Why do hybrid battery control module connectors get corroded?

I've looked closely at the PC board inside the Prius battery ECU. It does NOT perform any balancing activity. The IC chips which receive input from the voltage sensing harness are "receive only." When time permits I will attempt to identify the part # of these IC chips and get their datasheet. I cannot read their factory labels because my battery ECU PC board is thickly covered with an insulating/corrosion resistant coating. Although this coating is clear it obscures the part numbers on the IC chips.

 

Thanks!
I intend to search for more info about the circuit design of the Prius battery ECU. I am a retired electronic design engineer myself, although none of my design projects were as complicated as the electronics in a Prius.

Current progress with my HV battery pack repair:

I obtained a brand new voltage sensing harness and successfully cleaned the mating connector on my battery ECU. I found no visible damage inside my battery ECU. Its connector shows no signs of overheating or arcing.

I bench tested my original NiMH modules as well as a stack of "reconditioned" modules purchased on ebay. I have enough modules which test at 5600-6000 mAh to make a full pack.

BTW people rarely state what charging/discharging currents they use to "recondition" modules. After some experimentation I settled on a 2A charging rate from my SkyRC B6AC charger which I set to cutoff charge at a negative delta of 5mV per cell. For discharge testing I'm using 650mA (this is C/10) discharge rate with cutoff at 1.1V per cell. I started out charging/discharging individual modules (7.2V), but then I moved on to doing it in blocks of 2 modules (14.4V) which makes the process twice as fast. I do monitor the V of each module in a block so modules which aren't close enough to each other can be regrouped.

I cleaned/polished my original Toyota factory bus bars, many of which were absolutely black with corrosion to begin with. I used a solution of white vinegar and table salt followed by rinsing with baking soda solution and then plain water. After drying I polished them with a wire wheel.

Unfortunately I did not clean my busbars 10 years ago when I replaced a couple of bad modules. I should have done so...

RE: Prius busbars, I read several discussions about whether cleaning/polishing original copper busbars is better (or worse) than purchasing aftermarket nickel-plated busbars. I did buy myself a set of nickel plated busbars which came with nickel-plated nuts. Note: These aftermarket nuts are a different design than original Toyota nuts.

As of today I lean towards using the nickel-plated busbars and nuts, but I'm not going to discard my original busbars and nuts.