What are the effects of connecting an external battery pack to a laptop's power jack?
The answer is simple:  We don't know because the laptop is only documented to run at 19 volts.   It may work fine with less, or it might not work at all.  Although remote, it is possible that even if it "works" fine with less voltage, running it for an extended period could damage something due to parts of it's power supply getting too hot.  

We just don't know what is inside of your laptop.  We also don't know the internal workings of every laptop in the world.  Just because it might work with one laptop, it doesn't automatically mean that it'll work with any laptop.

If you want to be safe, take the output of your battery pack and bump it up to 19v.  Otherwise, all bets are off.