2001 Toyota Prius NHW11 battery swap

danger, high voltage

DANGER: If you make a mistake while working on this battery, you will die.

DANGER: Hazardous voltage. Contact will cause electric shock, burn, or death.

This traction battery pack can deliver over 100 amperes at over 270 volts DC.


  • These photos and descriptions are provided for information only. Do not attempt.
  • This Web page is NOT created by, sponsored by, endorsed by, affiliated with, or otherwise connected to Toyota Motor Corporation, Toyota Motor Sales USA Inc, any other Toyota subsidiary, any Toyota dealership, Panasonic EV Energy Co. Ltd, or Primearth EV Energy Co. Ltd.
  • The creator of this Web page, other than owning a 2001 Toyota Prius, has NO CONNECTION with Toyota Motor Corporation, Toyota Motor Sales USA Inc, any other Toyota subsidiary, any Toyota dealership, Panasonic EV Energy Co. Ltd, or Primearth EV Energy Co. Ltd.
  • The Prius shown here, and its original battery, were produced in June 2000 for the United States market. The parts in cars produced at other times or for other markets will vary.
danger, high voltage

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In early July, 2010, the original battery in my 2001 Toyota Prius failed, with about 122,000 miles (196,000 km) on it. I determined that it had a shorted cell in one of the battery modules. After evaluating the options, I decided to order a replacement battery from Toyota and swap it myself. The photos on this page document some aspects of this swap.

There are a few Web sites that offer Toyota parts at a discount from list price. I found out that one of them was actually operated by a local Toyota dealer, so I ordered through that site. The Toyota part number is G9510-47020, and I paid US$1,839.20 for it. Sales tax was about 8.65% or $159.10, for a total of $1,998.30. I was charged US$69 shipping, but that was probably because I was close enough to the dealer that they had the battery delivered to the dealership along with their other orders. They were going to deliver it to me on their own truck, but I picked it up at their parts department. If it had been shipped directly to me via UPS ground or FedEx ground, the shipping charge would probably have been much higher.

References used include:

Toyota manuals:
2001 Prius Repair Manual, Volume 1, RM778U1
2001 Prius Repair Manual, Volume 2, RM778U2
2001 Prius Electrical Wiring Diagram, EWD414U
Prius New Car Features, May, 2000, NCF182U
Technical Instructions for Special Service Campaign 40G

Other manuals:
How to Keep Your Volkswagen Alive: A Manual of Step-by-Step Procedures for the Compleat Idiot by John Muir
"Don't be fearful, be careful." "FRONT means the front of the car."

Discussion groups:
Prius Technical Stuff Yahoo! group

"Front" means the front of the car, when the battery is installed in the car normally. "Driver's side" means the left side of the car.

Tools used

NOTE: This list is incomplete.

Supplies used

NOTE: This list is incomplete.

New battery from Toyota

thumbnail of dscn0320.jpg This is the box the new battery comes in, with volume 2 of the repair manual for scale. The box measures about 12.5" H x 39" W x 18.5" D, or about 32 cm H x 100 cm W x 47 cm D. There are holes in the sides for a fork truck or pallet jack; the holes are about 2.5" H x 7" W on 20" centers, or about 6.3 cm H x 18 cm W on 50 cm centers. There are also four plastic hand-holds in the sides; the simplest way to carry it is with two people, one on each end of the box. The shipping weight is 124 pounds (56 kg).
thumbnail of dscn0321.jpg This is what I saw when I opened the box.
thumbnail of dscn0323.jpg And this is the packing material in the bottom of the box. As far as I can tell, the battery is not strapped in or tied down in any way; it just sits in the box.
thumbnail of dscn0324.jpg The "Nonspillable Battery" label is required for shipping only. It is held on with office-type clear adhesive tape. More about the nuts in the bag below.
thumbnail of dscn0325.jpg Warning label 1 of 3. This one is in English and Japanese. The English half has US addresses.
thumbnail of dscn0326.jpg Warning label 2 of 3. This one is in English and French. It has a Canadian address.
thumbnail of dscn0327.jpg Warning label 3 of 3. This one is also in English and French but it doesn't have an address.
thumbnail of dscn0330.jpg This is what I saw after I removed the "Nonspillable Battery" label. The plastic bag taped to the battery case contains the four module terminal nuts that are not already installed - two for the service plug cables, two for the main cables.

In the car, the big hole is where the blower duct sucks cooling air out of the battery case. Here, you can see a few of the modules through the hole.

thumbnail of dscn0332.jpg Hey, look, I really did get NiMH modules instead of old pinball machine parts!

The battery is shipped with the cover bolted on. You have to remove some 12mm and 10mm nuts, plus one T30 Torx screw, to remove the cover.

The modules are installed and clamped in. The front and rear bus bar modules (orange) are already installed and bolted to the module studs. The sense wire harness is already installed, and the orange connector for it is in the battery ECU / system main relay compartment, with a plastic bag over it. The black plastic bus bar module protectors are already installed on the bus bars.

thumbnail of dscn0334.jpg View with one of the bus bar module protectors removed. Note that the module stud on the far right has no nut on it; this is where the main cable (positive) bolts on.
thumbnail of dscn0335.jpg Same as the previous view, but a little further towards the center of the battery.
thumbnail of dscn0336.jpg The far end of the battery, with the second bus bar module protector removed. The module stud on the far left has no nut on it; this is where the main cable (negative) bolts on.

In the background there is a box full of lubricant for the qualified EV technician. It is made from renewable natural resources and all waste products are biodegradable. The containers can also be locally recycled (into fiberglass insulation). CAUTION: Only for use after working on batteries, not before.

thumbnail of dscn0337.jpg This is where the battery ECU and system main relay (SMR) go. The orange connector on the left plugs into the battery ECU; it has a wire to every other module in the stack, so the ECU can monitor the voltages of every two modules.
thumbnail of dscn0338.jpg View of the front of the battery with both bus bar module protectors removed. The two module studs in the center have no nuts on them; this is where the two cables to the service plug bolt on.
thumbnail of dscn0339.jpg Close-up of the two center modules where the service plug cables connect. There is a black plastic insulator around these cables that has to be transferred from the old battery.
thumbnail of dscn0340.jpg The inside of the cover. The black stuff appears to be foam weatherstripping.
thumbnail of dscn0341.jpg Another view inside the cover. The blue tape apparently provides some extra protection to a place where several cables end up.
thumbnail of dscn0342.jpg Another view inside the cover.
thumbnail of dscn0358.jpg The serial number label on the new battery; partially edited.

Getting the old battery out of the car

thumbnail of dscn0343.jpg The directions in SSC 40G sort of imply that the battery goes in and out through the trunk, but I took it out through the rear passenger door.

At this point I had already lifted the ends of the battery, one at a time, past the "humps" where the C-pillars meet the trunk floor, and had it sitting where the bottom cushion of the back seat would normally be.

I had to take out several bolts to get to this point: the ones that hold the rear seat back in, the ones that hold a couple of support bars for the rear seat back, and the ones that hold down the battery itself. None of them were particularly tricky to get to. I put each bolt or set of related bolts in a plastic sandwich bag, and labeled the bag with a felt-tip permanent marker. John Muir taught me to do this and it works.

thumbnail of dscn0344.jpg Another view. I left the seat belt for the center rear seat passenger installed and just put it around the driver's headrest to keep it out of the way.
thumbnail of dscn0345.jpg Starting to come out the door. Be careful, or the flange of the battery can cut the door gasket. It is also not hard to scratch the inner door panel with the battery.
thumbnail of dscn0346.jpg On the ground, next to the car. (I was by myself, so I couldn't take photos between the above one and this one.)

Note that in this photo, I have turned the battery almost 180 degrees from the way it sits in the car. Normally, the end with the big hole in the top cover is on the driver's side.

thumbnail of dscn0347.jpg Standing outside the rear driver's-side door, looking back into the trunk at where the battery used to be. You can see the two main cables at the far right side of the picture. The three places where the front edge of the battery bolts down to the floor are also visible, just forward of the shadow of the rear parcel shelf.
thumbnail of dscn0348.jpg Standing behind the car, looking forward into the trunk at where the battery used to be. You can see the two main cables at the far left side of the picture. One of the places where the rear edge of the battery bolts down to the floor is also visible, at the lower right corner of the photo.
thumbnail of dscn0349.jpg Moving the old battery into the house on a hand truck (dolly, two-wheeler). I first stood the battery on end, then walked it onto the hand truck. This is a relatively convenient way to move the battery around. Since I only had to go about 30 feet (10 m) on the level, I didn't strap the battery to the hand truck; if I needed to go up or down steps, a strap would have been a good idea.

Stripping the old battery

thumbnail of dscn0359.jpg This says "Kilroy was here." I really think so.
thumbnail of dscn0360.jpg This is the external part of the service plug assembly. Note that it is held to the case with three screws. The vertical white connector is for the microswitch that detects whether the service plug is locked in.

The white connector visible through a hole in the battery case is the battery ECU connector. There is one screw above and one screw below this connector; these hold this end of the battery ECU to the battery case.

thumbnail of dscn0362.jpg Another view of the service plug. The white connector hanging at the left of the picture is for the SMR. The orange plate on the end of the battery holds the secret charging connector; more below.
thumbnail of dscn0363.jpg Looking at the end of the battery on the passenger side. At the top, the white plastic pipe is the vent manifold that connects the two rubber vent tubes from the modules to the vent tube that goes overboard in the car.

The wires that run from the top left to the bottom center go to the thermistor that senses the battery intake air temperature. These wires are part of the thermistor harness that runs on top of the modules inside the battery. The wires are taped to the battery case.

thumbnail of dscn0365.jpg Two nuts removed and two cables to the service plug, complete with black plastic insulator, removed from the center modules. Normally the thick orange cables are each clipped into the two orange clips that hang down from the bus bar modules.

This battery had SSC 40G done to it, as can be seen by the sealing goop on every other module stud, as well as by the black squares below every other module stud.

thumbnail of dscn0366.jpg One nut removed and one main cable (positive) removed from the end module. Its insulator stays with the bus bar module.

Not pictured: the other end of this cable connects to the top of the SMR with a screw.

thumbnail of dscn0367.jpg One nut removed and the other main cable (negative) removed from the end module. Its insulator stays with the bus bar module. Normally the thick black cable is clipped into the orange clip that hangs down from the bus bar module.

Not pictured: the other end of this cable connects to the top of the SMR with a screw. There is also an "aluminum shield" on this cable that is screwed down to the battery case near the battery ECU / SMR compartment.

thumbnail of dscn0368.jpg SSC 40G implies that the cables can be detached from the back of the service plug assembly, but doesn't specify exactly how. I looked at the connections and I am not sure how they come apart; they might join together kind of like sticking a knife between the tines of a fork. I decided to see if I could remove the service plug assembly without detaching the cables.

The white plastic cable protector, on the right, is held to the case by a nut at the bottom (seen removed at the bottom right of the photo). Once that nut is removed, and all three screws that hold the service plug assembly to the case are removed, I could swing the bottom of the service plug assembly up, as if it were hinged at the top of the battery case, and then start feeding the white plastic cable protector through the hole in the battery case.

thumbnail of dscn0369.jpg A little further along in service plug assembly extrication. At this point I removed the secret charging plug to gain a little more room.
thumbnail of dscn0370.jpg This orange plastic plate is clipped to the side of the case with an "interlock" plug, just like the one on the cover for the main cables inside the car. Unclipping the interlock plug let me remove the plate from the side of the battery case.

Once I did that, I found the secret charging plug clipped into a cavity on the back side of the plate. This plug has a wire to the most negative module, a wire to the most positive module, and a wire to what I assume is 12 volt ground. The connection to the modules goes through a fuse on the bottom of the SMR.

I suspect, but don't know, that this plug is used at the factory and maybe at the warehouses to check on the overall battery voltage and possibly to recharge it, using a very well-regulated charger with overcurrent, overvoltage, and timed shutoffs.

Anyway, once I got this out of the way, I could get the service plug cables fully out of the battery case.

thumbnail of dscn0372.jpg Looking down into the SMR compartment to see one of the three mounting screws for the SMR...
thumbnail of dscn0373.jpg ...and the second mounting screw about an inch (2.5 cm) away. These are fairly far down there; I used a nutdriver to loosen the screws, and then magnet-on-a-stick to get them out.
thumbnail of dscn0374.jpg There is a third SMR mounting screw on the front (FRONT) of the battery case. Once it is removed, I could lift the rear end of the SMR up, and lift the SMR out of the battery case.
thumbnail of dscn0375.jpg The hole where the SMR was.
thumbnail of dscn0376.jpg The sense wire harness (orange plug) unplugs from the ECU first. Then, the thermistor harness (white plug) can be unplugged. Then, I could remove the nut that holds the "foot" bracket for the front edge of the battery ECU. I removed the two screws near the battery ECU connector at the rear of the battery case (seen above) and the battery ECU lifts out.

Removing the vent tubes (not shown)

My vent tubes were sticky to the touch on the outside. As soon as I felt the stickiness, I quickly washed off my fingers, fearing that there was battery electrolyte on the outside of the tubes; I did not develop any burns or discomfort on my fingers. I wore gloves for removing and reinstalling the tubes after that.

Removing the vent tubes was not too hard; basically I grabbed the black rubber tube with two fingers, and rocked it front to back while pulling up, to release it from the barb on the module vent tube. Every few modules, I could feel a hard plastic connector that joins two sections of the rubber vent tubes; I just left these connected. One of the rubber vent tube assemblies is slightly longer than the other one.

At the passenger end of the battery, there is a white plastic manifold that connects the ends of the two rubber tubes. I rocked the manifold up and down while pulling out on it to release the barbs on the manifold from the rubber tubes. Once the manifold is removed, I could squeeze in on the rubber lip at the end of the vent tube, and pull it out of the steel end plate of the battery.

Removing the thermistor harness (not shown)

I did not have to cut off any thermistors or connectors! The holes that everything has to go through are big enough to fit what has to go through it.

I started by removing the intake air temperature thermistor, seen above. It is held in by a rubber grommet through the sheet metal - I squeezed the grommet and worked it out of the sheet metal, being careful of the exposed thermistor. I peeled back the tape on the end of the battery, and removed the rubber grommet through the steel end plate by squeezing and pulling. I then carefully pulled the wires and thermistor towards the interior of the battery.

Next, I noted which modules on the old battery had thermistors on them, and marked the modules on the new battery that should have them. (I used a felt-tip permanent marker.) I also noted where the four white plastic clips are that hold the harness to the strut. I worked my way from the passenger side to the driver's side, unclipping the thermistors and pulling them straight up out of the modules, and unclipping the white plastic clips from the strut.

At the battery ECU compartment, I squeezed the rubber grommet and pushed it out of the sheet metal - from the battery ECU compartment towards the modules. Once the grommet was free, the hole in the sheet metal was big enough to pass the connector through.

Removing the plastic nuts for the air duct

The driver's side air duct attaches to the top cover with sheet-metal screws into four plastic nuts that are pressed into the top cover. The new battery does not come with the plastic nuts.

I used a pair of pliers to gently squeeze the plastic nuts and push them out from the inside of the old battery cover towards the outside. Each nut also has a tiny rubber gasket under the head, which sometimes stayed with the nut and sometimes stuck to the cover. I peeled off the gaskets that were stuck to the cover and reinstalled them on the nuts.

Then, I used my fingers to push the nuts into the new battery cover.

Additional photos

You can see some more photos of the System Main Relay (SMR) and the service plug assembly on the Components page.

Cleaning the cables

The ends of the main cables and service plug cables that bolted to the modules had some corrosion on them; mostly blue flaky stuff. I wanted to clean this corrosion off before putting the cables on the new battery. It is also possible for these cables to have a little alkaline electrolyte on them and I wanted to clean that up too. I soaked the cable ends in vinegar (the bottle says "distilled white vinegar, diluted to 5% acidity"), and when they quit bubbling, I took them out and rinsed them in tap water. I scraped off the remaining gunk with a plastic scraper, then used more vinegar and tap water. Finally I used a kitchen sponge (mostly soft yellow foam on one side, but with a thin green scrubbing pad on the other side) to shine up the ends of the cables, followed by a final rinse and drying with paper towels.

thumbnail of dscn0407.jpg These are the service plug cables. I put just enough vinegar in the bottom of the cup to cover the cable ends, and then let the ends soak in it. I was trying to avoid having a lot of moisture get up between the wire and the cable insulation, and I think I succeeded.
thumbnail of dscn0409.jpg Service plug cables soaking. SST QT-32OZ can be obtained from Mr. Lamar, Tulsa, Oklahoma. :)
thumbnail of dscn0412.jpg All four cable ends after vinegar and scraping but before scrubbing pad.
thumbnail of dscn0413.jpg This insulator goes around the service plug cable ends at the modules. The end with the two large holes in it goes under the cable terminals, and then the end without the holes in it folds up to cover the terminals. The insulator had some flakes of corrosion from the cable ends, plus some of the goop installed with SSC 40G on it. I soaked the whole insulator in vinegar, used a plastic scraper to get rid of the goop, and then rinsed with water. This shows the insulator after cleaning.

Putting parts on the new battery

thumbnail of dscn0396.jpg Second vent tube about to be reinstalled. I put the tube through the end plate first, and then plugged the vent tube on to the module closest to the end wall. It took two fingers and a rocking motion to get it over the barb on the module. Then, I worked my way back towards the battery ECU compartment. When I had the tube plugged on to all the modules, I went along the tube again, giving it another push at each module to get it seated down really well to the top of the module.
thumbnail of dscn0397.jpg First vent tube installed, second vent tube ready to install.
thumbnail of dscn0398.jpg Both vent tubes on. Thermistor harness installed. I started by passing the battery ECU connector through from the module compartment to the battery ECU compartment, and getting the grommet installed properly in the sheet metal. I then plugged the thermistor into the module next to the battery ECU, and then worked my way towards the passenger side, installing thermistors and wire clips as I went.
thumbnail of dscn0421.jpg Battery ECU installed, SMR installed, main cables installed, service plug installed.
thumbnail of dscn0422.jpg View of battery ECU and SMR compartment.
thumbnail of dscn0423.jpg View of entire pack, 1 of 3.
thumbnail of dscn0424.jpg View of entire pack, 2 of 3.
thumbnail of dscn0425.jpg View of entire pack, 3 of 3.
thumbnail of dscn0426.jpg View of cables at front driver's side of battery. The area where the cables in the orange conduit cross into the battery ECU / SMR compartment is where the blue tape is on the top cover of the battery.
thumbnail of dscn0427.jpg View of cables at front driver's side of battery. You can see the nut that holds the "aluminum shield" for the negative battery cable onto its stud.
thumbnail of dscn0428.jpg View of passenger end of battery. The new battery has a piece of foam on the end plate that the old one didn't have. There is a diagonal slit in the foam, I think for the thermistor wire, but I am not sure. I re-used the tape from the old battery to hold the thermistor wire into the slit in the foam.
thumbnail of dscn0429.jpg View of passenger end of battery.
thumbnail of dscn0430.jpg View of passenger end of battery.
thumbnail of dscn0432.jpg View of service plug and battery ECU connector.
thumbnail of dscn0433.jpg View of driver's end of battery.
thumbnail of dscn0434.jpg Kilroy was here, too!

Differences between old and new battery

thumbnail of dscn0417.jpg This is where the service plug assembly went on the old battery - the vertical slot on the left side. There are three threaded holes: one on the bottom right, one on the center left, and one on the top left.
thumbnail of dscn0418.jpg This is the service plug assembly installed on the new battery. There is no threaded hole on the center left. The bottom right and top left holes in the case match the service plug assembly. I used these two holes only, ending up with one extra screw.
thumbnail of dscn0420.jpg The left rear corner of the old battery. No big hole.
thumbnail of dscn0419.jpg The left rear corner of the new battery. Big hole, about 1" or 2.5 cm diameter. I'm not sure what this is for. Since the battery is air-cooled, I figured it would be good to close off this hole before I installed the new battery in the car. I covered it with overlapping strips of vinyl electrical tape, UL listed for 600 V and 80° C (176° F).

Putting the new battery back in the car

thumbnail of dscn0435.jpg I put the new battery on the hand truck and took it out to the car. This is where I put it before lifting it in to the car. The camera lens fogged up when I took it outside...

Installation was the reverse of removal. :)

thumbnail of dscn0438.jpg One of the two strange screws that hold the main cables (to the inverter) on to the battery. The end is spherical; is it a tiny corona ball?


After installing the new battery, I started the car and let it idle for several minutes, while I watched for warning lights and indications on the multi-function display. I didn't get any. I then drove it carefully around the block in my neighborhood, which went OK. I took a longer trip around the neighborhood - OK. I drove to the next town, staying off the Interstate - OK. I got on the Interstate and drove about 9 miles (14 km), turned around, and drove about 19 miles (30 km) back home - OK. A couple of days later, I started driving it on my regular commute to work (about 40 miles (64 km) one way) - OK.

The car now has 183,000 miles (294,000 km) on it - the new battery has been in service for 61,000 miles (98,000 km) and about 2 years, 7 months. I have not had any problems with the new battery so far. When I first installed the new battery, I did not notice any mileage improvement immediately, but after a few weeks, I thought I saw an improvement of a few miles per gallon. (I have the raw data on this, but I have not yet studied it to see exactly what the improvement was.)

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Last updated Fri Feb 8 20:11:22 CST 2013