Tom, I have a voltage buck converter of very similar appearance to the one Mr Wizard linked, but mine is rated for 10 amps, not 20 as in the link.
Your 2.0 volt differential claim, is simply absurd, and shows you have Zero experience with such a product. Perhaps long ago with older less efficient transistors a 2 volt drop could be expected, but certainly nothing like offered in Mr wizard's link.
My unit's output almost matched input voltage, to my great surprise, under a 0.65 amp load, when turned all the way up. I was trying to power a 24v fan and it would not go above input voltage but came very close. The fan draws draws about 0.65a at 12.8v iirc.
This unit I have, what I'd actually ordered from E bay was a 10 amp buck/boost module, and when I hooked it up I found they sent me a buck only converter, as output voltage would rise within a few hundredths of input voltage and would not go above, I was extremely surprised output voltage almost matched input, and changed swapped and used different voltmeters all with the same result.
The Ebay seller quickly refunded me, and I got to keep it.
I then ordered the buck/boost version from a different E bay seller, the difference in external appearance is the secondary trimpot, for current control, and 2 more transistors on the heatsinks, and one less capacitor. Its easy to see how the first seller sent out the wrong part as they look nearly identical at a casual glance.
The buck/boost version works great on a 24v Papst fan through its full speed range of 7.33 volts, where one can follow one fan blade spinning with their eye, upto 30 volts, where it is quite powerful and quiet for the amount of air it moves. I use the current control pot to limit current to ~2.42 amps, so then voltage cannot exceed 29.5 volts.
If I turn the current trimpot up higher, voltage can climb as high as 36 volts, so I limit max allowed voltage by the current trimpot. The voltage trimpots, I desolder and attach wires to the trimpots receptacles to a remote fingertwist potentiometer for simple speed control. This particular fan amp draw starts increasing nearly exponentially over 30 volts while the fan does not spin all that much faster so I actually limit it to 29.5 volts via the current trimpot.
I've ordered a second buck boost converter of the same design, but yet to employ it, but do have it with me while travelling on the other side of the country. I also have the 10 amp bucker and others too.
I employ many XL4015 based voltage buckers rated a 5 amps. These drop 0.29v across them under a 2.5 amp load, NOWHERE near your claimed 2.0 volts. I use them extensively in my rig to speed control powerful 12v computer fans and to also Dim LEDs. If they dropped 2.0v I would not employ ANY!!!
I previously used PWM motor speed controllers or LED dimmers, but unless 20KHz or higher, these would make the some LEDs Whine and maybe flicker when dimmed, and fans would also Whine at reduced speeds. I have almost entirely deleted them from my rig in favor of the XL4015 based buckers. Have 5 of them going right now and they have been going for weeks or months on end. These voltage buckers allow for much lower fan speeds, and dimmer LEDs, than PWM dimmers/motor speed controllers and there is NO whining from the LEDs or fans. The PWM controllers would shut fan or LEDs off at much higher speeds or brightness.
Previous to the XL4015 5 amp buckers, I employed LM2596 based voltage buckers rated at 3 amps, these drop 0.71v across them.... 12.0v in 11.29v out maximum. I employ just a few of these, for that 0.71v loss is inconsequential to the specific application. The XL4015s are so obviously superior I've replaced a few functioning lm2596's for the XL4015s for the higher voltage which can then reach fan or LED and greater efficiency and higher amperage rating.
I just got some new XL4015 based buckers of a slightly different circuit board layout, and these drop only 0.19v !!! Was suprised and pleased. 12.22v in 12.03v out with the 50k ohm voltage trimpot turned all the way up when powering a 1.5 amp fan.
These XL4015 based voltage buckers also have versions with current control trimpots in addition to the voltage trimpot. I've not had good luck with these, sometimes just stupidity and shorting them out while testing a powerful fan that would claw itself across the workbench, and others just failed for no apparent reason.
Using the voltage trimpot, worked better to control speed or brightness on leds and fans than did the current trimpot, and seemed to use less current when dimmed/slowed, so I only get xl4015 based buckers that have voltage trimpots only, like these:
https://www.amazon.com/Adjustable-Converter-1-25-36v-Efficiency-Regulator/dp/B079N9BFZC/ref=pd_rhf_se_s_pd_crcd_0_14?_encoding=UTF8&pd_rd_i=B079N9BFZC&pd_rd_r=6faf48bc-e4b7-4520-b4e4-ad1b5db6afd6&pd_rd_w=VzPdZ&pd_rd_wg=XU4s5&pf_rd_p=1b93de15-e9af-47f7-b562-4ce91b743ae9&pf_rd_r=Z7GBWE3E1TC0EKZKMZQR&psc=1&refRID=Z7GBWE3E1TC0EKZKMZQR Here is The buck/boost converter I employ on my 24v fan as a speed controller: The output voltage in my experience is indeed quite stable, until one's input voltage falls below 9.
https://www.amazon.com/Converter-Automatic-Regulator-Temperature-Protection/dp/B07QB74GM4/ref=sr_1_19?keywords=Buck+boost+converter&qid=1579280017&s=electronics&sr=1-19
There are also many 3 to 5 amp rated boost/buck converters available, slightly physically smaller than the link above. As far as i can tell they are all junk, go right for the 10 amp model even if you think 5 amps is more than enough. I've only one still functioning and perhaps a half dozen more which never worked or did not work for long powering a 2.5 amp potential load.
A 50mm computer fan would bridge the heatsinks perfectly of thebaove link, if one is going to run it near its rating for extended periods of time. I rarely exceed 1/4 of its rating and measured nothing on circuitboard over 92f ever in 80f ambients at 25% of its rating for 30 minutes.
Isn't actual data and experience great?
Earlier in the thread there was a 150 watt boost module linked. I have it, and have used it to recharge 19v and 24v powertool battery packs. both Lithium and NIcads it has exceeded that 150 watt rating by 25% for 15 minutes when one Nicad battery when into thermal runaway. got to to monitor those closely. I have it enclosed with a fan on it and anderson powerpoles leading in and out.
If I were wanting to do what Don wants, and which is very similar to what I am indeed doing with my 18Ah AGM, I'd use the Ideal diode I linked a few posts back, perhaps with an inline switch for when he knows the genny battery is chock full, and does not need to see any charging voltages via his solar or alternator.
I just unplug my 45 amp anderson powerpoles when my 18ah AGm is known to be chock full , and then reconnect them later if it sits for several days, or after I use any portion of its capacity as a portable 12v source. These Chinese AGMS self discharge much faster than does Lifeline or Northstar full size AGMS, in my experience.
