My cadillac shuts off when i come to a stop the battery drops from 14.7 to 12.1 everything will stay on lights radio , and my car wont start. if i wait a while and hit the breaks and gas it will start up bt wont stay on long.

Look for the light in the trunk, under good and glove box to see if they stay on. If the radio stays on after you shut off the engine but won't go off after you open the door then the body control module is faulty.

The voltage regulator built in your alternator is failing is usually easier to change the alternator and if I were you I'd change the battery too because you've been running the car off the battery and I'm sure the cells are weak

Who did the battery replacement? I'd take it back ask them to help you figure it out. Could be there is a security issue so the Radio stays on as an alarm. Many radios have a security code that needs to be re entered any time a battery is changed or other wiring is changed. or Fuses blow. Check Owners manual for troubleshooting and details about radio security code. Dealer may have code tied to Vin number.

Try jump starting it from another car. or that thing should have a volt gauge while trying to crank it if it drops lower then 11volts it wont start. or if you can hook up a volt gauge to the battery if not just try jump starting it.

Sounds like a bad battery

Auto Zone does not test the alternator under full load so the test result could be borderline.
start the car and turn everything on. If the voltage drops lower than 14.2
replace the alternator.
also the battery could be being drained overnight. to check use a test light by removing a battery terminal and connecting light between battery and cable with car off.
if it lights start be removing fuses.
when the light goes off the problem is with this circuit.
lastly if the light stays on take the wires off the alternator.
if the light goes out replace alternator.

If the security light is not staying on, I would check for voltage at the starter, the neutral safety switch, and the key switch.

Sounds like a dead battery.

Have your ignition switch checked. might be losing contact in the inside. its happened before

unlock driver door twice consecutively because that's how u disarm the car alarm.As for the radio, input the code that's in ur booklet. If u don't have it, call ur local dealer with the car's serial number and they will give u the radio code. It's 4 digits.