Have no Idea why it drains the battery?

Hello,

It sounds like there is excessive parasitic draw (current drain) to the battery.
Every electrical circuit with a battery will have some amount to parasitic draw, but when it becomes excessive it will drain the battery much faster.
The high parasitic draw can be triggered by a light staying on or a faulty circuit, like a shorted window switch etc.

A typical acceptable parasitic draw is around 50 milliamp.

Have a look at this video, it shows how to diagnose and find the parasitic draw:

https://youtu.be/z5F7WpgjaRs

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

Please let me know of any questions.
Thank you.