Battery keeps draining after a new ECM was installed?

Hello, does the battery drain down completely over night? It sounds like there is a computer module that is either staying powered up (awake) or something is causing a module to wake up and draw on the battery. It doesn't sound like the shop checked much, the PCM does control the Alternator, but if the battery has a bad cell or a couple bad cells in it, it will not hold a charge. How old is the battery and do you notice anything in the vehicle that is not working or staying on after you lock up the vehicle for the night?
Do you have any type of multimeters or scan tools? Are you comfortable doing some testing on the battery?

Ok I just came across multiple Technical Service Bulletins and Safety Recalls for the Alternator for your vehicle, this is most likely why you're having so many issues with this and why they couldn't find a solution to the issue, I will post all of them for you to read.

Pages 1-3 below go together, 4,5,6, and 7 are all Recalls on the Alternator. Apparently, there were close to 500,000 vehicles with faulty Alternators, some shorting out causing fires. There was a total of at least 10 recalls for the charging systems. The dealership will have an updated part on a recall this big. Chances are there are quite a few bad Alternators out there with the same part number as the faulty one and shops failed to look up any Technical Bulletins on this issue. There is also a chance that the battery was over charged or damaged due to some of these bulletins mentioning shorted internal components to the Alternator casing, I would also go through the fuses and main bulk connectors in the vehicle to make sure there are no signs of overheating.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

See if you can find a part number on the Alternator case. If the Battery light is on and its being drained your alternator is either not charging or the PCM is not commanding it to charge, having a basic multimeter to check the voltage at the battery at idle, at 2500 rpm, and then the battery rest voltage with the vehicle off would tell us a lot. I don't see how the shop let a vehicle go in that condition. The battery voltage really should be checked during cranking, so it has a max load on it. I don't see specific sub models here in all the recalls, but this is half a million vehicles with these faulty diode trio's that are failing due to thermal issues. It's not just Dodge Charger's with these Alternators. It's a Chrysler 300, Durango's, Jeep Grand Cherokee, Challenger's. So obviously there's a major issue here. If that shop has your vehicle for 2 months and couldn't figure out what was wrong with it, that's a red flag to me. The PCM was replaced, who knows how many Alternators the shop tried. I would get a basic multimeter and while the vehicle is idling in the driveway, start doing some voltage drop testing from the engine block, body(frame), Alternator case to Battery Negative and see what the voltage drop is on those Ground sides. If the PCM detects any faults in the Alternators field control which is used to control the charging voltage level, it will set codes for the charging system. There are only 3 wires on this alternator, the wire to the battery, the Gen Sense wire which is where it monitors the PCM monitors the output and the Gen Field Control. The shop should have checked these wires for open circuits or shorts to ground.
Now the big problem is that when the diode trio goes bad the Alternator outputs AC voltage, and PCMs and AC voltage don't mix. If there's no codes stored for anything, (no Check Engine light on), Something is going on that the PCM is not detecting or is not setting a code because it can't anymore. The diodes are built into the alternator to rectify the AC voltage to DC voltage so it can be used by the PCM. All these Recalls and TSBs are for the diodes failing.
There should be a code set for why it's not charging, this isn't a parasitic draw on the battery. There needs to be testing done.
The 2nd diagram is all the Recalls and TSBs for the alternator.

Have them do a voltage drop test from the positive cable connection on the alternator to the battery positive while it's running and with the headlights on. If there's an issue on the positive cable it will read a voltage. Thats the voltage being lost from point A (positive alternator cable) to point B (battery positive cable). Voltage drop testing while under a load is the correct way for them to determine where the loss is occurring. I will post a little diagram that shows the test better.
Excuse my crude diagram here, but this is a voltage drop test on the positive cable from the Alternator to the Battery +
If the wire happened to touch the exhaust manifold after it melted, check your fuses too, that would cause a fuse to pop if it shorted out on the manifold.
And make sure they either give you your money back, or do the repairs for free, because this isn't your fault. They misdiagnosed the problem to begin with.

https://www.2carpros.com/articles/how-to-check-a-car-fuse