OTRPU,
While that might've been true in years past, newer laptops that use Lithium-Ion batteries are much less affected by running on external power. Also, deep cycling is not necessary.
I've been using laptops on external power (110v/inverter/DC-DC) for years with very little problem. I typically wear the laptop out before the batteries die.

For me, the battery is there for the times external power is not available nor practical.
It's not that the screen is brighter on external power, rather that it is dimmer when on batteries. Check your bios power management. It's probably set to reduce screen brightness when running on batteries. Got to conserve that battery power!
A computer is a digital device. The internal voltages are supplied by regualtor circuits inside the laptop, whether you're running on batteries or external power. If the voltage from your laptop's power supply or your aftermarket DC-DC converter goes up, those regulator circuits will compensate and only provide the proper voltages to the computer. Of course, this only works up to a point, then the magic smoke comes out.
I've been using a Targus DC power supply (auto & airplane) for the last several years and on 3 different laptops. No power related problems at all.