Hi
Thank you for your query
No this doesnt have anything to do with your battery.
As you can appreicate the your screen needs alot of power so when plugged in there is a continuous source of power so its very bright, however as your battery has only a limited charge, the screen will go dark to conserve
There are 2 ways of changing this
1) Press the fn key + the key with a sun symbol on there this is usually the up arrow key or the right arrow key, but will will vary from manufacturer to manufacturer
2) Click Start> Control Panel>Power options
Inside here you will be able to set what type of power profile you want running on Plug and battery
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