Why is my iPhone 15 Pro Max battery charging itself?

22__22 wrote:

My phone was charged 96 or 95 % and I switched it off at night and after I woke up and switched on I found the battery full 100% and this happened twice. And this is really happened

When you powered the phone off, then back on, the battery meter (which is NOT exact) reset based on current battery state. The battery did not recharge itself. This would be normal to see.

Your post makes no sense, batteries can't charge themselves. Please give details on what you are seeing.