Generation II Battery - Repair it yourself!

I am a proud 2005 Prius owner and at 192k miles the Hybrid Battery finally crapped out.

I spent a couple weeks researching and pricing (I have a second car):
Dealership - quoted me $2,800 after the core credit
Hybrid Rebuild Companies - $1,200 to $1,800 to take out, rebuild and re-install
Used Batteries - $600 to $1,000 (installed by local shops)

All seemed pricey, so i researched what it takes to repair the battery pack myself:
28 cells in a prius generation II battery
Each cell is independent and strung in a series and easily replaced
Per cell cost ranges from $37 to $45 plus delivery cost.

I looked up various instructional videos and safety information available in the 'Googles'

I decided to DIY repair. 2 hours of labor and 1 bad cell - total cost $53

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EDIT to remove offers of services for a price: No self selling yourself or services on Priuschat
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Agreed, by all rights and specs it is a good idea to have all cells balanced and conditioned. However, in a rechargeable system and for the purposes of this repair, the only thing required is the voltage measurements to be the same. I and 3 others I know have performed this same fix now and the system charges all the cells up without issue.

As for capacity, over time, cells will fail and in just a year of use, cells are out of sync on total capacity. This is why single cells are failing and not all cells lost at once. The only real issue is the voltage which leads to the resistance and the heat if one or more cells are out of whack and exhibiting low voltage.

This was the case in my battery pack where all cells tested at 7.7v with the exception of the one bad cell testing at 6.4v and which likely had a short as my fan was blowing almost immediately once the car went under load (not to mention the Christmas tree of lights on my dash).

Bottom line, no risk to life, limb or property with all cells testing at 7.7v. While capacity issues may lead to other cells failing sooner rather than later as they age, so no real loss there. And, the state of charge of each cell will balance during the charging process within the vehicle - Take on a long drive up a mountain and let her coast down in Engine Brake mode.

I choose to replace cells as they become an issue as this is a used battery in a car with 192k miles and likely not something that needs another 10 years of battery life. I expect to get at least 2 years or 24k miles out of this $53 fix. And that is fine by me!

 

So, in a 1 year, 3 year, 5 year or 8 year old battery...are you saying all the cells are balanced perfectly? I have to disagree. The 'Modules' (to correct myself and adhere to proper technical semantics) each degrade over time at different rates and fail individually over time. This very fact negates the pro-shop's dire warning that somehow not balancing the capacity and the charge of each 'Module' will somehow lead to a catastrophic vehicle fire.

I stand by the fact that the risk of run-away heat leading to fire is variances in voltage (a voltage drop) within the series of 'modules' leading to resistance. The charge and/or capacity is not a significant factor in the creation of resistance and ultimately the heating of the 'module' and those adjacent to a capacity deficient 'module'.

This is a repair of a battery pack to extend the life of such. My opinion, based on my knowledge of electronics and research of these batteries, is that for this repair, it is far less economically efficient to spend $1,800 on a professionally reconditioned and balanced battery that they admit will likely fail in 24 to 36 months anyway; when you can simply replace the cells that are voltage deficient for $50 each as they die, especially when it is usually only 1 or 2 bad cells at the most.

As for using the engine brake going down the mountain...have you let your Prius coast down a mountain?...Unless you are looking to replace your brake pads at the bottom, I suggest the engine brake...and it will charge your battery to the top of the green and peg it, and continue juicing it after it gets there. P.S. If you search for Prius Battery Modules to purchase, you will find that they are being sold as Battery 'Cells'. The industry of resale has decided to use these terms interchangeably...lol.

 

Very much AGREE with your warnings. IF anyone here does decide to do this yourself, be sure to use proper precautions (gloves being a reasonable precaution) - By all means, REMOVE THE ORANGE SERVICE PLUG BEFORE OPENING AND EXPOSING THE TERMINALS! And, test the internal and external terminals with a volt meter AFTER you remove the service plug to assure Zero Voltage.