Exactly, the PC PSU coverts AC to 12v for the PC anyway, so 12v batteries should slot into that scheme, without a separate PSU and inverter.

You would still need a DC to DC converter as lead acids deliver anywhere from 12.8 to about 10v depending on SoC and load.

Remember that DC to DC power supplies are much smaller because existing AC PSU has to deal with the transformer aspect.

I worked for an ISP that was also a CLEC back in late 90s / early 2000s. All of the telco side was DC (massive 48v battery banks) - so when we were doing server implementation it made sense to get all DC PSU on the servers. Not sure why DC PSU aren't a more universal option as you'd think UPS providers could easily offer DC output models.

There are options out there, I ran across this [0] looking for typical ATX DC to DC PSU.

[0] https://lazer3d.com/dc-dc-power-supply-guide/

For anything larger than about 500W that'd become untenable because at 12V as you'd need cables that can carry in excess of 50A.

UPS systems start connecting the batteries in series to provide higher voltages for this reason

Or multiple cables, just as PC power supplies of 500w+ have, and in fact some of their power is delivered at 5v.

Even more annoying is why aren't decent USB-only ups systems?

just ditch the 110 outlets for usb-a or usb-c ports and skip the inverter.

I'd love to power a pi or 3 from ups-based usb ports.

Where I live, this is actually extremely common now - typically referred to as a "Mini DC UPS" [1].

These cheaper ones just provide 9V and 12V output to power a fibre and/or WiFi router, as well as 5V USB-A, and start at around 30Wh capacity - some for less than USD 20. If you pay a little more, you can get 24/48V PoE included as well.

Unfortunately, ones providing USB-PD are quite rare. You do get "power stations" [2] with USB-PD (in addition to an AC inverter). These are typically USD 100 - USD 1000, and have much larger capacity.

Examples (I haven't tested these and do not specifically recommend them):

[1]: https://www.amazon.com/Lithium-Battery-Appliance-Monitor-uni...

[2]: https://www.amazon.com/Portable-Solar-Panel-Power-Station-Ge...

Yeah that would be great. Pimoroni makes a version of the Pi Pico that takes a LiPo battery and automatically switches to battery power when the USB power goes away, but I don't know of any full computers that support that. I think adafruit makes a LiPo addon for picos. I wonder if it could be modded to work on a zero or something?

That's where USB-C PD is nice. It's all DC at several different voltages the recipient can request. The only downside is it's not quite powerful enough for many desktop PCs, but it's getting better.

Pretty much expect the Mini to end up this way within a few generations. Somebody in the Mac team really hates wall warts, but given all their other hardware is going USB-C one would think they’d want to standardize & eliminate the space/heat of the internal power supply.