Not complicated at all. Turn the engine on and put the red lead of your VOM on the large red wire on the under hood fuse box. and the black lead on one of the strut bolts. The voltage with the engine running should be more than 12.4 volts, if it is not the alternator is not working.
Battery voltage with the engine off should be more than 12.5 volts, if not the battery is bad..

Check for continuity between the starter and the alternator with your ohm meter to test the fusible link.

All of the big box auto parts stores will test the charging/starting/battery for free.

 

I replaced the fusible link, and there is no issue with the battery or battery supply voltage.

With the car running and the lights on, the voltage drops to under 12 volts.

When her batt light came on I "assumed a bad alt. so I replaced it first, but no change. Just to be safe I replaced the battery as well, but same issue.

I am wondering if the alt. is getting, "on" voltage from the ecm? Do you know what the "turn on" signal is from the ecm" I dont know if its 12 volts, or less?

I dont think the new alt is defective as the issue is the same with as I started with.

Thanks
Jeff

 

Battery just sitting there with nothing hooked up to it will last a good while unless it's sitting on concrete. That will drain it.

 

Just a bit of an update. I had some time this afternoon and I tried checking for voltage at connection on the alt. where the small plug is and there is apparently no being supplied to the "turn on terminal". There 2 wires in this connector and from what I can find one is duty cycle the other is "turn on". As I said earlier, what is normal at those terminals?
.

 

That concrete thing has been debunked. The only problem is acid leaking onto lime based concrete making explosive gasses.

 

The small connector at the alternator supplies current to the windings based on the need that the ECM determines. Alternators produce voltage based on the current.
The problem could be in the ECM, and there may be no problem at all.
The battery light has been coming on and then off in my 2007 HHR for years, and the car has never gone dead or failed to start. I used a voltmeter that plugs into the lighter and determined that the voltage was always good, and did fluctuate normally. My Fluke VOM verified this. A scanner revealed a code that I can't remember the number, but it had to do with a connection somewhere. In my case this is a harmless gremlin, and I'm not going to change the ECM, either.
If the battery isn't going dead just live with it.

 

The sensor that the computer uses to determine how much current is needed is the plastic ring around the negative battery cable. My alternator output would vary between 11.9 and 13 volts
I think many people think "I don't know what that is so don't plug it in".

 

Check the connection to the frame, shown in the previous photo. It may affect charging, might not. If the connection is loose it will become more resistive and affect the current reading.