I went to start my 89 coupe yesterday and the aftermarket alarm
started giong off but i didnt have it engaged as the car stays in the
garage. I turn off the alarm go to start up the car and nothing. I
have lights in the car courtesy,dash, and headlights but no ignition.
The voltage on the dash only reads 9.1 so I suspect its the battery
(an Interstate) which was on the car God knows how long when
I bought it. The car currently sits in the garage and is not driven
much. I start it up every couple of days mainly to move it out of
the way to access other stuff in my garage but I never let it run
long. It will be a couple of days before I get a chance to check the
battery so any opinions would be greatly appreciated. Thanks in
advance...........Biggie

 

I have typed this so many times that I can do it in my sleep. First, you have a discharged battery, charge your battery up with a battery charger. Short durations of running the engine will not keep the battery up. Lead acid batteries self discharge 1% every day, so even disconnected, your battery will discharge. When batteries discharge, lead sulphate forms on the plates and lead sulphate is a good insulator. A sulphated battery will not take current from a battery charger and you will have a door stop. If you don't drive your car often, then get an interval timer (turns your lights on each evening when you aren't home) and have it charge your battery every day for its minimum interval (usually 15 mins). You will then experience normal battery life and the battery will be fully charged.
You can tell the state of charge by measuring the no load battery terminal voltage. 12.0 volts and below, discharged. 12.9 volts and above, fully charged. NEVER use the alternator to charge the battery up, first it takes too long and it will shorten the life of the alternator!
When you find your battery discharged, charge it up with a battery charger for at least 24 hours!