High power voltage stabilizer for my 13.8V powered ham radio

I am looking for any resources on how to construct a relatively high power (25A, 13.8V) voltage stabilizer (or voltage booster if you like) for my 13.8V powered ham radio.

The problem I try to resolve is powering the radio from both only batteries, and from car battery. So I would like to be protected against both voltage drop especially in first scenario and voltage spikes in the second one.

There is still a plenty of charge in a AGM 12 battery when the voltage is around 12,.4 volts but the radio PSU cuts off when the voltage drops that low.

I know that there are ready to buy solutions, but they are not very accessible in my part of the world, and I would like to at least know the principle they operate on

Many thanks in advance!

UPDATE:

What about LM2587(datasheet:http://www.kynix.com/uploadfiles/pdf2286/LM2587S-12.pdf） in flyback regulation mode in parallel? Is it possible? I have only found a description of boost converter mode parallel application.

UPDATE2:

design a SEPIC since it works well for inputs both higher and lower than the desired target.
View attachment 1083

A couple options come to mind: What you're describing is a DC voltage regulator. Depending on the current draw, there are plenty of regulation circuits out there. That said; if you're running off batteries, ultimately there is no better way to regulate voltage. It's whatever is charging the batteries that the regulation is important. Cheap battery chargers will have poor regulation, both in the cleanliness of the DC (1/2 wave vs. full wave DC plus filtering), but the voltage and current regulation.

If it were me; I'd go with AGM batteries and a good quality marine charger/inverter.