This is crazy stuff, but it's still got scientific basis. This would be very guerilla/MacGyver/ducttape way of going about things, but I'm monitoring this because of a lot of forum members on very bad Eastern Europe / Southern America electricity grids. To the point where I am seeing people trying to power a PC off a car battery to get away from electrical interference built into grids.
A lot of computer monitors use 12V-20V AC adaptor bricks, so I am not surprised that some gaming monitors works fine off unregulated car battery electricity. There's switching DC voltage regulators built into computer monitors so monitors have its own way of regulating power, to support the wide variety of DC power bricks used.
Even the 18V-20V computer monitors can usually be successfully undervolted to 12-14V (aka direct voltage from a car battery) and still function, but it varies from monitor to monitor. This is because most monitors have a built-in DC-DC switching power supply, that will regulate the barrel input jack.
However, the computer is going to fry very easily -- due to the 3.3V and 5V rail -- unless you thought of it. I'm really curious how you did it to a computer PSU. Most have 12V, 5V and 3.3V rails. Applying 12V to the 5V and 3.3V rails, would blow up a lot of components, unless you're using some kind of voltage regulator or modifying a 120V PSU as a 12VDC power supply splitting to 12/5/3.3. I am not crazy enough to try that, but it would be quite cheaper than a Jackery Power Station or similar clone --
the recommended offgridding route. Be careful. The battery can explode if short circuited. I don't recommend it. But if you're doing it anyway, I'm monitoring -- just everyone here disclaim all risks of following advice here. This isn't stuff for the inexperienced!
For those successfully hooking a PC to a car battery -- you said "DIRECTLY". Do you mean through an inverter? Or without one?
If without an inverter, how are you handling the very dangerously sensitive 3.3V and 5V rails? Hopefully you are actually using an inverter, and just simply mean "directly" as the battery+inverter system.
500 watt power-socket inverters with car-battery clips, are
nowadays cheap at under USD $50 on Amazon/Alibaba/ebay so that would work much more safely -- you're letting a computer PSU do its job. Some local stores in some countries may sell 500 watt inverters at the equivalent of USD$20-$30 in your currency (no Amazon premium). Pay a few pennies more and get a 750/1000W capable inverter instead. So you have a cheap car battery, a cheap inverter, and you're offgrid for a little while depending on battery capacity.
Even the 4000-watt solar inverters (compatible with car batteries) have fallen to under USD $400, so you could even use those with a car battery for now. (And add solar panels later, win-win). And works with multiple car batteries (expandable) in parallel for longer runtimes. Some even also double as car battery rechargers when plugging the inverter into the grid.
All very Macgyver-style duct-tape stuff. Monitors/routers/etc would usually easily work fine directly connected to a car battery (12-20V DC barrel input), with some mitigatable risks, but the computer is complex to power! Ideally if it's not an indoors-safety-rated battery, then the battery needs to be outdoors (but still weather-protected), hooked by extension cable, if you try this. You'll be safer.
And, if possible, consider the 12.8V 100aH lithium iron batteries with built in BMS (safety system!), a bit more expensive but you get 7000+ charge cycles and they're much more explosion-proof than the common lithium-maganese or lead-acid batteries. Make sure the charger is compatible with lithium (and configure the charger correctly)!
I'm lucky I don't have to do it in Canada with a (comparatively) cleaner electric grid, but offgridding is indeed getting cheaper and cheaper. A good-quality spare car battery, a $30 inverter, and you're offgrid with a gaming PC rig roughly one hour-ish (give or take).
Disclaimer: Not For The Inexperienced. Batteries Can Explode. Batteries (Lead or Lithium) Are Highly Toxic. You Can Fry Computer Components. Inverters Can Electrocute You. Do At Own Risk!