Reconditioning & reordering modules in a 2005 HV battery

My mother recently bought a 2019 Prius to replace her old 2005 one. The old one had a very weak battery, to the point it could not go up the mountain near where we live (in a rather mountainous area). This was making it hard to sell, so I figured I'd try my hand a reconditioning the battery.

Being almost 20 years old at this point and with nearly 200k miles, heavily sun-damaged paint, bad suspension (very bouncy and wobbly), bad bearings, and an intermittent check-engine that nobody can get to go away, it's obviously not worth much. Therefore I'm not looking to dump much money into the car. So far we've just spent about $30 on a Tenergy hobby charger plus some electricity from the grid.

Starting out, I was hoping I might find one or two modules that were markedly worse than the others. A couple did stand out during my initial voltage check after pulling them out of the car:



My initial module discharges (before grid charging) also showed some significant discrepancies:


After cycling the modules a number of times (charge current below 2A, discharge 0.7A), I managed to improve the numbers, but they're still not great:

(If that graph isn't legible here, you can view it in full, along with all the source data, in this Google Sheets doc.)

I understand the stock/factory capacity should be around 7500mAh per module, but isn't that when discharging to 6.0V? (Assuming a linear relationship between voltage change and discharged energy, that would mean discharging about 13% more energy [2.5/2.2], which would suggest that my strongest-performing cells are more like 5600-5700 mAh -- still well below 7500 but not quite as bad as depicted by the graphs. If the relationship isn't linear then I could discharge one to 6.0V to see what I get out of it, but that takes time and I'm not sure how much I'd really gain.

I also have some internal resistance calculations, but they were only made under the 0.7A discharge current that my Tenergy charger will pull, so they may not be super useful:

If it would help to have better numbers here I can try to rig up a higher load.

We could replace weaker-performing modules, but my take is that the capacity variance seems pretty evenly distributed across the modules. I worry about adding outliers on the other end (overly high-performers), and about sinking money into a sinking ship. My plan at this point is just to reorder the modules to create better blocks, unless those who are experienced think I should do anything else first.

When it comes to rearranging modules to create stronger blocks, I've been reading that there are various opinions out there. Some say it's better to pair lower-capacity modules together, others seem to say the opposite. Some say to pair based on internal resistance, which I suppose I could do pretty easily (though module 13 appears to be an outlier and might need another test/reading). Maybe I should also move the weaker inner modules to the outside for better cooling.

I'd appreciate some pro/experienced takes on what I've got here and what I should try doing next. I'd like to wrap this up in the next week or two, as we do need to sell the vehicle and get it out of here.

Thanks a million

 

I understand these modules are old and worn. However, they're not as old as the car, as they were replaced about 5 years ago (I forgot that when writing my initial post). I do not know how old the replacements are, but we have yet to have any warning lights come on whereas the original modules were triggering warning lights. The only "problem" we're facing now is that the battery drains rather quickly when ascending a hill, so you have to stop and let it recharge before continuing on big slopes. (Maybe that's because the engine isn't putting out as much power as it should, since my 2012 Prius seems capable of ascending mountains indefinitely, but that's a different topic.)

Right now I just want to know if there's anything else I should try before putting the battery back together. If not, then should I rearrange the modules or leave them as-is?

Thanks