Amazon and AliExpress sell sealed buck boost regulators from 3 Amps to 30 Amps from about $5 to $50. They are pretty much bulletproof. Output voltage is chosen by the unit selected and input is from about 5 to 50 volts. Run your zBitx from a solar panel!
I think you'd be best served by using a regulator to turn 13.8v into 9v, rather than trying to redesign the radio to natively run on 13.8v. I imagine a suitable circuit could be installed in the 18650 battery case on the side of the zBitx case, rather than putting such a heat source inside the case.
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Personally, I plan to use the battery powered option, but I do understand the desire to operate from a standardized 13.8v power source...
