After going camping years ago and having a dead car battery when we tried to leave because the family used the car to charge phones, etc., I started a project to power the camping trips on solar power. This was many years ago before all the battery pacs and solar systems hit the market. Got all the devices covered with a 12V solar panel/battery system I designed except for the inflating/deflating of the air mattresses. The ones we had used 4 D batteries and consumed a significant current and seemed to be designed to work with the voltage sag a D battery has under heavy load. I found that you can buy step down converters on sites like ebay.com and aliexpress.com by searching for terms like “DC/DC Power Converter Regulator Module 12V/24V to 6V 5A Step Down Buck Adapter”. My 2 concerns were that the converters might running at or over their rated current, and they will not sag like a D battery will. So I worked out a circuit that allows two converters to work in parallel and has a sag similar to a D battery.
R1, R2 serve 3 functions: help to balance the load between the converters, add some sag similar to a D cell, and you can put a meter across them and monitor the current sharing. D1, D2 may not be needed, but they are there to make sure no current back feeds a converter.
I got my converters at aliexpress.com and their product title was:
12V 24V to 5V 6V 10A DC DC Converter 8-36 Volt to 5 Volt 6 Volt 10 Amp Step Down Module Car Power Supply RCNUN CE RoHS
Below is the prototype I built. You can see the converters mounted back to back on a piece of scrap aluminum I had. The diode tabs are bolted to the metal to remove the heat they generate and the tabs have power so the sheet metal must not touch ground. This “temporary” prototype has been working well for many years. Maybe someday I will find the time to come up with a better case for it.
4 D Batteries Emulator Prototype