I have an HP laptop, which has just returned from service. It was having problems charging the battery and they put in a new motherboard.

Now, it is charging fine but the minute I pull the power cord out and the laptop is on battery power, the screen goes darker. Not dark enough for me to not see anything, just simply darker.
If I plug the power cord back in, the screen goes brighter straight away.

Any idea why it's doing that?