How would you pair modules into blocks when rebuilding a Prius battery?

I finally bit the bullet and took my battery apart to rebuild it. As expected, the middle blocks have failed (< 1Ah) weaker (2-3Ah) modules, and the outer blocks have the strongest (3Ah+) moudles. Note that these absolute numbers are low because I'm only testing from 8 to 6.5V, the real capacity from 8.4 to 6V is about 50% higher.

I see several options for putting the battery back together:

1. Just replace the bad modules. These is the fastest way, as you don't have to measure the capacity of each module, just the voltage. The bad modules usually have a much lower voltage than the others. Problem is that the weaker modules are still in the middle, and will probably fail soon.

2. Pair the strongest/weakest modules. Idea is to even out the average capacity in each block, minimizing the voltage difference between the blocks. You have 2 ways of rebuilding the battery. First is to put the strongest/weakest pairs on the outside, and the average pairs in the middle. This is gentlest on the weakest modules, and should make them last the longest. Problem is that you are not wearing out the modules evenly, as the strongest would remain strong as they are on the outside, and the average ones in the middle would get weaker.

3. Second way is to put the strongest/weakest pair in the middle, and the average pairs on the outside. The strongest modules would gradually wear down, and the weakest would fail. The idea being that you would get the most use out of each module as they wear down towards average, and you keep replacing the weakest module.

4. Put the strongest modules in the middle, and the weakest on the outside. The idea being that the weakest moules on the outside would last longer, while the strongest modules in the middle wears the fastest, and you would gradually have the capacity even out between all the modules. Problem is that the capacity difference between the inside/outside modules could cause enought of a voltage difference to throw a code.

5. Divide the modules into 2 sets, with the strongest in one set, and the weakest in the other. You pair the modules, one from each set, strongest to weakest. i.e. if the modules were numbered strongest to weakest, you would pair 1/15, 2/16, 3/17... 14/28. Idea is similar to (4), with the strongest pair in the middle, but with much less capacity difference between the pairs.

I'm way over thinking this, and will probably end up replacing the entire battery with a new Lithium one instead of playing wack-a-mole once more modules fail.

But I thought this would make an interesting taught excercise and discussion. How would you put the modules back together if you had to rebuild the battery?

 

@sam spade 2 @alftoy If you don’t mind, I’d rather not turn this discussion into whether to rebuild with new vs reconditioned modules. There are a lot of other threads about this which people could read to help them make a decision.

What I really want learn about is what is the prevailing thought about the order to put the modules back together again. I’ve read that I should “match” the modules in each pair, and that it’s important to replace the failed modules with ones in similar condition to the other modules in my battery. But no details on exactly what this means.

Also @alftoy, thanks for the links, especially the one to 2ndlifebattey. I bought my 5 modules from eBay, and the results were mixed. 1 leaked (and was promptly replaced by the seller), 3 were in average condition (3Ah from 8 to 6.5V at 3A), and 2 were below average (2Ah) which I probably won’t use since it’s worst than the weakest non-failing module in my pack. I should have asked here first for a reputable seller. I will definitely use them if I end up needing just a few more modules. More than that, I’ll probably take @sam spade 2 ‘s advice and get a new battery.

 

Just an update. I've decided to match the modules using method 2 (strongest/weakest from outside in), since this is the first time I'm rebuilding a pack, and this seems like the most straightforward way. Attached are the spreadsheets of the initial and rebuilt pack. As you can see, even thought the capacity of the modules varied quite a bit (2.6950 to 3.8024Ah, 1.1074 diff), method 2 evened out each block pair (6.4974 to 6.7201Ah, 0.2227 diff). 3 of the modules were clearly outliers, and those were replaced by the strongest eBay modules.

I equalized the module voltages by shorting them together before reassembly, but from reading the other rebuild threads, it looks like this doesn't really do anything, and I should really balance/recondition them by doing several cycles using my grid charger. I will do it this weekend and post an update if it changes anything.

Also, I have the 3 bad modules I replaced, as well as the 2 iffy ones from eBay that I decided not to use, on my bench right now. Is there anything you want me to test? I'm thinking on "reconditioning" them by cycling them from 8.4 to 0.6V (at 0.05C) several times, to see if this method from the insight forums really works.

One other weird thing I noticed about my particular battery is that the serial numbers are all over the place, even though I'm the original owner and bought the 2010 new from the dealership. Usually, this indicates a rebuilt pack. Does anyone else ever see this in their original battery?

Went around town for about 50 miles this weekend, and the pack seems to be working fine. I did not get a hybrid error (I previously always got it after 10 miles or so), idle delta voltage is about 0.07V, though it did peak at about 0.3V briefly under hard acceleration/braking. Block 14 (with the strongest/weakest modules) was the outlier, with consistently higher voltage than the other blocks, so putting it on the outside seems to be the right choice.