[Al Skierkiewicz]

In most cases, leaving AGM batteries for several months without charging does not cause death. As to the cold myths, very cold temps (i.e. less than 30 F) will simply slow the chemical reaction. The colder the battery temp the less chemical activity hence lower charge is available. As pointed out above, the engine conditions simply worsen this already compromised battery. An old trick is to turn the headlights on for 30 seconds to get the juices flowing and warm the battery. Don't know if that is enough but on really cold days in Chicago I have been able to start the car. ( I have never had a garage.)

Many smart chargers sense low battery voltage as a dead battery and they will not attempt to charge it. So the bigger question here, have you measured the terminal voltage on the batteries? Are they zero volts, 4 volts or something odd like 6.8? If the batteries measure one of the low voltages you can try the parallel trick or you can use a variable voltage power supply. Monitoring of the output current is essential so a supply with meters helps a lot. Adjust the output of the supply to 12 volts then turn the power off. Connect the output of the power supply to the battery, positive to positive please. Turn on the power supply and begin slowly increasing the voltage while monitoring current. The current should start out low and increase as you increase the output voltage. Do not raise the output voltage above 14-14.5 volts or stop operation if the current suddenly spikes. In this range, charge current should remain in the 1-3 amp range. Monitor for heating of the battery and wait about 15 minutes. Remove the battery from the supply and measure the terminal voltage. If it has gone back up to 12 volts, then connect to the battery charger and see if it will charge normally. If not, try repeating this procedure for another 15 minutes. If after this charging period the battery terminal voltage does not change, or if the battery is at zero volts, it may be time to recycle.
The biggest problem team face is storing dead batteries. Self discharge for these batteries is measured in months from full charge. A discharged battery left in storage can only get much worse over the months.