I agree with Maxwedge- sounds like you need to get a different battery for starters. If your battery was down to 6 volts, thats not a good sign. 12 volts doesn't mean a good battery. Battery's capacity are actually measured in current or Amperage, i.e 600 Cold Cranking Amp Battery. This means a battery can provide that much current/amps to a load or draw and still be at 12 volts when it does it. There is more to it insofar as how long it can provide (Amp/Hour rating), under what temp conditions, and others. Battery reserve rating on a car usually indicates how long a battery can probably keep the car running if the alternator stops charging (i.e. can you make it to safety before it dies).
A better test is to put the volt meter on the battery before trying to start the car, and measure the volts- presumably 12 Volts DC. Then keep measuring the volts as someone tries to start the car. A good battery will stay very near the first 12 volt reading- maybe drop a half volt or so while the starter is putting a load on it. A bad battery- you will see the voltage drop down anywhere from a couple volts all the way to almost zero volts, while the starter is putting a load on it. This is the poor man's battery load tester- a DC Voltmeter and your car's starter as the heavy load.
Batteries setting in the cold for a period of time with a low charge, due not recover very well. You should keep your battery charged if its going to set. Batteries are resistant to cold and freezing only when they have a good electrical charge.
Good Luck!!