Charging a laptop using 12 volts.

Hello,

I have a latitude E5470 and would like to charge is my car.

I have a cable charging lead that is

7.4 mm circular outside (Negative)

5.0 mm circular inside (positive dc) 19.5 volts

but there is the centre pin which checks the battery or charger?

I work with electronics and have a 12 volt to 19.5 volt dc converter.

When this is connected to the - + plus of the charge lead when plugged in to my laptop is says power connected but not charging. It needs something to happen using the centre pin to charge.

Does anyone have an idea what I need to do with the centre pin as I will create a circuit to control this and let the laptop charge if possible.