Not sure if Repair Core works as same as Corrosive Core, I think they work similar.
It is charged by every damage (except “missing hero” tiles which do 1 dmg, so we can throw them at Construct Hero and it doesn’t charge)
Here is that about 650 dmg for full load:
I checked it also on one construct hero and I had similar damage to full. It was about 6-7 dmg per 1%.