|Some Laptop Batteries degrade when not used as intended. 30 minutes run time on a full charge is an indication of a weak battery.|
Try running the Laptop on the battery only till it goes into Hibernation on it's own, then connect the AC and allow it to fully charge. Repeat a few times. If the Battery only run time stays the same, you may need a new Battery.
Toshiba Rates Battery Runtime on a Model by Model basis. You could check what is normal for your LT/battery combination.
Generic Laptop batteries may not perform to Toshiba standards. Same for AC adapters . Check to see if Battery & adapter are appropriate for your M24
A weak battery can affect the AC operation because the AC is always trying to charge the battery.
Run with the Battery out, on AC only, see if performance increases, Or backlight problem persists.
I suppose it could be the inverter, but I would think Inverter problems would not be time dependant, waiting 2 to 3 hours to develop. The other possibility would be the Backlight Tube itself.
Heat, in an electrical circuit, equals increased resistance. Laptops tend to have Hot Spots. If some part of the Inverter or Inverter wiring resides in a Hot Spot, it could affect the Inverter circuit only. Hot Spots can occur in different locations than expected due to dirt & dust accumilation in the Processor Fan ducting.
Problem with Laptops is, nothing is cheap. Not parts, not service and who just happens to have a new Laptop Battery hanging around? Makes troubleshooting a real PITA and wallet.
What I would do:
Check for matching Battery & AC adapter.
Run on Battery only/Recharge, repeat.
Replace the battery if Run time stays Low
Run on AC only to see if problem persists. If YES.
Open the Case, clean Dust/Dirt out of system.
Retest on AC with cleaned LT.
Retest with new Battery & cleaned LT
Or have a shop clean the LT & test Inverter
There is nothing to learn from someone who already agrees with you.