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How To Recognize A Failing Toyota Prius Traction Battery

The Toyota Prius traction battery can be a tricky thing to understand. The more I research, the more I uncover hidden information on these batteries. Here is what I found most recently, that helped me gain a better perspective on new vs reconditioned batteries.


When it comes to a replacement battery for your Toyota Prius, the options and information you get hit with can seem overwhelming. As my quest to understand the nickel metal hydride battery continues, I find myself starting to go a little crazy with all this information.

I called a colleague of mine who is a hybrid trainer, and asked him why my Prius, even though the battery was not faulting, seemed to run more often, and not hit the desired fuel target the car is rated for. His answer shocked me.

Your Prius Battery Can Fail And Not Set A Trouble Code

There are obvious signs of a failing traction battery, some of them are easier to detect than others. What about the ones you cannot detect though? How do you know if you are having a battery on the brink of failure just waiting to rear up and bite when you least expect it. This burning question had to be answered.

Craig and I spoke about battery life expectancy and how as the battery modules age, they become unbalanced. This unbalance between the modules causes the others to try and "pick up the slack" where the weak ones are. These modules are trying to get the pack balanced again by over charging in hopes that the balance can once again be restored. What we want to avoid setting a trouble code, so we can avoid breakdown.

I was not sure if this was true though. To better wrap my head around this idea, I plugged in my trusty Carista OBD2 reader and opened Dr. Prius. I first did this on a known good 2004 2nd Gen.

2004 toyota prius hybrid battery

I was blown away what I saw. Here was a pack that has seen less use than mine, has never been balanced and had perfect balance across the module blocks. Looking at the resistance values, I can quickly see the pack balance. The car started went through a quick cycle and shut off. Mine does not do this, it keeps running until it sees what it wants.

I then went to my 2007 and plugged in. I had seen these results before, but now I had a different perspective on why my car was doing what it was doing. Sure enough, the resistance values were not at all consistent like the 2004.

2007 toyota prius traction battery resistance values

The ICE was running more often because it is trying to bring back balance to that battery pack, and even though it is not setting a trouble code it still is not shutting of soon enough to really gain that extra mpg. This is brillant, and very much needed to know information.

How You Can Tell If Your Prius Pack Is Weak

There are a couple signs you should be aware of when driving your Prius with a weak pack. One of the signs is that the battery monitor will drop and charge very rapidly. If this occurs, your pack is in really bad shape. As I mentioned before, you may not even know your pack is bad, but here is a sure fire way to tell.

When you turn your Prius on, it should do a pretty quick warm up then shut off. If it is cold out, of course it will run longer, so it is best to really get a good idea of how long your vehicle is running by having it parked in a garage or place that runs a constant temperature. This is what I did to understand how long my car was running initially before it shut off. I found that my control car was running for about 2 minutes or less, while the suspect car, mine, was running for 3 to 4 minutes.

This may not seem like much, but in reality it is. Not only that, but because the pack is less balanced, your ICE will start and stop more while you are doing regular driving. Not to mention it will stay running longer when you come up to a stop light or sign. All of this adds up to fewer miles per gallon, which is not what we want.

The next thing you need to look out for is acceleration. I did not think about this until it was pointed out to me. Which is why I think many Prius drivers do not catch battery degradation until it sets a trouble code for it.

During acceleration, you should feel very snappy and quick throttle response. This means that your traction battery is delivering maximum power evenly. If you feel a slight delay in acceleration heavy or moderate, you are looking at a pack that has become subject to pre-failure. It cannot deliver the power evenly if it is not balanced, hence the delayed response.

Other signs of course include trouble codes and terrible fuel economy. I know I should be getting at least 45 mpg per tank average, and at 41 to 42, which mind you, is not bad, but it is not 45 plus. These cars were meant for better fuel econ than what I am getting.

Use the Carista OBD2 Tool and the Dr. Prius App to help you determine this.

Toyota Prius Carista OBD2 Tool What Can Be Done To Help Your Prius Pack

A pre-failure pack is ripe for one of several things. You can of course replace the pack, but with what becomes the next question. There are loads of "rebuilders" out there that claim a "like new" battery but how can you really know for sure?
If you have the data that I have given you today, you will be able to know if the pack is good or if it is not.

Another option is to recondition the pack yourself. Hybrid Automotive can help you with that. They have a consumer line of balancing and reconditioning equipment to help weak packs. Best of all, you can "recondition" your pack every 6 months or so and really, squeeze a ton of extra life out of it. I highly recommend this option because it yields great results and does not cost as much as a replacement battery.

Last but not least, you can get a new OEM battery from the dealer. If you can install it yourself, you will save a ton of money. Best part is that Toyota OEM packs have come down significantly in price to where they are finally comparable to the reconditioned units out there. Not too shabby.


Remember that I am not suggesting a replacement pack unless it is necessary. I am suggesting you pay closer attention to how your Prius is running and accelerating. We want to find a solution to the problem before the problem actually happens, that way we can get the most out of these cars while minimizing costs. Yeah the reconditioning equipment is somewhat spendy, but it is still less than the cost of a battery. Plus with some people getting 300k plus out of an original battery, even if you gained another 100k from it, the cost is worth it.

This story will more than likely keep having regular updates as I gain more information and learn how to best serve all of you in the Prius community. Feel free to reach out to me on social media. I wanted to tell you all be sure to check out my favorite 3rd gen accessory right now which is the Nimbus Phone Mount, found on Amazon.

Check out my other story Why the first generation Prius is better than either generation Nissan Leaf.

See you in the next story where I am discussing why Carista is the best $20 tool for your Toyota Prius.

Also Watch New tech means more MPG from your Toyota Prius and Click to Subscribe to Torque News Youtube Channel for Daily Toyota Prius and Automotive News.

Peter Neilson is an automotive consultant specializing in electric cars and hybrid battery technologies. He is an automotive technology instructor at Columbia Basin College. He holds a Bachelor of Science in Automotive Service Technology from Weber State University. Peter is also an Adjunct Instructor of automotive technology at Columbia Basin College. Peter can be reached on Linkedin and you can tweet him at The_hybrid_guy on Twitter.


jg (not verified)    September 18, 2019 - 11:31PM

I got around 330,000 miles out of my original battery with a 2001 before it quit keeping up with uphill freeway traffic and I traded it in. The dealer wanted $900 to replace a computer just so the could get their computer to talk to the car for troubleshooting. I wish some of the battery information that is available now was available back then.

Ty Thompson (not verified)    May 16, 2020 - 3:06PM

"Last but not least, you can get a new OEM battery from the dealer. If you can install it yourself, you will save a ton of money. Best part is that Toyota OEM packs have come down significantly in price to where they are finally comparable to the reconditioned units out there. Not too shabby."

Thank you for the article. I have a 2005 Prius. The quote above generates many questions in my mind. Why has the cost come down on "new" "OEM" "genuine Toyota" (fill in your favorite identifier here) HV battery packs only recently? That is not usually how older "OEM" parts work (and certainly not for the brake actuator on the gen 2 Prius, for example.) Simultaneously, the warranty offered on such batteries has fallen off a cliff. The original warranty (why can't I get that too?) was 8 years 100K I believe, and there is nothing like that offered with any "new" battery anymore. Just a few years back (like approximately 10 years after the gen 2 Prius ended production ahem) both prices and warranties dropped, and now those that used to sell only refurbished batteries are selling "new" "OEM" options for only slightly more but never say who actually made them or what is going on. I consider the HV battery pack to be a serviceable item, a fact which I considered carefully when originally buying the car, and I am surprised that there seems to be so much resistance to this notion - when people think "battery" they think it either works, or you need a new one. But, after so many years, obviously you will never be able to truly obtain a genuine, new replacement. As times goes on, production on "new" "original" parts will decrease without the original production cost decreasing whatsoever, and I worry that the "new" "OEM" "genuine Toyota" HV battery market is becoming muddled. Add to this the fact that Toyota is famously secretive about how "original Toyota" parts are produced, and I don't understand how anyone realistically believes they can get a new battery for a gen 2 Prius as it was originally manufactured for way less money 10+ years on, and anyone offering a refurbished battery for the comparable or lower money with a reasonable warranty and conditions is somehow offering a thing that is "too good to be true." But that is the prevailing advice, though generally not on this site. At some point new parts cannot be obtained anymore, and the first indication is big price and warranty drop. Common sense, it seems to me. Where am I wrong? It is still possible to buy a gen 1 Prius HV battery made within the last year fresh off the boat from Japan for way below the original price although the manufacturing process is pricey and the market is nil?

Jim (not verified)    July 21, 2020 - 4:59PM

In reply to by Ty Thompson (not verified)

You have no idea what you are talking about. Toyota hybrid battery warranties are and have been 8 years/100k miles for a very long time. In 2020 they have actually extended that warranty to 10 years/150k miles.

Ty Thompson (not verified)    July 23, 2020 - 3:49PM

How very not helpful. This is a gen 2 Prius, you are referring to newer models. I have talked to dealers, battery shops, hybrid shops, and Toyota themselves. If you think you will be able to find any 100% new, manufactured by Panasonic, as originally manufactured hybrid pack for a 2005 Prius with the original off the lot warranty it is you who doesn't know what he's talking about. Anyway I went with a lifetime warranty reconditioned unit and the problem is resolved (for anyone else who might be reading).

Ty Thompson (not verified)    July 23, 2020 - 4:00PM

I am talking about an HV battery replacement for a 2005 Prius, you seem to think I'm discussing the warranty Toyota provides on the original part, possibly you did not realize the article you are commenting on is about replacements, and thus my comment obviously would be too. Or possibly you are just wrong, and erroneously believe you can recover that same warranty on a replacement somewhere, which you cannot (at least I was not able to after some extensive searching), and was the purpose of my post. You are not going to get any 8 or 10 year warranty on a HV battery replacement anywhere period, "OEM" "Original Toyota" "New" whatever label any dealer or shop may choose to use. If I am wrong, please supply a link and or contact information for that source so as to concretely contribute to the discussion? Thank you.

Cate Janert (not verified)    September 24, 2020 - 5:00AM

For car testing, an environmental chamber is needed, which can test the performance of the vehicle in different environments

Brian W (not verified)    March 3, 2021 - 12:25PM

The Toyota dealership I talked to told me their service guys have never seen a production date on the "new" "official Toyota" batteries. So, the price drop may be due to these new batteries simply being unused, and just as old as the battery you are replacing. Having said that, I still intend to go that route and test the "new" battery with the Dr Prius app, returning it if it is not in good shape.