At a high level, all a person needs to do this step is a multimeter. No one would ever confuse me with someone who knows what they’re doing with a multimeter, but for this project I think a cheap (not 5$ cheap, 30$ cheap) multimeter will do the job just fine. Without knowing anything about anything, here’s a list of the battery voltages as I initially measured them, see if you can spot the bad module(s):
To be clear, all the modules are 7.86 – 7.89v except for one. Module 13 is at 6.61 volts. Apparently it’s quite common for the center modules to go first, as they’re the most stressed by heat, being buried in the center of the pack. That is fundamentally the whole problem that this effort is trying to address, that one module is not at the same voltage as the rest. A “healthy” module would be anything over ~7.2v, although this reading is not conclusive. A module can appear healthy by this metric, but still discharge quickly or not recharge completely.
I personally have a cheap multimeter, so I retested every module twice just to be super sure, and the readings were completely consistent, so at this stage I was happy to conclude that my battery was generally fine, module 13 was toast. Not knowing anything about battery configuration, chemistries, or electronics, this is readily apparent.
The kind of cool insight comes when you learn a little bit more than the bare minimum. The Prius battery is made up of 28 modules, as measured above, and each module is made up of 6 cells. So in the same sense that the modules are hooked up to make the battery pack, the cells are hooked up to make a module. As a property of the chemistry behind a NiMH battery, the nominal cell voltage is 1.2v. 6 cells x 1.2v = 7.2v, which is what each module is rated. So looking at a healthy module, somewhere around 7.88v for me, subtracting the nominal voltage of a single cell (1.2v) yields 6.68v. Not an exact match to 6.61, but close enough that it looks like this whole debacle is because of a single dead cell in a single module in the battery pack.
THIS is the problem with Toyota’s vision for long term maintenance of their battery packs. 168 relatively cheap cells (+/- some rounding and geographic availability, but in the neighborhood of 10$ per cell, or 60$ per module is what one would pay buying in single module volumes), one of these dies and their answer is “you should probably spend a few thousand dollars, buy 168 new ones, despite the fact that 167 of your cells are still doing just fine”. This is a bit of an oversimplification, but not much.
Note that this is the nominal voltage, so exactly half way between completely charged and completely dead. The charge to voltage level is roughly linear within the operating window, with sharp spikes just outside that window. This explains why my modules are all around 7.88v – they came out of the Prius *mostly* charged.