It was crazy cold the night before , about 3 Farenheit. It wouldn't start so I ended up buying one of those portable starters,it strated right up. I drove it for 30 minutes, all seems ok.
Not it starts but I notice every time I turn on the light the battery gauge drops down or goes up when you turn the light on. It also goes up if you rev the engine.
I took some measurements with voltmeter:
car off: -11.70 and droping
car on idle: -13.80
car on idle light on -13.15
Obviously the battery is dead, but is the alternetor on the way out too. Shouldn't the voltage stay the same with a good alt. even when the battery is bad?
Thanks!

 

Bad Battery - Maybe even frozen from not having a charge. Alt looks like it's doing what it should......

 

Just put a new interstate battery today, with the car off it's reading -12.20 but the arrow on the voltmeter gauge still moves depending on if you turn the lights on or off between 12.5 and 13.5v. Is it normal or it's my Alternator. The old battery was titanium autocraft with a stamp 05/07.

Only one year old, too bad I don't have a receipt for it.

 

You are testing the voltage with a multimeter at the battery and not using the dash voltmeter right? Don't trust the dash voltmeter, its never 100%. Test the voltage at the battery with the engine running, it should be around 13.8-14.0V's with no electrical loads.

 

I'm using a voltmeter. Could it be a faulty gauge if it's constantly moving up and down?The arrow on the volt gauge is at 14v when I first start the car and once it warms up it's moving from 12.20-13.80, I hope it's not the alternator, not putting constant charge.