Quit messing around and take it to your local autoparts store! Autozone, Advance, etc. will all test it for you, with the right equipment. I have found them invaluable for identifing the problem. Their test will save you from unnecessarily buying an alternator or battery. Good luck.

A good charging system will maintain the voltage under almost all loads. While needing to get a jump could be because of a bad battery, once running... the charging system is the primary source of electrical power. Low runnig voltage is a charging problem, not a battery problem.

13.7 is really at the low end of acceptable charging voltage, especially in cold weather. 13.2 is barely 0.1 volt charge per cell (each fully charged cell is 2.10 v). Recheck the voltage at the alternator output stud and its case. A higher voltage there would indicate a voltage loss in the wiring or ground cables. If you happen to find 14+ volts at the alternator, you know it is good.

The battery on the wifes 96 850 was claw hammer dead after sitting a week while we were on vacation. After a boost charge to get it runing, I was getting 14.2v alt output across the terminals. I slow charged it overnight. Next day it was reading 12.85v just sitting with the engine off and~ 13.2v with engine running the lights/heater blower on which I think is reasonable. Do you have reason to suspect your alt and or battery?