plastic holder that could easily strip.

10) Use a 10mm socket to remove the 3 bolts circled in red, the bolt on the battery post just needs to be loosened. Now the clamp on the battery post will probably still be tight but at this point you need to wiggle the entire hold down clamp up and off. The black hold down clamp is attached to the positive clamp. Just don't pull on the fuse block. Now twist the battery vent hose and pull it out.

Torque on the battery clamp bolt = 53 in-lb (6.0 Nm or 61 kg-cm). Torque on the 2 hold down clamp bolts = 48 in-lb (5.4 Nm or 55 kg-cm). When reinstalling after you have the new battery situated and pushed all the way back, I did the hold down clamp bolt in the floor first, torqued it, then did the hold down clamp on the other side but didn't torque it all the way to spec as it was plenty tight. I just went by feel. Lastly, I torqued the positive clamp bolt to spec.

11) Remove the battery. It comes out following the left curve of the black battery bracket.

As you can see this Optima is exactly the same size as the OEM battery. I even got one made in the same month I bought it!

12) Install your new battery, assembly is reverse of removal. You can see any special assembly notes if you work your way back. A perfect fit! Make sure to wash your hands when done as the terminals are made of lead. Recycle the old battery.




If you found this information useful please support the site by clicking the button below and making a donation. Thank you.



: home :: info :: tutorials :: support :: links :: contact :: site map :
This is an enthusiast site and is not affiliated with Toyota in any way.
Site best viewed at 800x600 or higher using Mozilla Firefox Get Firefox!
Copyright © 2012-2015 C. Pearson. All rights reserved.
Last revised: Saturday, 11-Oct-2014 10:55:16 CDT