Engine Light on 2004 PT Cruiser

I inherited a 3004 PT Cruiser Turbo from
my Father-in-Law who passed away. He
hardly ever drove it and it only has 36,000
miles on it. Most of the time it would sit in
his garage and he would hook up a charger
to the battery while it sat. The engine light
has gone on even though the engine is not
acting strange. Took it to the shop and
they said they were getting a "Low
Voltage" code on their computer and that's
why the engine light was on. But, they said
if it wasn't giving me problems not to
worry. They reset the light. I drove for
about 80 miles and the light has gone back
on. Does anyone know what could be
causing this? It was also suggested the car
might have a short somewhere that is
draining the battery...but then wouldn't the
battery light go on and not the engine
light?