Why do hybrid battery control module connectors get corroded?

Way back when I had my first car, the rubber-sealed weatherproof connectors now used under the hood were not a thing yet, and terminals were more exposed, and I noticed a pattern to which ones seemed to grow the most malachite (the green stuff). They would be the ones on constant-powered circuits, 12 volts 24/7.

I think malachite and azurite are both compounds of only copper, carbon, oxygen, and hydrogen, so all the needed stuff is there with a copper terminal in humid air, but it must help to have a voltage gradient to grow the stuff.

There are voltages constantly present on those orange-connector wires. They're the same wires at the other ends where they attach to the battery modules, but those ends are spaced out farther apart. The ends in the orange connector have the same voltage differences in a tight group with much smaller distances, so the voltage gradients there are steeper.

 

So I just asked Google AI and then Asked ChatGPT this question and as usual GPT one was better (enclosed below):

Specific to Prius packs there lots of different humidity-based and electrochemical and akaline processes involved that lead to this corrosion from trace amounts of NiMH electrolyte leaking from the seals of the modules. No doubt severe changes in temperatures play a roll in these very tiny leaks that Toyota can't prevent in a cost effective way. But your original voltage sensor wires act a means for the corrosive process to reach the pins of the ECU and you can get a new higher quality voltage harness at a 10% discount if you use this link: http://www.acenbay.com/?ref=priuscamper

If the corrosion on the pins inside the ECU goes on for too long you'll start getting micro-arcs between that first pin and one next to it and you'll see it first as a sudden voltage drop down to one or two bars on your charge screen even though pack hasn't lost charge, it's just the ECU sensor getting scrambled. Once it progresses you'll get smoke and melting of the voltage sensor plug at the ECU which usually destroys the ECU.

Anyways, here's a taste of how this corrosion happens according to chat GPT:



ChatGPT - Electrolyte corrosion process

 

What was the prompt you used to get that ChatGPT response? Did you hint that you wanted electrolyte leakage to be part of the answer?

I've seen malachite dependably grow on copper terminals given just exposure to humid/road environment and constantly-applied voltage.

A lot less of it in modern cars because the sealed, weatherproof connectors used under the hood have become much, much better.

The orange connector in the battery isn't that sealed kind though.

 

The vapourised electrolyte from the modules reacting with oxygen and copper is the major catalyst to promote this sort of corrosion, a bit of heat generated by resistance across a joint helps to speed it up.
The real problem leads back to the pressure relief system in each module. They are supposed to release at around 80psi and vent outside the vehicle through those rubber hoses and joiners that connect every second module.

These rubber bungs get stuck and the internal pressure in the modules climbs enough to force the electrolyte out through the terminal seals.

It will be interesting to see if your voltage sensing harness has corrosion along the length of the wire, two of mine did, so I'm interested to see if this is common or just than my two Priuses had second hand battery installed during their long life, so the corrosion problem is exaggerated in my case.

I know I had to push the terminal pins further through the plug in the battery computer so they made a deeper contact into the pins in the plug ..... this appears to be a common problem resulting in a burnt pin and terminal in the plug, caused by a poor contact and gradual increase in resistance, heating the pin and it retreating further into the computer housing .... heating the pin housing from both sides with a hot air gun, while pushing the pins through, seemed to sort the problem ...... as a test, use a pin or small diameter wire and measure the depth of the socket in the new harness plug, then compare that with the length of the pin sticking out of the computer plug ......

T1 Terry