When a car battery looses voltage on a cold day

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A new battery that has a 3 year free replacement warranty and its voltage drops from 13.7 volts to 11.5 volts, it could be the cold but mainly it might in fact be a lose accessory belt tenssioner pulley assembly. In my case, it may well be the case. That Duracell battery I bought about one year ago, still has 2 more years under warranty. That is good but no sense in just charging it on a cold day only to wait and see if it loses voltage in a day or two because it just might if cold temperatures cause the neutral safety switch prevent it from starting in the PARK position, on a very cold day. I was instructed to put the car in Neutral with my foot on the brake pedal. It started. This was done after doing a jump from an electric jumper device. The fact is, cold weather may be the reason the battery would not start. It is sometimes a fluke of temperature being very cold to both drain a good battery of some voltage and / or cause the neutral safety switch to Think the car is in gear when it is in fact, in PARK. So, after doing a two hour charge from an AC to DC line charge directly to the battery, the voltage went up over 14.3 volts at first then after the car ran a few minutes, dropped back down to a normal voltage of about 13.7 or 13.8 or 13.6 variating from time to time as the car remained at idle speed of about 850 RPM. After that, the charger was disconnected and the car ran a few minutes at 13.6 volts to 13.7 volts back and forth. I let the car sit for about 10 minutes, then cranked it up in PARK and it started with no problem as the indicator read from 12.6 volts to 13.6 in a matter of just a few seconds after reaching a level constant voltage. The thing I found to be the case with older cars that are over 20 years of use, and a Cadillac Northstar, is that from time to time, in VERY COLD temperatures, these things occur but the main thing is, after all this time, over 20 years since purchase, it most likely has a bit of slack in the tensioner pully assembly. So I ordered one and will get it put on in a few days to a week. meantime. I don't even want to bother trying to drive if there is any possibility that the tensioner pulley is not fully taught. This would lead to fast voltage failure over a day or two and would result in not properly bringing up the best possible voltage level of the battery. Cars are a thing that need to be checked out through the process of TRIAL AND ERROR. And to rule one thing out at a time to come to an informed conclusion as the the cause and effect of performance issues. I recommend the following link for persons experiencing symptoms of a loose belt tensioner.