My laptops backlight turns off when i plug it in.it works fine when running off batt power

try external monitor, maybe the lcd backlight or inverter has stopped working.

your laptop probably in power saving mode, it is a preset to makes battery last longer. your power adapter might not being detected shown by red light, try to plug it firmly or use other power adapter if available

Remove the main battery, now try the power supply, is there any change?

You have a bad backlight or inverter. The purpose of the inverter is to turn on the backlight. I suspect it's a bad inverter because that is a common problem on most LCD displays and you mentioned it turns on briefly - the backlight lamp is likely good..

I'd try:

Updating the bios to the latest version
Uninstall video driver and check for newer version
Replace the inverter.

Let me know if any of those suggestions work, if not we can try something else.

First you want to check and see if your back light inveter is not giving out or you can change both the inverter and the back light in the lcd.

Check the inverter first and replace as needed.

thanks and please leave postive feedback

Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/edwy_1a56970cf1ea4620

turn off- unplug power- remove battery 1minute replace batt power cord press and hold zero restart while continuing to hold zero 30 to 45 sec screen will flash when you see cursor blinking let go... now you will have option to restore to (out of box state) select each time.( finish when prompted) computer will restart several times DO NOT TOUCH MOUSE OR KEYBOARD until you are sure auto config is thru approximately 1 hr 45 min

remove battery check if laptop works fine with ac adapter, if not then download the latest bios version first if fails then replace ac adapter. If laptop works fine with ac adapter. but cannot work with battery alone, download the latest bios then charge the battery for 6 hrs. if still batt not charging then replace batt. but as you mentioned you already replaced the batt and your still getting the same issue. replace motherboard.

when you leave the computer for a long time inactivity it goes to hibernate or standby
you can change it on control panel power options

backlight needs replacing, can be done