most laptops power from both the Batt and Ac (not 100% of the time though). as the AC adaptor cant provide all the power required, eg a 65watt power brick cant provide full load to a laptop when the laptop is running at 100% and will also take power from the batt if available, if its not, then it will change the cpu/gpu to a lower power mode. If you run your laptop batt flat and plug in the charger, do something cpu/gpu intensive and you will notice the laptop will stop charging and use the full 65watts and then the batt as well (if there is any capacity) . So im guessing the 787 works similar, rather then float charging the li-on all the time , unless there is some special way to float charge a li-on.
any idea if they charge the batt from the 115Vac ? or from the 270Vdc ? on the 787 ? im guessing its easier(cheaper?) to covert from 115Vac to anything that the charger needs, rather then a DC to DC converter ?