Hi - I am having a bit of a perplexing problem lately with my car

The last time I drove it I noticed the voltmeter gauge reading was
dropping lower than normal

The next day the car would not start - nothing when I turned the key
and no power going to accessories

My battery was quite old so I got a new one and installed it - I ran
the car at idle for a few minutes - at first the gauge came up to the
middle point but then after a few minutes it started dropping not quite
into the red zone but it was dropping quite low

I went to start the car next morning - once again everthing is dead - I
took the new battery in and got it checked and was told it was fine -
put it back in the car and it started up fine - ran it at idle for a
bit and again the gauge gradually dropped - however I took it for a
drive and when the car is in motion the gauge moves closer to the mid
point - I turned the interior fan on high and the reading drops down
quite a bit - ( it is getting cooler where I live so I am going to be
needing the defroster in the morning )

I visually checked the battery cables and they look ok

I did the test for an electrical short by taking off the negative cable
and then used a test light between the cable and the battery post and
the light came on - according to the haynes manual I am using that is
indicative of a short - from what I have read on discussion groups like
this one in a lot of cases that is caused by a bad ground ( besides the
negative cable to block connection how many ground points are there?)

The manual also said to unplug the electrical plugs from the alternator
and if the test light comes on that means a bad altenator - it didn't
come on so that test at least would seem to indicate the alternator is
not the problem ( note my dash alternator light does not come on BUT it
doesn't seem to come on under any circumstance so I am not sure if it
is working - for example when I turn the key to the on position it
doesn't light up on the dash)

I want to do some more testing using a hand held volt meter - my dumb
question is  what setting should I be using?

Sorry I took so long with my sad little tale - also if any one has any
other ideas or steps or experience with something like this I would
greatly appreciate your advice

I am about 6 weeks away from having to say good bye to the car for the
winter when it goes into storage so I want to enjoy it while I can

Thanks again