As far as I know, there is no software/configuration or other trick that would do what you want just because laptops are designed so. To force it to using AC you have to take the battery out, which, as a side effect, also increases the life of the battery due to reduced warming (given that you store the battery in an appropriate place).
When plugged in, the power plug will both charge and simultaneously run the computer. It won't/can't run from the battery while simultaneously charging. It has to do with the voltage differential. The power adapter has to have a higher voltage than the battery in order to overcome the battery's voltage and cause charging. Net charge INTO the battery.
And unless there is something critically wrong with the laptop's battery, it will stop charging when fully charged, meaning that it basically disconnect the battery altogether.
In other words.... DON'T WORRY!!! It is smarter than you are!