Porsche Repair: Electrical Problem on a 1981 928, emergency flashers, charge voltage

Question
Dave, I forgot to tell you that I just put a new battery in this car last week,  I put a voltage meter on the battery and it read 9.12 and when I opened the door, everything just shut down,  alll the interior lights just went dim,  then off.   Greg
Followup To
Question -
Dave,  I have a problem ,  when I started my car up, it ran ok,  then when I turn the headlights on, the entire car just shut down as if I removed the battery,  I started it up again about 20 mins latter and I cut the emergency flashers on and it shut down again, everything just went off when I opened the dorr,  the interior lights went from bright to dim then off,  could it be a problem in the dash pod?  Please help me with this promblem. Greg
Answer -
Hi Greg.

I don't know exactly WHY yet, but let's look at the obvious first.  There is not enough battery charge to power both the coil and the headlights simultaneously.  This could be because the battery is undercharged or because the alternator is not re-charging for what is used.  Check the charge voltage by putting a voltmeter on the two battery terminals when the car is off.  The reading should be very close to 12.8.  If it's not, at least part of the problem is an undercharged battery.  Then take a reading in the same manner when the car is idling AND after you have revved it to say 3500 rpm a couple times.  The reading should be near 14.5 or even higher.

Please write back to me and let me know what you find.  If the car won't start to even do the second test, re-charge or re-place the battery.  Let me know.  

Dave

Answer
OK...so that's good info.  You have a new battery...but it's not fully charged.  You can't check or do any of the next steps until your new battery is refreshed.  Don't put it back in the car until someone re-charges it back up to 12.8...or very, very close.  None of the rest of your testing will matter until this key factor is in place.

Dave