You need to measure the Alternator output (at the Battery terminals with engine running. Here are some Battery/Alternator basics...

The Battery's main job is to supply power to the starter. Period.

Once the engine is running, the Alternator supplies power for Everything Else — plus a little extra to keep the Battery charged. (The Battery is just along for the ride at this point.)

In simple numbers, the Battery is capable of supplying no more than 12 Volts. The Alternator can put out about 14 Volts. (Anything over 12 Volts is what keeps the Battery charged.) A meter on the Battery terminals shows Battery voltage with engine off, and Alternator voltage when running. If it drops to about 12.5 V when running, you're reading Battery again.

If the Alternator output falls below 12.5 Volts, the Battery starts taking up the slack (not its main job) and "running down". The fewer the electrical demands, the longer the running down will take. I once went about 40 daytime summer miles after leaving a wire off the (V8) Alternator.

Voltmeter readings can be misleading, however. For example, six flashlight batteries connected end-to-end will read 12 Volts on a meter. But they would hardly power your headlights, let alone turn the starter motor. They wouldn't have the "Capacity".

Alternators sometimes lack Capacity too. They may show 14+ Volts with the engine idling, but less and less as lights, fan, etc. are switched on. The usual suspects in this case are:

1 - The Voltage Regulator/Brush unit attached on the back of the Alternator
2 - Worn/grooved commutator (in the Alternator, where the brushes ride)
3 - Worn/Glazed/Loose/Slipping drive belts
4 - Poor connection(s), Alternator-to-Starter-to-Battery + terminal
5 - Poor "Ground" wire connection (between Alternator body and mounting bracket)

If the Alternator output never rises above the Battery's 12 volts, it could be due to:

A - More severe versions of any of the items listed above
B - Failure to "pre-exite" the Alternator's charging circuits (current flow from the Battery Light on the Instrument Panel via the thin red wire to D+ on the back of the Alternator.
C - An internal failure within the Alternator itself (least likely)

--
Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.