I had a drained battery this week and had to jump my truck a couple times to start. Saturday I pulled the battery and alternator and had both checked at local AutoZone. The alternator checked out good, but the battery needed to be charged before a test could be run. I had put a charge on it the day before, so I just bought a new one. I got home put the alternator back in and installed new battery. When I drove the truck, my voltage would drop at idle and several times the engine would kill. The truck would start backup with no problem. I took the truck back to AutoZone and had them check with the alternator in the truck. They said that the cables where pulling the bare minimum and suggested changing the alternator connector and positive battery cable. I changed the alternator connector, but they did not carry the cable for my F150. After changing the connector I drove around without killing at idle, but still showing voltage drop on gage.
It did kill when I got home and put in reverse (this is why I am wondering if there may still be a problem).

I checked voltage at the battery and wanted to know if this is ok or do I still have a bad cable or connector somewhere. I can not remember if I had this voltage drop before the drained battery.

Engine off: 13.0v
Engine idle: 15.0v (w/o a/c) 14.9v thru 14.2v (w a/c)
Engine ~20000 RPM: 15.0v (w a/c)

 

Those voltages seem to be right on.

 

The voltage at idle will be dependent on the battery state of charge at the time you measure it.
Fully charged battery would be close to 13.5, while a battery taking a charge will measure higher, assuming no other high current is being consumed at the time.
The dash gauge is not very usefull to see what is going on except seeing a major problem.
You need to monitor the system with a better device over time to see what is happening.
A drain test would be good at this time, to see if there is something else that is drawing more current than it should, with the engine off.
Drain measureing will see the draw from the PCM, clock memory, radio memory, anything in the cluster that has memory and any thing else that has memory.
A large draw of much more than 50 to 75 milliamps will suggest a problem that will discharge the battery in a short time.
Ever notice the car lots with the hoods up and a charger sitting there hooked up? If the vehichle is not run enough for a couple months, the battery will run down from normal memory draw.
An alternator can charge fine but discharge the battery when at rest due to a rectifier stack fault. I don't know the extent the auto stores test this but would hope they can and do. This is easy to test as well, right in the truck.