This is what happens with a hybrid bios when your using the 180w power supply unit. Hybrid bios is A01 through A05, A00 doesn't do this. Since you have the 980m graphics card you can call dell and they will send you a 240w power supply for free (if you live in canada/usa, other countries do not have access to this option for free I'm told). The 980m requires more power than the 180w provides during high load.
A hybrid bios will use power from the battery to augment the power supply when needed during high load. This is why your seeing the drop.