What is going on in a mini inverter?

[sternwake]

Getting from 12.x vDC to lesser DC voltages requires a buck converter, also called a step down converter.

I make extensive use of these, for speed controlling 12vdc fans, and for dimming 12vdc LED lights, and most recently, to charge a single 1.2v Nimh Eneloop battery within some beard clippers. I found setting it to 2.36v yields about 0.15 charge rate into the when new 1900 mah battery. A bit slow but I dont need to monitor it closely for fear of overcharge.

For 5 amps or less of fans/leds, I like the XL4005 based buck converters.

https://www.amazon.com/XL4005-Adjustable-Module-Supply-Converter/dp/B09SWLFH3J/ref=dp_fod_1?pd_rd_w=HpoVZ&content-id=amzn1.sym.68174014-ed7a-4e35-badd-6d3576b85c0b&pf_rd_p=68174014-ed7a-4e35-badd-6d3576b85c0b&pf_rd_r=19WC22EPYJCQ8ZX6BSVV&pd_rd_wg=zWaMB&pd_rd_r=d15bebb7-1cb0-4646-8508-153d25f53b4f&pd_rd_i=B09SWLFH3J&psc=1

Here is a 300 watt version. it has two adjustment potentiometers, the second one controls maximum amperage allowed.

https://www.amazon.com/Anmbest-Converter-Adjustable-Regulator-Protection/dp/B07R832BRX/ref=sr_1_13?crid=32D1LQXQZPP42&keywords=dc+to+dc+buck+converter&qid=1678586835&sprefix=dc+to+dc+buck+converter%2Caps%2C135&sr=8-13

I have a 150 watt version I've yet to employ full time.
Note that to maintain these max output wattage ratings these should have 50 or 60mm fans attached to their heatsinks.

i also have 150 watt step up boost converter. input 12.0v dc, output anything higher upto about 33vdc. got one of these slow boat from China at least 8 years ago. I use it fairly regularly. I gutted an old ventilated inline power brick from I cant remember what, installed it inside with a 60mm fan, and have modified it with a external potentiometer, and a 2 decimal voltmeter capable of handling 100 volts. I use it for battery to battery charging, but also to charge some power tool batteries that are 18 or 24v. Anderson Powerpoles for input and output, but also a 4 pin computer fan connector too. It has greatly exceeded 150 watts unintentionally.

In my weaker moments i wish I had a 300 watt buck boost converter inside the casing with fans and voltmeters and ammeters and dual fingertwist dials for voltage and amperage adjustments, but I really don't have an immediate need for such.

I have several 24vDC fans. For these, I speed control them with a 10 amp combo buck/boost converter. My papst fan as a range of 7.44 to 30 volts and I use the amperage potentiometer to limit voltage to about 30 and retasked a small blue pot to keep minimum voltage above 7.33 for the one fan and ~9v for the other. The fans can handle more but they dont really spin faster, just consume more significantly more amperage . There are not a lot of these combo buck boost converters available. I've had no luck with the smaller ones rated for 5 amps. Smoked them all. no issues with the unit shown here:

https://www.amazon.com/XINGYHENG-High-power-Converter-Adjustable-Protection/dp/B08LZ2RWC1/ref=dp_prsubs_2?pd_rd_w=Dfhdn&content-id=amzn1.sym.c283371f-7974-465c-b809-831433e1ef42&pf_rd_p=c283371f-7974-465c-b809-831433e1ef42&pf_rd_r=H0Z27X27XFMF7RD311NE&pd_rd_wg=ZbgcT&pd_rd_r=4bcaf56f-f4dc-454f-8148-a939232f5fad&pd_rd_i=B08LZ2RWC1&psc=1

I've got one of these^ powering two 24v fans, and one powering my clamp papst fan, and one backup waiting to be employed.

Here is the other 8/10 amp buck boost converter I own. It works and has been reliable, but is bulkier and.

https://www.amazon.com/Converter-1-25-30V-Regulator-Constant-Charging/dp/B08HS76XG1/ref=pd_bxgy_img_sccl_1/138-4661372-1575558?pd_rd_w=ifIFy&content-id=amzn1.sym.6953b182-f957-43e6-9fe3-866e180e1e05&pf_rd_p=6953b182-f957-43e6-9fe3-866e180e1e05&pf_rd_r=JGGR4ZVA45TR347FSRQ6&pd_rd_wg=cYG1V&pd_rd_r=65a1fae5-041c-4669-aec5-81711b5990a3&pd_rd_i=B08HS76XG1&psc=1

I've not found any higher amperage/wattage buck boost converters. many product descriptions say they do both buck and noost, but then they are either just buck or boost, not both. I've even been sent one which was buck only and the seller said keep it then sent me the combo unit with an apology, they look the same but the buck only had just one potentiometer..

Removing the little blue potentiometer to solder in wires to circuit board for finger twist voltage adjustment, takes some practice and soldering skills but is not really difficult. I think I elaborated on this process somewhere here, a few tricks to make it go simpler.

I avoid inverter use, whenever possible.

This seems to be a new offering, 20 amp buck boost step up step down converter. I suspect is buck or boost but not both, despite the description.
https://www.amazon.com/Dc-Dc-Converter-Adjustable-Charger-Module/dp/B07RBKVDHW/ref=sr_1_20?crid=257D3T2P9JX38&keywords=buck+boost+step+up+step+down+converter&qid=1678589443&s=industrial&sprefix=buck+boost+step+up+step+down+converter%2Cindustrial%2C113&sr=1-20