What could be draining the battery if the alternator is good?

How often does the car get driven, and how often do you start it and let it run?

The first problem is all generators are very inefficient at low speeds, particularly at idle speed. The next problem is even when the car is driven normally, a battery that needed to be jump-started is very run down, and it takes some time for the acid to become electrically conductive before it will start to take a charge. If you have an amp meter on your charger, you'll see it starts out pretty low, then goes higher after a few minutes before dropping down again when the battery is fully charged. From nearly dead, it can take a good two to three hours to charge a good battery at a slow rate. If you just jump-start the battery to get the engine started, then let it idle for a while, you'll end up with just as little charge in the battery as you started with. Recharging the battery doesn't really begin until you've been driving for a few minutes, then it can take a few hours to reach full charge.

The third problem becomes a concern if the car sits for weeks at a time. As far back as the 1980s, Chrysler used to allow up to a 35 milliamp, (0.035 amp), drain to keep the many computer memory circuits alive, including the clock and radio presets. At that rate they said a good, fully charged battery would still be able to start the engine after sitting for three weeks. Today that is the industry standard unless specified otherwise by the manufacturer. Most cars can go longer than three weeks, but there's no guarantee

If the engine only idles, and if it can be weeks before it is run, consider connecting a battery maintainer. They use very little power, but will keep the battery fully charged. Some plug in to house current. Some run on a small solar panel.

If you charge the battery fully, then it's too low to start the engine after two or three days, there is a procedure for measuring the current drain. It requires a digital volt / ohm / amp meter and some small jumper wires, or "clip leads". Harbor Freight Tools has a perfectly fine meter for around $7.00, and a pack of clip leads for less than that. You can find voltmeters at Walmart and hardware stores too. If you aren't sure how to set up the meter, I can help with that.

Due to the age of your car, there are likely to be some computers that need time to go to "sleep" mode after the ignition switch is turned off. Performing the current draw test the way we did on older models may blow the meter's internal fuse, or at least give incorrect results. I can post the steps to prevent that along with some drawings I put together just for this purpose.

These articles will give you an idea of what we might need to do:

https://www.2carpros.com/articles/how-to-use-a-voltmeter

https://www.2carpros.com/articles/car-battery-dead-overnight

https://www.2carpros.com/articles/how-to-charge-your-car-battery

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

Be aware they're using an "auto-ranging" meter in these articles. That's an expensive option you don't need, and is why these little $7.00 meters are such a good value. Also, the charging system test shows just part of the full test. If this part passes, it just means it's okay to perform the rest of the tests, but that requires a professional load tester.

I'm here every day, usually later in the evening.

If you go to a Harbor Freight Tools store, the photo below shows the meter I use. The clip leads are shown in the second photo. They come in a pack of about ten. We only need three for the current drain test.

The problem with this test on cars as new as yours is the need for some computers to time out and go to sleep mode. That might take up to 30 minutes during which time up to three amps might be flowing out of the battery. If you insert the amp meter as I'll have you do later, that can blow its internal 2-amp fuse. Testing becomes valid after that 30 minutes.

The bigger problem is anything that is done to interrupt that current flow will reset the timer, then we have to wait again, for up to another 30 minutes. Interrupting the circuit includes turning the ignition switch to "off", disconnecting and reconnecting the battery's negative cable, moving the red meter lead from one jack to a different one, and even switching ranges on the meter. These will be solved with the third clip lead. The first two are just used to connect the meter's probes so you don't have to stand there and hold on to them.

If you just want to use a battery maintainer, Harbor Freight Tools has those too. I have five of them. Only one stopped working for a few days after it got wet. They come with instructions, but hookup is very easy. They will have a red clamp that goes on the battery's positive cable. If yours has the miserable side post terminals, those little bolts are never meant to be used as connections for jumper cables, even though they do appear to work, but for such a low-current device like the maintainer or small battery charger, they work okay. The black clamp goes to the battery's negative cable or to any paint and rust-free point on the engine, or body sheet metal. Plug the unit in, and most of the time it just starts working on its own. Mine have five red lights to indicate the state of the battery's charge. When all five lights stay on steady, the battery is fully charged.

I've never used a solar-powered battery maintainer. My batteries sit indoors out of sun light.