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How to Change A Prius Battery - 12V Battery Replacement

Updated on September 7, 2017
How to install a new 12 volt battery in a Toyota Prius
How to install a new 12 volt battery in a Toyota Prius | Source

Changing a Prius 12 volt battery

The Toyota Prius has two batteries in it; a large 200 volt battery that is used to drive the car in the electric mode and a small 12 volt battery that is used to operate the accessories such as lights, radio, etc. This article is written to show how to change the smaller, 12 volt battery.

Unlike most cars, this battery is not a very simple thing the change or install. It is buried in the trunk, not under the hood, and there are many items in the way of getting the work done. Nevertheless, it is not particularly difficult as long as you understand where it is and what needs to be done to give access to it. The task is well within the capabilities of the ordinary homeowner and should not require too much time to accomplish. The changeover shown in the photos was done in about ½ hour by someone fairly experienced in working on cars but that had never done a Prius battery change.

Like every car battery, the one in the Prius will wear out - it is normally good for around 5 to 6 years but can last longer. Unfortunately it loses it's strength when left in place too long and will go dead much quicker if the car isn't driven daily. An overage battery can also negatively affect the gas mileage your Prius will get as the car will continually attempt to charge it back to full status but the battery simply won't accept a full charge any more. That takes gas; gas that could be going into moving the car but is instead going into a futile attempt to charge a battery that is partially dead.

Finding a replacement battery for the Prius

The Prius battery is an unusual one - it is smaller than normal cars carry and a glass mat construction rather than the normal lead acid type. It can be purchased from a Toyota dealer, of course, but there is also one other option.

Just one company makes a battery that is a direct replacement for the Prius battery and it is considerably superior to the one from Toyota at about the same cost. That company is Optima, a well known battery manufacturer that produces top quality batteries, and the model number as of the time of this article is DS46B24R. This battery is a direct replacement for the 2004 and newer model Prius that is equipped with a smart key system, as most cars in that range are.

It is slightly too large for cars that are not equipped with the smart key; it fits into a depression in the trunk that in other models is just a bit too small. There is another Optima that is described as fitting into earlier models or cars without the smart key system, but this writer cannot guarantee that it will fit properly. That battery also requires some modification to the battery terminals to be used, and installation kits are available for that.


Make sure, then, that if you are purchasing one of the Optima batteries (highly recommended over the stock Prius battery from Toyota) that the correct model number of DS46B24R is purchased; while other models may fit without a special installation kit, they may not.

Tools to remove the Prius battery

There aren't a lot of tools necessary for this task; it just isn't that major. Make sure you have the following on hand, though:

  • Either a 1/4" or 3/8" drive ratchet.
  • An extension for the ratchet, about 5" long. A 3" is too short; an 8" probably too long.
  • 10 mm socket, preferable a deep socket
  • 12 mm socket, shallow or deep.
  • Large flat screwdriver or small crowbar. It will be used to spread the battery terminal for easy removal.
  • An old glove, rag or some other method to cover the battery terminal so that it doesn't short out on surrounding metal.

Removing the Prius Battery

Before starting the work you will need to decide whether you want to keep power to the vehicle at all times. Failure to do so is not a major problem but you will lose such information as the radio presets and power window limits if you don't maintain power. Neither is a big thing to correct - input the radio information again and run the windows up and down a few times.

If you want to avoid this small problem, you will need another battery (perhaps in another car) and jumper cables or a batter charger. Instructions are in the section below, and once the power source is connected you can proceed. Do take notice of the caution given there.

  • Open the hatchback and if you have a floor mat there, remove it. Remove the carpet and the cover over the hidden storage space.
  • Remove the plastic storage bin, exposing the spare tire.
  • The battery is located under a plastic molding on the far right, in the rear of the trunk. Unsnap the molding and remove it. This will expose the battery to view.
  • Crossing the top of the battery is a squarish air duct used to cool the primary drive battery. It is far too close to the battery to permit removal and must be removed before the battery can be taken out. There is a 10 mm bolt on the side of the car, behind the carpeting on the side, and another nearer the center of the car. Pull the carpeting up enough to reach the bolt and remove it as well as the second bolt; the duct can then be worked out and set aside.
  • It will be far easier if the brake controller, just to the inside of the car from the battery, is moved aside. There are three 12 mm bolts holding it; remove those bolts and carefully swing it aside and out of the way. There will still be wires attached to it, so take care not to tug hard on those wires.
  • Remove the positive cable from the battery, including the plastic housing over the terminal. Loosen the 10 mm bolt and, with a screwdriver or small pry bar, open the terminal a small amount. Lift the terminal straight up, being very careful not to allow it to touch any part of the car. Cover it with an old glove or wrap a rag around it to prevent shorting to the car body and set it out of the way.
  • Remove the battery hold down, a bar across the top of the battery. There is a 10 mm bolt on one side and a 10 mm nut on the other; take them both out and remove the hold down.
  • It will be easier to remove the negative terminal from the car body rather than from the battery. There is a 10 mm bolt holding it to the car, and that bolt is much easier to reach than the bolt on the terminal itself. Remove that bolt and fold the wire over the battery.
  • There is a vent tube on the forward side of the battery. Pull the tube straight out of the battery and, without pulling it from the car, set it out of the way.
  • Lift the battery out of the car and remove the negative battery terminal and wire from the battery.

You are ready to install the new 12 volt Prius battery.

Keeping power to the car

While not absolutely necessary, it will be advantageous to keep some power to the car during the battery change in order to keep the settings on the radio and windows.

To do this, open the hood. On the far drivers side is the fuse box; pop the lid open. Inside is a large red plastic cover over the positive jump start terminal. Remove the plastic cover.


Connect either a battery charger or jumper cables from another battery or car. The positive wire goes to the bolt terminal under the plastic cap, the negative (black) wire can be clamped to a nearby bolt head on the body near the windshield

Caution! Connecting the wires backwards may cause several thousand (yes, thousand) dollars worth of damage to the electronics of the car. Make absolutely certain that you know which wire goes where before making any connections! Your Prius manual has further directions; the wires are to be connected exactly as if you were going to jump start the car.

Click thumbnail to view full-size
Floor mat has been removed from the trunkCarpet and storage cover have been removedThe plastic storage bin removed, exposing the spare tire.  The arrow points to the molding to be lifted out.Bolt holding the square cooling vent in place.  Remove this one and one nearer the center of the trunk and work the vent freeCooling vent has been removedArrows show the three bolts holding the brake controller in place.Brake controller has been unbolted and swung out of the way for easier battery removal.  Without this step it is difficult to lift the battery out.Location of the positive battery terminal (remove the entire plastic assembly in one piece) and the battery hold down bolts.The positive terminal has been removed, wrapped in a rag and set out of the way.Pull the vent tube straight out of the batteryBolt location of the negative terminal wire.  It's easier to remove this bolt than the terminal itself while still in the car.
Floor mat has been removed from the trunk
Floor mat has been removed from the trunk | Source
Carpet and storage cover have been removed
Carpet and storage cover have been removed | Source
The plastic storage bin removed, exposing the spare tire.  The arrow points to the molding to be lifted out.
The plastic storage bin removed, exposing the spare tire. The arrow points to the molding to be lifted out. | Source
Bolt holding the square cooling vent in place.  Remove this one and one nearer the center of the trunk and work the vent free
Bolt holding the square cooling vent in place. Remove this one and one nearer the center of the trunk and work the vent free | Source
Cooling vent has been removed
Cooling vent has been removed | Source
Arrows show the three bolts holding the brake controller in place.
Arrows show the three bolts holding the brake controller in place. | Source
Brake controller has been unbolted and swung out of the way for easier battery removal.  Without this step it is difficult to lift the battery out.
Brake controller has been unbolted and swung out of the way for easier battery removal. Without this step it is difficult to lift the battery out. | Source
Location of the positive battery terminal (remove the entire plastic assembly in one piece) and the battery hold down bolts.
Location of the positive battery terminal (remove the entire plastic assembly in one piece) and the battery hold down bolts. | Source
The positive terminal has been removed, wrapped in a rag and set out of the way.
The positive terminal has been removed, wrapped in a rag and set out of the way. | Source
Pull the vent tube straight out of the battery
Pull the vent tube straight out of the battery | Source
Bolt location of the negative terminal wire.  It's easier to remove this bolt than the terminal itself while still in the car.
Bolt location of the negative terminal wire. It's easier to remove this bolt than the terminal itself while still in the car. | Source

Installing the new Prius 12 volt battery

Installation of the new battery is a reversal of the steps already taken.

  • Install the negative terminal wire onto the new battery, making sure it is pointed in the same direction as it was on the old battery.
  • Set the battery into the floor cavity, with the positive terminal towards the front of the car.
  • Bolt the negative terminal to the car body.
  • Set the hold down bar into place and bolt it down. Just snug the bolt and nut; excessive tightening will do nothing but bend the bar and could, if grossly over-tightened, crack the battery.
  • Attach the positive terminal assembly, being careful not to twist the wires from the position they were in originally.
  • Replace the vent tube by pressing it into the hole in the side of the battery.
  • Replace and bolt the cooling vent for the main battery pack.
  • Replace brake controller, bolting it all three locations.

Now is a good time to remove the jumper battery or battery charger, if used, and test start the car. If it does not start, carefully check that the positive and negative wires are in the correct location on the battery and, if necessary, provide a jump start. Once you have started the car, turn it back off and the rest of the installation can continue.

  • Replace the plastic molding covering the battery assembly.
  • Replace the hidden storage over the spare tire.
  • Replace the flooring over the storage area.
  • Replace both carpet and any floor mats.
  • Disconnect the jumper battery or battery charger, if used, in the engine compartment.

Most batteries will come with sufficient charge in them to start up the electronics of the Prius, but will need additional charging before long. You will need to take the car for a short drive, a few miles, as soon as convenient.

Installing a 12 volt Prius battery

The new battery.  The negative wire is attached and it is ready to set into place.
The new battery. The negative wire is attached and it is ready to set into place. | Source

If you're satisfied with the results of the quiz, it's time to start the project. Return to the beginning and carefully follow each step. You will save yourself around $100 by doing the work yourself.

© 2013 Dan Harmon

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    • wilderness profile image
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      Dan Harmon 5 months ago from Boise, Idaho

      I doubt the main battery was damaged by running the car on a dead or nearly dead 12 V battery. Could be the charger for the 12 V battery, could be a sensor somewhere or even one of the electric motors. I'd have to say to take it to a Toyota dealer. Sorry I can't help you more.

    • profile image

      Linda LK 5 months ago

      12 volt battery replaced. Now inside lights dim with headlights on and cell battery not recharging well.

      I did notice decreasing mpg from 49-46 prior to failure. I now get 57-58 mpg. Could the Cell batteries have been damaged by running car on dead battery or during installation, some issue that is causing the cell battery to be engaged to the point of poor recharging?

    • wilderness profile image
      Author

      Dan Harmon 17 months ago from Boise, Idaho

      Removing the negative cable first makes it less likely that the positive cable will short against a ground (body). However, in the case of the Prius there is the second, drive, batter to consider, along with the electronics to charge the smaller battery. At least some of this will be grounded as well, meaning that touching the positive cable to the body even after the negative cable is removed will likely produce a large spark.

    • profile image

      macondo100 17 months ago

      When removing the battery, don't you always want to disconnect the negative terminal from the car body first and then disconnect the positive terminal?

    • wilderness profile image
      Author

      Dan Harmon 19 months ago from Boise, Idaho

      You might try it and see if it starts the car, but that would not be proof that it's OK - it could be broken inside and making electrical contact until it vibrates a little and loses it. I would be afraid to depend on it, but if there is a way to use it safely for a month or two (carry a second battery and some jumper cables?) I'd be happier.

      It's also possible that it was defective from the manufacturer - it should have taken considerable effort to bend that terminal. Although I've seen terminals damaged on other batteries with hammers or pry bars, the small wrench used to loosen or tighten that terminal nut should have been unable to bend it.

    • profile image

      karmakid64 19 months ago

      i purchased the new optima and i bent the negative post it did not take a lot of strength to bend i think it was too easy to bend. My question is: is the battery now bad? i think i may have heard a little air escaping. ugh i hope i didn't break it.

    • wilderness profile image
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      Dan Harmon 2 years ago from Boise, Idaho

      Can you re-state your question? Not sure what you mean by "ran out" - both the Prius C batteries are charged every time the car is ran.

    • peachpurple profile image

      peachy 2 years ago from Home Sweet Home

      prius c battery is dry type right? How long do you use it before it ran out?

    • wilderness profile image
      Author

      Dan Harmon 3 years ago from Boise, Idaho

      No. There should be no connection between replacing the 12 volt battery and the smart key system. Certainly not in a simple reduction in range. Nearly a year after replacing the battery, I am also seeing a reduction in range of the electronic key but only one of them - the other works fine. If the battery change was at fault somehow both keys would be affected.

      I suggest another key battery replacement and if that fails to help, another key. Expensive at around $200 but it might be worth it.

    • profile image

      ThePaz 3 years ago

      When our 12 volt battery died after 6 years of use in our 2008 Toyota Prius we had it replaced. Now we're having problems with the electronic key working as well as it has in the past. We replaced the battery in the electronic key but the problem persists. My wife would go shopping & when she approached the car with the electronic key in her pocket the doors wouldn't open when you came in close proximity to the vehicle as it had in the past. Does replacement of a 12 volt battery require something being done so the electronic key works properly?

    • wilderness profile image
      Author

      Dan Harmon 3 years ago from Boise, Idaho

      Good point. As long as that positive wire is attached to the battery, any wrench used to loosen becomes positively charged as well and could short out if touched to the car body. It is wise to insulate it so that it cannot short if it is inadvertently touch to the body.

    • profile image

      Thank you! 3 years ago

      THANK YOU. So much...the photos marked with the bolts were especially useful.

      You might want to add that you also have to disconnect the bolt holding the positive wire to the pole - I knew that, but people who haven't changed a battery might not. They also need to know NOT to allow any metal tool sitting on that bolt to make contact with the car body - I wrapped a sock around the metal brake controller bracket at the back of the car, or you could use a rubber cover for your socket handle if you have one.