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.

 

Another issue related to my Prius having a corroded orange battery ECU connector:

The corrosion inside my orange connector is between the pins which connect to battery blocks 1-4. These blocks are on the passenger side in a gen 2 Prius (battery blower side).

At first Dr. Prius occasionally showed discrepancies with voltages from blocks 1-4. This gradually worsened. Sometimes the voltages would jump around randomly and rapidly. At this time the car was still drivable after resetting occasional "hybrid system" error codes.

Then after the car was parked for 2 weeks it wouldn't start at all.
Dr. Prius displayed this:

• Block 1: 3.1V
• Block 2: 5.6V
• Every block from 3 to 14 was normal: 15 to 16 V.

I discovered these issues after disassembling my HV pack:

Module #1 is weak. Only 2,000 mAh and internal resistance is 50 milliohms. It does hold a charge OK.

Curiously modules #2 and #3 both test OK after being charged on the bench and thoroughly tested, They hold their charge for weeks. Both test above 5,000 mAh on the bench.

Module #4 is totally fried. It accepts a charge and rises up to 8.5V, but then decays to 6.4V within 15 minutes after disconnecting it from the charger. It may contain a partially shorted cell. Or perhaps a cell with little or no electrolyte? It does stay at 6.4V for a long time while resting.
The pan under module #4 displays a considerable amount of electrolyte leakage The worst leakage is near some of the bolts which secure modules to the pan.

I suspect the corrosion inside the orange connector was sufficiently conductive to discharge modules 1-4 over that 2 week period where the car wasn't started.

 

For the case of two electrical terminals which have a conductive path between them due to the presence of water or other liquids, both electrons and ions will flow through the conductive electrolyte.

While electrons (and negatively charged ions) are flowing from - to +, positive ions are moving in the opposite direction, from + to -.

Unlike electrons, ions are composed of chemical elements or compounds. This is how electroplating and chemical batteries work, as well as what causes electrically induced corrosion.

Flashing back to the early 20th century when the telephone entered the scene, early telephone companies who provided a pair of wires energized at 48V DC from their central office and switchboard to the customer's location soon learned that the polarity of the 48V needed to be negative with respect to earth ground. One wire was at earth potential while the other was at -48V. The selection of negative polarity for the 48V was found to greatly reduce corrosion of the cables and connectors which were outdoors. It worked well enough for this technology (aka the "land line") to have a lifetime of over 100 years.