When the engine is running and the alternator is charging the battery, the voltage should indicate anywhere between 13 volts to about 14.6 volts.
Answer 2 - A wider Acceptable Range
13.5 −16 volts is acceptable with the engine running.
It can put out too many volts and destroy the battery but not drain it. The alternator should put out between 13.5 to 16 volts.
With the engine not running you should read 12.6 volts on a fully charged battery. With the engine running you should read from 13.5 to 15.5 volts at the battery posts.
Fully charged battery should have 12.6 volts. 75% charged 12.4 volts, 50% charged 12.2 volts, & 25% charged 12.0 volts.
Engine running at idle you should read 13.5 to 15.5 volts.
If the vehicle is running you should see between 13.5 volts and 14.5 volts. If the engine is off, you should have at least 11.5 volts. Voltage alone is not enough to determine the "health" of a battery. If a battery may be the cause of a problem then have it load tested by your local parts store or automotive shop.
12 volts with the engine off. 13.5 to 15-5 volts with the engine idling.
With engine running at idle you should read 13.5 to 15.5 volts at the battery.
1.55 volts
13.6 to 14.5 volts with all accessories turned on, depending on temperature.
With a fully charged battery (12.68 volts) and engine running at idle with no accessories on, 14.5 volts. You will not get a accurate reading if the battery is not fully charged.
Most alternators should put out between 14-14.5 volts, and this would be the reading if you have everything else turned off in your car (no Air conditioning, no radio, etc.)If they get too much over that they can damage the battery causing it to literally explode.
A good AA battery has about 1.5 volts across it.