Charging indicator showing problem in iphone 13

I am a user of iPhone 13. This is a second-hand mobile phone, purchased from a renowned shop in Chittagong, Bangladesh. It has official Apple warranty valid until February 2026. Currently, it is November 2025. I bought it last month (October). Its battery capacity was 100% at the time of purchase, and it remains 100% to date.

After using it for a few days, I noticed that the charging indicator (🔋⚡) was showing even without connecting to a charging port or adapter. At that time, the battery level fluctuated irregularly, and the phone became very hot. Restarting the phone temporarily solved the problem. However, this issue has happened several times since the first occurrence. Today, it has already occurred twice. Restarting fixes it temporarily; otherwise, the problem continues.

I went to the shop, and they reset the settings, but the issue persists. I use an Apple-certified 20W original adapter and an official Lightning cable. I have also cleaned the Lightning charging port, but the problem is not resolved. My device was updated to iOS 26 when I bought it, and now it is on iOS 26.1. However, the issue still continues.

I am very concerned about the battery health and the functional reliability of my phone. I would appreciate guidance on what can be done to permanently resolve this issue.