Charging system Problem !

I noticed that the needle goes to about 11-12 volt during stop or 900 RPM.
the weather these days very cold about ~ 4 c. night.
I changed my alternator before year with new one!
when the engine runing in normal codition say above 1000 RPM , the voltage goes and stay ~ 14 V.

q: is it possible IF the battery is weak ,even with engine runing the voltage can be drop as my situation??!
where is the problem you think guys?
GUess why i post this thread in Parts wanted??









I WAS SMOKING! , But i swear of god its truth what my eyes seeing at gauges!