Compliments of QRZ.com....

https://forums.qrz.com/index.php?threads/13-8v-30a-4s-lipo-battery-power.787300/#post-6014375

which looks like a winner, but not with Lipos. U1 form lawnmower battery.

I'm minded to find a regulator, but I'm leery of delivery. Does anyone know a firm here in the US
that has this:

https://www.amazon.com/dp/B097DK4PV6?ref=ppx_yo2_dt_b_product_details&th=1

or similar?

And will deliver by USPS Priority Mail? I've not the time or resources to risk packages unsupervised.

de KI7CIL

Might want to check in to this:
https://mfjenterprises.com/products/mfj-4403

ETA#2: I have a 4416, I just looked at the 4403 and it looks like it's just a fancy surge suppressor and not an
actual regulator. The 4416 is a regulator but it works on the wrong side (it corrects low voltage on lead acid,
not high voltage on the LiFEPO4 batteries.)

ETA:
I have lower wattage chinesium voltage stabilizers and they've worked fine. I do a search for
units with prime shipping with short wait times as that usually means it's coming from a US
warehouse.

(Grumble) The local 2 way radio shop ordered the unit
shown on the opening post.

It will be able to boost the mower batteries up to 13.8V
or reduce the Ryobi 18V batteries to that same level, as
intended.

This, after much furious scouring around for US firms
that would carry or order similar, and then turn aroundI
to ship via USPS Priority to a USPS mail box.

Frustrating.

The GOOD news is.... You don't need any of that stuff. Buck-Boost is not required, and adds nothing to your signal. Power-poles,  rig runners, all of that junk will improve your portable  radio in NO way.   String up a  full length half wave  antenna and connect radio to the nearest 12 volt lead acid battery, and operate.

A $40 lawn and garden battery ( U1 ) or the $90 dollar AGM version in  the same form factor   will run your HF radio all day without any  intermediate accesories.
 Just hook the  radio's RED wire  to positive terminal, Black wire to Negative.. Turn the   rig's output power down to 40 or 50 watts and have fun.
Charge with conventional  automotive battery charger when you get home.

That's true if it's a lead AGM.

If he's using a 18V LiPo as he mentioned in the OP he definitely needs some kind of regulator, as 18V that's well past what a mobile rig is rated for.

Why use an 18V battery??  Get a LiPo3 battery that puts out a lower voltage. Any time you use a converter, more energy loses are introduced.
Personally, I use a LIPo3 Bioenno, 15 AH battery with my portables. I've used it with both KX3 (15 watts) and IC7300 (running at about 40 watts). No issues.
For longer operating times, like a Field Day, I use a 100 Ah AGM battery and recharge it with solar. The battery size and weight is not an issue since my vehicle is nearby.
This upcoming winter Field Day, I'll be using a new BattleBorn 100 AH LiPO3 battery. It puts out 13.8v and charges at 14.4V (for a full charge). 13.8V or an automotive alternator will charge it too.

I use 12V equivalent LiFePO4s myself, but I'd really like a nice 18V solution as well.

The benefit to 18V is that there's a crapload of packs out there for power tools. I'd love a box
I could slap a couple Makita 18V/5A batteries on that fed out 13.8V to my radios.

The Xiegu G90 has a peak of 20W output. That 20W only stays there at 13.8V or
higher. Hence the boost if the U1s are being used, and the buck if the Ryobi
Li-Ion (not lipo...error there) 18v are being used.

The G90 will cut off completely, IIRC, if input voltage exceeds 15v or drops below 9v.

Thus the extra equipment.

The concern here is well noted.

I am still a little fuzzy on what you are trying to do, but a basic $20 solar charge controller will take 18 volts ( what solar panels put out ) and charge / supply a battery with 13.8 volts and keep the voltage at 13.8 volts during the process.

ETA: there are solar batter chargers / controllers that will output 14.4 volts to charge LiFePO4 batteries ( they are switchable to type of battery). Just make sure the current output of both the 18v battery and the solar charge controller will meet your demand.  You would have an 18v battery pack to a solar charge controller to either a LiFePO4 battery or even a AGM or SLA battery as a buffer to the radio. that would allow you to use 18v battery packs to power your. I don't know what the efficiency would be, but the controllers are all solid state with no fans so I would expect the efficiency to be good. You would have to research to get one that is RF quiet. I have one that is that I use for a portable solar panel battery setup, I will get the name and model for you

I have a X-108G that I run on a 12 volt LifePo4 battery at 15 watts with no problem

The idea behind the buck/boost is to have a wide variety of DC power sources
available.

I want that 13.8V so that I can run at the G90's highest design wattage, being
20W. YES, i could hook up my U1 directly, but would not get above 15W.

As to solar power components... Many have attempted to foist onto me the
virtues of solar. Fine, point out to me a reliable American firm, and I just might
consider thinking about it.

I have my own pedal powered generator that will keep my U1 happy, and not
have to worry about clouds or weather to do the job. And yes, the company it
was made and sold by is American. (K-tor. DYODDD finding the links.)

Pah. Drifting my own thread...