Installing a Prius HV Battery

GEN 1 Battery Replacement

“Oooh I just knew they were going to break down and be expensive to fix”.

Don’t get too excited my Hybriphobic friends. Yes, some first generation Prius battery modules have failed. However, they fail at fairly high mileages (over 100K) and replacement cost is comparable to, say, a similar vintage Honda Odyssey transmission. Yet when an Odyssey owner’s transmission fails, friends and aquaintances don’t get giddy and announce to anyone who will listen that they knew this was going to happen. Nope. They would probably say something along the lines of “bummer dude”, followed by something like “should be worth fixing though, Honda makes a great car”. Well Toyota makes a great car too. It’s likely a Gen 1 Prius will last longer with fewer major repairs than a conventional Subaru or Mazda, but that does not dovetail with commonly held preconceptions, so it doesn’t make for the sensational I-told-you-so story.

OK. Done with the preaching. On with the replacement.


If you are more interested reading aa bit about how an HV battery’s health can be assessed, click here.


Before making repairs to the HV system, the first step is to remove the interlock at the battery pack and wait five minutes. One should wear lineman’s gloves to guard against shock until one is certain no voltage is present.

That reminds me. I forgot to warn y’all not to kill yourself mimicing the procedures outlined in the article. This article does not provide all of the information you’ll need to replace an HV battery, or work safely with high voltage. This article is only intended to provide the curious with a general overview of the procedure, not to provide the step by step information necessary to embark on a battery replacement.




Once the interlock is pulled the rear seat and trunk trim must be removed from the car. I’m using a set of KTC trim clip pliers. KTC (Kyoto Tool Company) is a Japanese tool manufacturer, and they make several Japanese application specific tools that are not available from any other source. I have not broken a single Honda splash sheild clip since I bought the KTC tool for that purpose. Anyway, I love their tools so much I thought I’d give them a plug and provide our friend Glen with a link. You can purchase KTC tools from this vendor in the US.



Once the seat and trim are removed, set them off to the side where they won’t be damaged or greasy. The cream colored interior is a grease magnet. If you so much as look at it without washing your hands, you’ll be spending some time on cleanup duty.



Next, I remove this plate. It covers the two high voltage lines that go from the battery to the inverter. Should you be wearing your high voltage gloves even though you disconnected the HV battery interlock? Yes. Until you have verified that there is no voltage present, you should have gloves on. There is no reason there should be voltage here with the key off. Disconnecting the 12V battery provided another layer of protection. And removing the interlock prevents the SMR (System Main Relay) from closing and spilts the battery into two 137V sections. Still, it’s always better to be safe than sorry.


Once the nuts are removed, there is a small plastic retainer that holds the cover in place and prevents removal. The battery interlock is used to release this retainer. The clever engineers at Toyota required you to use the interlock to remove the retainer, thus preventing you from doing something stupid, like removing this cover without removing the interlock. One of the many ways Toyota prevents you from killing yourself with their products.



Check for voltage across the two cables and from each of the two cables to the body. A CAT III meter with good leads should be used for this measurement. Hey, where are the linemans gloves? Oh well, do as I say, not as I do.



The cables are secured to the battery module with an special bolt with a ball end. I don’t know why.



Next the vents are removed. The Prius uses a blower motor to pull air from an inlet above and behind the rear seat, across the battery pack and out behind the rear quarter window.



The vents and ducting are removed, the BCM (Battery Control Module) unplugged, and the cables are removed. Now it’s time to unbolt the battery pack.



There are two brackets and several bolts that hold the battery to the body.



There’s probably a hoist or sling available to remove the battery from the car, but it’s easy enough for two people to move the battery from the car. It’s just a little akward.



The trunk with the battery out.



The old battery and the new battery are now side by side on the ground, but it’s not time to reverse the removal procedure and reassmble. The majority of the job still lies ahead.



The battery comes from Toyota with just the battery modules (28 / 7.2V batteries clamped together with a bus bar attached). It does not include a BCM, SMR, vent manifolds, temperature sensors, or battery cables. All of that stuff needs to be transferred from the old battery.



This is the compartment in the new battery where the BCM and SMR will be installed.



This is the old battery with the BCM, and SMR still in place.



A few bolts and out it comes. Do you need a set of insulated tools, or to wrap your tools in electrical tape? Nope. Save your money. Is it possible to kill yourself with a Prius. Yeah, it is, but you’d really have to work at it because Toyota has not made it easy. You are WAY more likely to kill youself wiring a 3 phase compressor or other shop equipment, when there are no safety systems in place.



All of the battery temperature sensors must be removed and transfered to the new battery.




The vent manifold connects all of the individual battery module vents so that any gas or caustic mist can be vented outside the cab.



This is one of the vent manifold outlet grommets.


This is another vent manifold outlet grommet we found during a battery replacement. It looks like the mechanic did not line the plastic elbow up properly during the bus bar recall, and the grommet got folded over. Toyota was not be able to supply a  replacement manifold for quite some time; it’s not a part that would be commonly replaced. Carolyn over at Luscious Garage offered to lend us a manifold from one of her projects, but we’d need to brave bad traffic and the bridge. I was kneading the grommet between my fingers (with Nitrile gloves!) while considering whether I wanted to spend 2 hours stuck in traffic and surprisingly it actually returned to its original shape. Problem solved. There were no signs of damage or corrosion around the elbow joint, so I suspect it was actually sealing relatively well.




This is the where two of the battery cables connect. The green stuff is corrosion. The gooey stuff is the sealer from the bus bar campaign.



Luckily the new battery comes with a new bus bar, so there are only 4 battery connections that need to be cleaned. If there were 28 connections like this to be cleaned, it would probably be more cost effective to just buy a new bus bar.



Here again, the green stuff is corrosion.


The cable needs to be transfered to the new battery, so it gets a cleaning.



The battery terminal torque is very low, about 48 in/lbs. That’s too low for my 0-250 inch/pound torque wrench to measure accurately, so I use this beam type wrench normally reserved for bearing preload.



I found a bit of a surprise when I went to put the interlock holder back on. See the threaded hole marked on the picture?



I had some trouble putting the bolt back on the new battery pack due to the lack of a threaded hole. I figured they might have re-engineered the part and done away with the hole on purpose. However on two subsequent battery jobs, the hole was there. Are manufacturing robots affected by Fridays?



The cover reinstalled and the battery is back in the car.




Plugging the BCM connectors in followed by installing the brackets, vents, etc.



The first time I saw a cable cover retainer taped like this, I though there was something fiishy going on. As it turns out one turn of painters tape makes installing this retainer a whole lot less frustrating, and will not interfere with the retainer function so long as the tape is weak and a minimal amount is used.


Seats back in place.



Ready for the road.