If the problem is only happening on battery operation, the pack may have one or more failed cells, reducing the available voltage below the level that the display inverter needs to keep the fluorescent backlight at full brightness.
The display is the power hungriest part after the processor itself.
The pack should have a rating on it and any multimeter (everyone should have one-digitals are dirt cheap) will tell you if the pack is charging up to its rated voltage.
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You may have a defective CCFL backlight, it may have the following symptom: Screen flashes on and off or displays reddish pink hue. Picture stays on in red and slowly be come normal. Picture flickering with dim display or appear black. Picture flickers on and off. All these symptoms indicate that the LCD backlight lamp (CCFL Lamp) has reached the end of the life and all you need to do is replace the CCFL lamp. To determine if the video card or the LCD is faulty, connect an external monitor to your laptop and power the monitor up first then the laptop, if you see the normal Windows images then the video card and laptop is OK and the problem is definitely the backlight. Dim image and/or dark display on the laptop's LCD screen indicates a faulty LCD backlight. It could be the inverter that supplies high voltage to the CCFL lamp or it is the CCLF lamp is nearly burnt out, this is the most likely case. The inverter can be replaced easily but the CCLF lamp is more time consuming and requires soldering skills. Check out LCD Parts for service, parts, and DIY info Use this link to their WEB site :- www.lcdparts.net
Try connecting an external monitor to the your laptop and see if the vertical line is also present on the monitor. If the monitor display is fine, then your laptop’s LCD display panel is defective. If the monitor also displays the vertical line, then your problem is with either the video card or motherboard located inside your laptop.Not sure if the Compaq Presario has an external monitor plug, but if it does the above procedure can be applied to that one as well.
Check the "Power Options" in your "Control Panel" (classic view). For whatever option you have picked, click "Change plan settings". There you can either "Restore default settings", or manually "Change advanced settings". If you choose "advanced settings", look for "Display" / "Display brightness".
It miight be the backlight for this unit. It all depends on whether the screen came with the backlight. There is a remote chance that the display circuitry on the motherboard is faulty too. Try to plug a monitor into the VGA port and check to see if it is dim there too.