Car starts, but the battery loses it's charge

"Lead acid car batteries don’t like being discharged and tend to “expire” in ~5 years. Check when your battery was made and if it’s 5 years old or older, try replacing it and see if that helps first. In some cases they're still reasonably strong even after that so in some cases a battery test may reveal your old one is still serviceable. Usually the battery is near the end of it's life after 5 years but you sometimes get lucky.

If you do not get a better result with a replacement battery, you may need to try another alternator (especially if you used a rebuilt unit in the past) that is OEM or new but not rebuilt. This is the second common reason for no starting.

If you have issues with it even with a new battery and a quality alternator, you likely have a parasitic draw somewhere that will take some time to find, or something like the anti theft system or ECU may be failing. With the anti theft system it may be bypassable, but I am not going to discuss that here or provide links on how - you're on your own. If it's an ECU, they usually need pairing to the BCM and anti theft system and can be over $1k in some cases. At that point you probably want to go to a mechanic unless you own a scan tool which can do the pairing already ($$$)."