Run a laptop off of 12 volts safe?

My laptop battery is 10.8 volts. The charger I assume inverts ac to dc current to charge the battery. I also assume that if there are any ac voltage requirements  in the laptop it must have an internal invertor from dc back to ac.
The charger input is 110-240volts 1.7 amps. The output is 18.5 volts 3.5 amps.
My question is why can't a 12 volt source like a jumper box be used to run the laptop, when the battery is low, through the port that the charger is plugged in when charging the battery.
My thinking is that the battery being 10.8 volts dc is charged with up to 18.5 voltsdc then it would be safe to plug in a 12 volt battery source.
Thanks for your time!

orksecurity6 years ago
You could, of course, use a DC-to-DC converter to boost 12V to 18.5V, which is what the official "car charger" for this laptop undoubtedly does.
Re-design6 years ago
It might work but I would not try it it.

The 18.5 volts will be regulated down to what the battery needs to be changed properly. If you supply less than enough then regulation will suffer and may damage the charging system. If the computer does run, it probably will not be dependable.

If you want to power it externally then build a battery that is at least 18 volts.