Samsung netbook NP300E5A dimming on battery

You most likely have your power or display settings located in Control Panel set to have your screen dimmer when it is not connected to the charger.

That would make sense because the screen being less bright = more power conserved. When the computer is plugged in, it doesn't matter because it is actively charging thus, it can use as much power as it would like.

So, to resolve this navigate to Control Panel > Power Options > Change Plan Settings > Click Change Advanced Power Settings > Expand the Display section, then expand the Enable Adaptive Brightness option. You can turn this off or on, and as I stated earlier, it is for conserving power.