Peter643 and Cool Guys:
Simple:
1) The external supply has a greater voltage than the battery.
2) The battery receives some of the current from the external supply.
3) The load (laptop, mobile, etc.) receives some of the current from the external supply.
PS
I am operating my laptop right now without itīs battery.
. If i connect it now, the external supply must be capable to feed the battery too.
The mobiles (most) not operates without the battery. I think is just for cost reduction of the charger. I modified some mobiles using long endurance external batteries connecting them directly to the mobile instead the use of itīs internal battery. When recharging it (recharger set to 4,2 V directly connected to the external battery) the current (from the recharger) divides, part to the external battery and part to the mobile circuitry. A 18650 provides energy for several days of heavy use.