If you can see images then there is a backlight problem. They will be very faint if they are there so trying this in a darkened room may help. Try shining a torch at an angle close to the screen to check if there are any images at all on the screen when the laptop is on and there are images on the external display. Once you have replaced the RTC battery, re-assemble the laptop (don’t forget the main battery do it as late as possible to prevent any problems while the laptop is still open), connect the charger and try to turn the laptop on. Here’s the part number for the battery 741976-001 While the RTC battery is out you may as well test it to ensure that it is still OK. hold Power button for at least 30 seconds to hopefully restore the BIOS to default values. Once the RTC battery and the main battery have both been disconnected do the residual battery discharge procedure i.e. Ensure that the main battery has been disconnected as soon as it can be accessed. (Note the orientation of the battery before removing it so that you put it back in the correct way). Use it to gain access to the systemboard so that you can remove the RTC battery. Here’s a link to the service manual taken from this webpage. Try resetting the BIOS by removing the coin cell RTC battery as well as disconnecting the main battery from the system board.

Hi you say a 5.3 blink code is a system board problem - “The embedded controller times out waiting for the BIOS”.

Hp pavillion power button