I’d much rather do that than purchase a NUC with a built-in battery. Keeping the two pieces separate makes them easier to repurpose later.
Regardless, the market for such a device is relatively small.
I think it’s possible to build one. Get a bunch of 18650 cells from a reputable brand (or just an old laptop battery, if you’re brave enough), then look for a 18650 powerbank kit on AliExpress – some of these would be with slots you can just plug the cells into, without welding/soldering.
this is a supposition, but i don't think the numbers are very far off. Most powerbanks are 18650 or 26650 inside - flatter or "better" ones are lifepo or lipoly or whatever, instead of cylindrical Li-Ion. anyhow those are 4.2V nominal and USB wants 5V so all single cell or parallel'd 18650/26650 power banks are going to use a boost converter to get the voltage to 5.1V nominal at the USB terminal (assuming a dumb power bank, a power bank with power delivery will boost that even higher but also probably has multiple batteries in series, but that doesn't matter, it makes efficiency worse for our daisy chain regardless!). So these boost circuits "charge" something and store it until there's the correct number of electrons to equal whatever charge/joules is required to run the load. There is >15% wasted as heat either in the capacitor or the inductor (depending on the style of boost converter). There's another ~15% or so lost in the charging circuit, as it has to take 5.0VDC in and run a charging circuit (similar to mppt) on the battery at different voltages and/or amperages which again waste is generated as heat. Cable losses in the daisy chain probably account for a percent or two each, the indicator lights across all the power banks, plus all the microprocessors/etc inside of them probably waste another couple percent.
Look for "PD in" on this sheet for some examples (columns BW-BY): https://docs.google.com/spreadsheets/d/1SWqLJ6tGmYHzqGaa4RZs...
i left it at stock bios settings and did not put it into higher TDP mode though, since i use it as an htpc and it mostly idles, i'd rather it stay at more comfortable operating temps.
the nvme cost me more than the pc, which for $207 includes 16GB ram, 512GB nvme, and a Win11 Pro license. insane.
[1] https://www.amazon.com/MeLE-Mini-Quieter-4C-Astrophotography...