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When does it go dead ? Over night , while driving ? Two different diagnostic's ! If it goes dead while driving the alternator is not charging the battery . If it goes dead over night there is a parasitic drain on the battery , something is on , using current . You don't say what make , model and year vehicle but most all vehicles from the mid 80's on have a number of control modules (computers ) . They power down after a set amount of time , an sometimes they don't power down . There are videos on youtube showing how to find a key off power drain .
Without a heat strip on the battery and a block heater, at that temperature the battery isn't going to be optimal at supplying power. Look for a draw on the battery when the engine is not running. It coud be a light in the glove box , trunk or under the hood causing it or your alternator may not be charging to it's fullest. If just one diode in the alternator is shorted you'll drain the battery over night. If one is open, the alternator has lost 1/3 of it's rated output. If your hrad lit switch is on the steering column, it's possible it will turn the head lites on when it gets cold..Happened to me.
IF THEY CHECKED FOR A DRAW ON THE BATTERY AND THERE WAS NONE DID THEY CHECK ALTERNATOR OUTPUT? IF IT WAS IN SPECS THEN YOU MOST LIKELY HAVE A BAD BATTERY. YOU MAY WANT TO HAVE IT CHECKED AGAIN. THE DEALER IS NOT ABOVE MAKING MISTAKES
take your vehicle to Autozone they can check the batter for free with the batter in the truck they can also check the altanitor, Most likely you have a dead cell in the battery-if not change out the Altinorter
Never saw an ammeter that had a "normal" reading on the gauge face...You aren't reading the dash gauge are you? Normal procedure is to place an ammeter between the battery and either lead and read draw that way. Then remove each fuse till the draw diminishes substantially. An alternator can draw a bit but it should not be excessive. Checking charging volts, the meter should not read below about 12.5V even with all accessories on and should not continue to drop regardless of running time. (accessories off reading should be between 13 and 14V). Also have battery load tested if you haven't done that already just to make sure it is good (even when new) and make sure something simple like a visor mirror light or glove compartment light isn't on all the time.