Battery life Issues

I don’t know. I bought this device on the fifth batch, probably two years ago, and I still don’t understand everything after all these updates. What is the issue with power management? Sometimes, in standby mode, it takes, I don’t know, probably one to a few hours, and the device is completely dead. It’s not clear to me what the issue is. Probably it’s a software issue. Probably it’s a hardware issue.

Any ideas how I can improve, or how I can actually test this situation? I don’t know. Probably after two years I’m not covered by the warranty, and I can access developer mode to use some utilities to check battery life and understand what can be done.