Low voltage reading after re-starting car

[Resistolcowboy]

My 1997 volvo 850 base mode has voltage issues, when started in the morning the voltage from the alternator as well as battery reads 14.3 volts at idle and when driving with lights on, radio cranked up to 10, heater blowing at full, windshield wipers on high trying to knock the sick Oregon rain fall away, it keeps at that reading with no voltage drop. "But" after turning the car off as well as all of the above turned off. i return from getting coffee and start the car and voltage drops down to 13.3 to 13 volts and when i turn on the above again it can go down to 12.7 and back up to 13 to 13.1 / 13.5 volts after turning everything off! i replaced the much needed alternator "21 years old" and replaced the battery. still the same reading. "NOTE" Serpentine belt is new" but i noticed that the belt tensioner is bouncy! a little more than 1/32 of a inch. 85% of the forms i have read say voltage should always read between 13.8 to 14 volts. This is abnormal behavior and have never seen this before in other cars. Another note is that I have driven this car in this condition with the old parts for over a year and a half and never had starting or stalling problems. Any ideas that might be the cause. Thanks Resistolcowboy

[misha]

You will need 13.5-14.4v with headlights,heater blover(not on 4th speed!),radio and rear defroster ON.
Anything below 13.5v will not keep the battery fully charged when car is used.

The usual problem is positive battery cable which goes from battery to starter and then the other end...from starter to alternator.
It must be replaced because it makes problems...and replacement MUST be the same thickness as original or slightly thicker.

Perform a voltage drop test on both...positive and negative side AFTER you drive a car for at least 30min but without turning the engine off because weakness of the positive cable will show up ONLY after it warms a bit.
As heat builds up in the engine bay...the resistance will also go up if cable is bad.