where is this "jump block" located ?

so today i put the battery in - had 12.8v on it

cleaned the cables--they had some light corrosion , but ive seen way way worse . Installed new bolts into batt. cleaned up a ground wire from batt to chassis. cleaned all the pins on all the relays .

So, at idle with lights / radio / ac / fan on high -- it drops to around 11.83 v , when the 2nd fan kicks on .
If i rev it up to 1500-2k , it goes to 13.8v or so .

I would think if the alt was bad i would have seen a big drain and all the symptoms would come back right away .

Im going to drive it today / tommorrow and see what happens. ive got jumpers in the trunk