It is dead or not connected. A light bulb draws a quarter amp in a car. Your draw and connection problems may vary or add up to a draw, over 12 hours, that reduces the charge enough that it wont start.

Components that don't work can often draw power and have it going to ground.

The lights on, the key on, the door ajar, the trunk light, the dome or opera lights, these all can be in play.

Wiring and switches and controls for aftermarket add ons, and their components are more likely to fail or be installed in a way that leads to this. It is also a possibility that you have a defective new part, a battery that self discharges, very rare, but I have seen a couple in my life. even high quality brand name. But any component has the capability and till it's found you have to increase your deletions of circuits by unplugging components as well as their fuses and relays.

A DIRECT SHORT BY A CABLE will get very hot and likely start it burning so some of these answers are stupid on this page. or not well worded. A parasitic draw in a high amp user can be just the control or solenoid. or the circuit board within.

What I would do is get between the cable end and the post on either cable and measure in amps.

This being after a logical visual inspection and check of everything i could turn off.

Most high quality meters are protected with a 10a fuse and the leads are attached by the operator to use that function, then read for out of spec draw, eliminate fuses, wire plugs and relays till you find the one at fault. It is going to be a low amperage circuit because you did not mention a fire, a smell, or say you have been experiencing fuses blowing or saw smoke.

Google your problem and spend a little time familiarizing with what service bulletins may affect your vehicle.

safety check your electrical system to be sure you don't

cause damage or hurt yourself.

There is always the automotive electrician you could hire to simplify your problem from his experience.

If your car has a warranty and insurance, call your dealer.

Alternator

It was not the battery, but having a new battery is not a bad thing.

First, with everything off, doors closed remove one battery cable and look for a spark. This would reveal a parasitic drain which is caused by a short somewhere--and possibly in the alternator.

Second, ther alternator might be putting out nothing. I have had an alternator bad but the light (working light) did not come on.

While you are at it clean both cables and tighten properly. Dirty oe loose cables could be your whole problem.Or have someone else do it.

Most likely a short circuit in your alternator. When the alternator functions normally, it runs the electrical system of your car while the engine is running, as well as supplying a trickle charge to your battery to keep it fully charged. If there is a short circuit in the alternator, your car will be running on the battery at all times, draining the battery's power and, when the engine is shut off, the battery will continue to discharge through the alternator to the car's chassis...effectively killing your brand new battery.

Check all the battery cables and ground connections. Touch all surfaces/run your fingers over them to detect breaks in insulation that could ground out including on your alternator.It may be on the underside so you must be through. On the battery cables look for one that is rubbing against a metal part and the insulation is worn. This may only occur when driving due to vibration or movement of the engine. Be sure to look at the cables that go to the starter motor. As others pointed out look for courtesy lights that are staying on including in the trunk and under the hood, glove box etc. You might systematically try removing a few fuses for things like the sound system. Also are you leaving a phone charger plugged in at all times? Or another device?

You'd have to test for excessive draw in the wiring circuits of the car...should not exceed 50 milliamps...most often it's a bad power port or lighter port that has bent terminals shorting out from a cell phone or other charger plugged into it....very common on modern cars and the increase in modern devices plugged into power ports. You can simply pull the fuses for the lighter or power ports and see if the battery is good the next day...if so, then the issue is likely with the port. Other issues could be a light staying on, bad windings/internal shorts in fan or wiper motors, bad alternator/internal short, frayed wires shorting to ground or corroded wiring connections shorting out, etc..

Pickup a multimeter or DVOM if you don't have one....basic models run around 10-15.00 at home centers... and run some parasitic draw tests of the wiring circuits ....see video below on how to do it or have a shop check it out....