This has confounded me for about 3 months.

Car ('88 944Turbo) had battery replaced with an Optima about 2 years ago.

In the last 6 months, I have had to jump start from battery pack all the time, once started voltage has shown 11-12 volts on gauge.

I checked engine rail grounds & battery ground = OK, as the car started well from battery pack I assume the engine grounds are good.

I replaced the alternator yesterday, have driven about 60 miles, but the battery is still flat & the gauge still shows about 12v!

So I got a DVM, with car not running battery terminals have 11.12v across them.

After starting the car from the battery pack I have 12.7 --> 13.16 v across the battery terminals. After 10 minutes the +ve battery cable is hot & the battery terminal is too hot to touch.

Car will not start from it's battery, does anyone know what's wrong here?

Why does the gauge show 1 v low? Any way to fix it?

 

Sounds like your 2 year old Optima battery is cooked. Could have been caused by a faulty regulator, so you should have the problem licked by now. Pull the battery out, take it to a parts store and have them load test it for you.

When you changed the alternator, how was the heavy wire running back to the starter? This thing needs to be in good shape as it has to carry 100+ peak amps under high temps in your turbo. If it were my car, I'd want a 6 gage wire there to do the job.

 

Your battery has a shorted cell.

Each cell should have about 2.2 volts. 2.2 volts/cell X 6 cells = 13.2 volts.

Conversely, 11.12 volts / 5 = 2.224 volts/cell