Nickel Metal Hydride chemistry is very stable!

I recently repaired a hybrid pack from an 2002 Prius. When I saw it, the check engine light was light and the master warning light (red triangle of death) was displaying and the following ODB II codes: P3000,P3006, P3020, P3023, P3024. It has been driven after the the warning were displayed. Probably very extensively. (recent change of ownership so missing data) All of the modules were within a tenth of a volt except two. The two modules were reading about 1 volt low. When I pulled them out I saw that one cell in each module had exploded and and emitted a black substance. However there was no obvious deterioration to the surrounding cells even the other cells in the same module. They were just blackened.
For pictures on see the paragraph below. On the same page I have a picture that explains ODB II codes: P3011, p3012 etc. These are the codes that indicate out of spec differences in internal resistance in parts of the pack.

I think you will like the pictures just go to: http://www.elearnaid.com/reyouhythivo.html and scroll down to see them. If you click on them you will get more detail.