The cooling fan rotates slowly when only connected with mains supply, but rotates normally after being connected with mains supply and turning its switch on.
The switch has not been controlled entirely by the control part of the cooling fan. There is something wrong with U3 optocoupler and N6 triode.
Solutions and steps:
1. Remove all wire from BP series with charge, include battery and AC output.
2. Clear surface dust.
3. Uninstall screws from Front and rear panels.
4. Open the top cover.
5. Find the damaged parts.
6. Change U3, N6 parts.
7. Close the top cover and run it.