I want to make a very simple external battery pack for a laptop, connecting to its 19V DC power jack. I plan to use a series of four lithium batteries that should provide between 11V and 16.8V of power.
The internal battery pack consists of 3 Li-Polymer cells, which I assume provides the laptop with approximately 8.1V to 12.6V of power.
When I connect the 4-cell external battery pack to the 19V DC input, what are the effects? Does it make sense to assume that the laptop is fine with voltages below 19V? Will it take whatever voltage it needs to run, which should be related to what the internal battery provides, i.e ~8.1+V, and direct only excess voltage to recharge the internal batteries? Are there any terrible inefficiencies going on? Does this even make sense at all?