Checking the battery in a Cherokee

How often do you check the battery in your PA28? I don't remember ever
checking it except at annual. We just had paint and new interior done.
When the interior shop pulled the back seat they found where the battery had
overflowed and begun corroding. I wire brushed everything to bare metal
and sprayed some corrosionX on it. That should last until the annual and
we'll get a new battery box.

Side question; There is some expanded fiberglass insulation under the back
seat, next to both the port and starboard wing root rib. It looks like
standard construction insulation. The insulation on the starboard side had
absorbed some of the battery overflow so I threw it away. Am I going to die
because I just tossed some hypercritical piece of the airframe away? Does
anyone else have this insulation or did this come from a previous owner?
Anybody know specifically what the purpose of the insulation is (thermal,
sound, ???) ?

Thanks in advance,

Eddie
N40008