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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.

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.

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.

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.

Conclusion

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.