How to Change a 12V Prius Battery - 12V Battery Replacement
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 to 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.
Update: Since the writing of this article, ACDelco, a part of General Motors, has begun offering these batteries as well. Amazon carries the Delco brand as well, and it, too, is an exact fit for the Prius in the years discussed, at a considerably lower price.
Prius batteries from Amazon
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 battery 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.
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.
Ready to go?view quiz statistics
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.
Questions & Answers
Does a 2009 Prius need to be reprogrammed after a new battery is replaced?
If you disconnect all sources of 12V power to the car a few things will need re-programmed, such as the windows and radio settings. They are all very simple (run the windows up and down, input the desired radio stations, etc.) and there is nothing that will keep the car from operating fine. (I'm assuming you refer to the small 12V battery, not the one that moves the car.)
© 2013 Dan Harmon