Here are several reasons why a car battery might drain after starting the car once and sitting overnight:

Parasitic Drain: Certain electrical components in the vehicle might draw power even when the car is turned off. These can include clocks, alarm systems, or poorly installed aftermarket accessories.

Faulty Battery: The battery itself might be faulty or aging, leading to overnight charge loss. This could be due to sulfation (accumulation of lead sulfate crystals), internal short circuits, or other internal issues.

Corroded Battery Terminals: Corrosion on the battery terminals can impede the flow of electricity between the battery and the vehicle's electrical system, leading to a drain.

Alternator Issues: If the alternator fails to recharge the battery while the car is running sufficiently, the battery might not have enough charge to last overnight.

Electrical System Malfunction: A malfunctioning component within the vehicle's electrical system, such as a faulty relay or switch, could be drawing excessive power even when the car is turned off.

Poor Ground Connections: Inadequate grounding can lead to electrical problems, including excessive battery drain.

Extreme Temperatures: Very hot or cold temperatures can affect battery performance. Extreme cold can reduce a battery's ability to hold a charge, while extreme heat can accelerate internal chemical reactions leading to faster discharge.

Short Trips: If the car is only driven for short distances, the battery might not have enough time to recharge, leading to a complete drain over time.

Old Age: Even if the battery appears to be functioning normally, it may be nearing the end of its usable life, leading to decreased performance and eventual failure.

Ignition Off Draw (IOD) Fuse: Some vehicles have an IOD fuse that is designed to prevent excessive battery drain when the car is not in use. If this fuse is faulty or incorrectly installed, it can lead to overnight drain issues.

To address the issue effectively, it's essential to diagnose the specific cause of the battery drain. This often involves testing the battery, checking the vehicle's electrical system, and inspecting for any signs of corrosion or damage.