Grill blocking causes engine to flip out

[joeventura]

I have a 2004 Mits Galant ES 4 Cyl 2.4l

I did a partial grill block of the top grills and there was no impact,
the bottom grill (intake) I tried blocking and on my drive to work as I expected the temp of engine where the gauge usually reads at the 50% (halfway point) was now at the 3/4 mark. Which I expected.

On my 16 mile jaunt to work which is mostly highway driving the Check Engine Light comes on, then the Battery indicator comes on.

I check the scangauge and the battery is like at 17.8 to 18 volts.

Now the temp gauge is no where near overheating but I forgot to check the scangauge for engine temp.

In any case, what the heck can cause the alternator to flip out like that?

Needless to say I removed the blocking on the lower part of the car and we will see what the ride home does.