Had been using 'deep cycle' mode on battery charger for a couple of years when 71 Olds not in use... Had been in garage Friday, nothing unusual. But Saturday and through yesterday I noticed rotten egg smell, which I learned was from overcharging the battery. I disconnected the charger and today the smell is gone.

Is this battery somehow damaged or dangerous so that I shouldn't use it anymore ?

 

Rotten egg smell (sulfur dioxide - lethal in any quantity) means your battery has a dead cell, it's toast, you need a new one.
Batteries dont last forever, infrequent use shortens their life just as often churning over an engine that is reluctant to start.

A decent charger shouldn't overcharge a good battery.

Roger.

 

For constant charging for a stored battery, I would only recommend a 1A (or less) trickle charger - much more gentle. You do not need much current flow to maintain a charge.
Smaller chargers are safer, too should the battery plates decide to short out.
I once had a battery short before when on a 6A charger and charger current went to max. Battery boiled internally, but no drama since I has home and heard the louder buzz and killed it. After then, only small chargers for me.

 

Batteries should be charged in a well ventilated area. A battery will emit hydrogen when being charged, very explosive. Its also a good idea to remove the filler caps if equipped to prevent any pressure and to let the gases escape. I've had more than my share of battery explosions over the years, both when charging and a when starting cars. Be very careful!

 

I always thought the "deep cycle" setting was for marine type batteries, or batteries specifically listed as deep cycle.
Not sure if that's correct, but that's my story and I'm sticking to it!

I usually just hook up a battery tender for extended periods of storage. Not much chance of damage and keeps a good battery in good shape.

 

Is this a maintenance free battery?

Is it bulged on the sides? If it is deformed in any way-the battery has had it.