Re: charging AGM batteries in cold climates
While I agree with the concern about charging a frozen AGM, my experience differs... somewhat
1055Ah AGM sat in an unheated but modestly insulated ( 2" polystyrene), 4x8x8 foot, power room at the cabin all winter for 4 years.
It was always fully charged when unattended.
Temps regularly went to - 30 F* every year, and up to freezing at times.
During winter visits I found the room temp to be near, or above freezing to my amazement! A cheap I/Outdoor thermometer showed the AGM to be above freezing .
Charging time would only be 3 - 4 hours max, with ~1 hr of max input from 4 120W mono panels in Parallel.
So the short version is 'don't sweat the petty stuff'... (from George Carlin) As Long As the AGM's are fully charged and you have a good minimum Array size.
100% Off Grid @ 51* 46' N lat 124* 44' W long
New House system: coming - 2 arrays @ 2240W
CL150&Epanel 2 @140 W 12v PVs , 24V C&D AT-15P AGM @ 950Ah
+ CL150 2@120W 12 V panels, 24V C&D AT-15P AGM @ 950Ah
Cotek ST1500W inverter, TBS 30a-24v Omni-charger, Honda Eu3000is,
Guest cabin system: 3 - 70W panels to SB 2000e CC, with 2 - 100 ah 12v SAFT wet NiCd's , 600W TSW Inverter