Dial *#*#6485#*#* . Check "MUH" (Magic Unit Health). If below 70%, replace the battery. Also check "Temperature" – if above 40°C idle, an app is overworking the CPU.
If the code doesn’t work, Xiaomi has likely blocked it. Alternative: Install from Play Store (requires root for some functions). How to Use Secret Codes on XIAOMI Poco C65
Tap 7 times until you see "You are now a developer!". Dial *#*#6485#*#*