If I change only the bad cells/modules in the battery, how long would it last?

Nobody knows. It could fail again at any time, usually when it is least convenient. Most people need a reliable car. The only way to do that is either to find a pack of modules from a low mileage wrecked Prius or buy a new Toyota pack

newpriusbatteries sells a kit of new cells I would usually recommend but they are backordered.

 

37 days, plu or minus how many days you drive it times miles.

 

As mentioned, there is no solid answer for this question. With no true testing, other than taking voltages, it's a crapshoot. I have personally witnessed this exact scenario done to a 2005 prius with the original battery and 235,235 miles. One module was replaced. It lasted several months and then I replaced it with a battery built using 2013 modules. Then that 2005 battery was placed in my garage on a piece of plywood. About a year later, I bought a white 2005 with about 260k miles that had a failed battery. I said 'what the hell' and installed the garage battery into it. The car fired right up and I drove it for close to a year before I was bumped off the road and the car was totalled.

So.....can it be done and last a long time? Absolutely. Can it be done and last 2 days? Absolutely. Most likely scenario? 1-4 months. Rolling 2 dice or picking a number out of the air have the same chance of being an accurate guess though.

This can be mitigated by using an app like Hybrid Assistant/Hybrid reporter to perform a HV battery test to determine if other blocks are weak. At least, it will keep you apprised of the condition of the 14 blocks so you can easily see if one is getting weak. That can keep you from eating a surprise failure.

 

this is not a very reliable repair. There are 28 modules in every battery pack. There are many variables on how long it can last. From not working at all to possibly a couple years.

 

Take a look through this thread if you have time, it may give you some ideas.

Just Another HV Battery Thread and Experiments | PriusChat

 

This was early in my Prius life. The car was owned by a family friend who's daughter was using it for college when the battery coded. I had never even driven in a Prius when he and his son ordered a module from ebay and swapped it out. They couldn't get the car to work again, so he decided to sell it to raise money to buy his daughter a Gen 3. I bought it from him to help him out. Figured I'd use my years of "ET ness" to figure it out. A couple days later, discovered they merely forgot to perform step 3 of the safety interlock installation procedure and the car came alive.. He had already bought another car, but was happy to hear he and his son had done a good job. I used the car for a while and decided I wanted to build a better battery for it so my son could use it for college. Bought 2 2013 Prius C batteries (only a couple years old at the time) and a $100 Gen 2 core. Used the 2013 modules to build a new battery using the Gen 2 core case and electronics. Installed it in the car and put the 2005 in the garage. And so started my Prius adventures.............now I have 5 gen 2s and a 3 car garage FULL of Gen 2 components.

Please read through the link I provided in post 7. It answers your questions. There are a handful or two of members here who have similar or more "hands on experience" than myself. There's many with some first hand experience and a thousand with opinions.

Even the 'rebuild' companies, who have mega gear to test and build batteries, get burned by early failures. The fly-by-night craigslist rebuilders are multitudes worse. I've replaced many rebuilder 'rebuilt' batteries that never got out of warranty period or have had multiple replacements before the owner finally gave up. Even the big names can't always predict when a rebuild will fail.

Ultimate individual module capacity isn't nearly as important as the individual modules being fairly well matched.

 

One thing that's interesting about the battery in that link is how, after installing the module from my shelf inventory, it starts showing it's superior capacity during the load tests. This is a great example of why a well matched replacement module is important.

The ecu is always monitoring the highest and lowest block voltages and codes out when a specific threshold is exceeded. Many people say it's 1.2 volts, but in reality it varies, depending on many factors. .This is why the "whack-a-mole" happens. On the later load tests, where the Block 9 replacement module is performing head and shoulders better than the others, it can potentially cause a problem. The other 13 blocks are in a pretty nice range with each other, BUT since the ecu is always looking at Block 9 as the highest, it is much easier for the lowest voltage to exceed the threshold. It could easily just have one module that weakens a little, and it causes the battery to code. If a slightly weaker module (that would put block 9 in the middle of the range of the other 13 blocks) was used instead, the battery would be tremendously reliable. This is why I feel that app is probably the most useful available for a DIY guy rebuilding a battery. If you rebuild it, install it, test it a bit, you can easily see where the next weakest link is and correct it before it's a problem. Do that a few times over the first month or so and you could probably have a very well balanced battery, with no real test equipment.

 

You have equipment to charge and discharge the pack to balance it though. That is not just replacing a module and calling the pack repaired which was the original question.

 

The battery in that link was 100% identically repaired as the OP asked. The failed battery (from a fellow members car) was disassembled for inspection, no individual module testing/cycling or anything was performed. A replacement module was installed merely based on the obvious failed module and the battery assembled and installed into the car. The only testing performed was by using the Hybrid Assistant and Hybrid reporter app.

The goal of that thread was to do a layman's DIY repair and show the results. Later on, the battery was put through a cycle routine to show improvement that can be had.

 

it's like rolling the dice, how do you predict it? some have gotten well over a year, reportedly. others, a day or week.
it is likely a combination of the state of the remaining cells and the voltage of the new ones.

you have to go into it hoping for the best and prepared for the worst.