if you put power into the back of a laptop by what ever means you try, either the manufacturers dc adaptor to a cig socket or via an inverter and laptop psu . it doesnt matter. the laptops power/battery management system will use that power and also charge the laptop battery. in other words it is part of how the laptop is made and unless you modify the power circuits it cant be altered by software alone. the laptop battery wont be used unless external power is removed. you could make a circuit with a relay that is controlled by software and monitors the laptops battery level and then switches to ext power at the point you want . but as stated before the laptop battery isnt going to take a lot of deep discharge cycles ie down to 10% or less too often. i am looking at a dell d620 dual core laptop right now and its battery is 11.1 v and 85 WHrswhich means a capacity of 7600 mA approx. this gives me approx 3 hrs of use. your car battery would probably power the laptop for over 24 hrs straight. then when it gets low you would then have the laptop battery kick in and have 3 hrs to get the main battery recharged.. the laptop battery would act like a UPS.... if your second battery is charged via the car as per normal then you would probably never need the laptop battery. so why do you want to do it the way you said?????