I have owned my 84 vette for 5 years and one theme seems to be repeating itself. The Battery continue to die. This happens sometimes after sitting for a week+ or after sitting for a few minutes. Nothing repeatable.

The last new Battery I bought was three years ago, so I bought a new Optima Performance Battery at the begining of the summer, thinking I could enjoy the summer with the new Battery. Typically, the Battery charges fine and I had the alternator checked out a couple years ago and it was given a clean bill of health.

About a week Ago, I noticed the volt meter dipping from 14V to 11.9 V while I was driving it on the highway. I have never noticed this before. Usually it is charing between 13.5v to 14 V. Nothing was different about this day, but the temps were well in the 90s.

I am getting ready to give up on this this problem, as I do not want to be stranded on a regular basis. Any advise you have will be apprecitate.

Thanks in advance for your help!
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I just took it out today after I jumped it. Another hot day 95 degrees. The Battery Completly drained while I was driving... So much that it stalled. Fortunatly, I was able to coast back to my drive way.

Is it possible that the electric fan is draining the Battery? Just a thought.

 

Nothing is forever. An alternator (or even you, for that matter) that was given a clean bill of health a couple of years ago may not be so healthy, today. Your volts reading dropping from 14 to 11.9, would suggest that the alternator has died. It's life may have been shortened by recharging a battery that had been allowed to discharge, too far, too often.

All computer cars have a constant drain. Depending on the outside temps, I consider 30 days as the point where my car may not start. It will be longer with fresh battery, in moderate weather than with one on it's last legs, in the dead of winter. In fact the constant discharging and recharging is a major factor in how long the battery will live.

It is possible you have something in your car that is creating an excessive current draw. If the charge will last a week or less, it's time for some trouble shooting. Find out how large that draw is. If too much, track down the source.

RACE ON!!!

 

If your dash voltmeter indicated below 12.0 volts, then you should have noticed that the red alternator symbol lit up on the dash which means your alternator is not charging the battery. Car batteries are not deep discharge batteries and each time you fully discharge it and recharge it, you lose amp hour capacity. NEVER use the alternator to recharge your battery, ALWAYS use a battery charger and keep in mind that it takes at least 12 hours of charging to fully recharge your battery. Jumping the battery and driving the car for even an hour does not charge your battery up very much, but it does heat up your alternator and shorten its life. Car batteries last longest when they are kept fully charged!!!!!! If your car sits for long periods (4+ wks) , then buy a battery tender and keep your battery charged.
You need to measure the leakage current by disconnecting the negative battery cable and placing an ammeter between the battery terminal and the negative battery cable. After the courtesy lights time out, switch your ammeter to lower scales until you can read the current. GM says leakage current should not be more than 50 milliamps. My 87 measures 27 ma.
You can tell the state of charge of your battery by measuring the voltage at the battery terminals after it has set for 24 hours of no use (discharge or charge). 12.0 volts or lower, discharged. 12.9 volts or higher, fully charged and linear in between.

 

Your edit came while I was typing my first post and I didn't see it until now. Your experience, today only reinforces my thinking that your alternator is shot. I doubt that is the only problem, however.

"Is it possible that the electric fan is draining the Battery?" Anything is possible, but the fan is unlikely. If the fan were running with the engine off, the battery would discharge much more quickly. There MAY be a problem in any one of the circuits in your car (including the fan) that is causing an excessive current draw as jfb describes. Charge and test the battery as he outlines. Also have the alternator checked. I'm betting it's bad. Then, check the current draw.

RACE ON!!!

 

I second the dead alternator idea.

Check the car in a dark garage to see if the vanity mirror lights are on. This happened in my 85 - on passenger side.