get some raw usb a ports (could use extension cables with the plugs cut off and data pins capped off) and hook them all up to a 5 volt powersupply.
this ought to work as a powersupply for the whole rig. for the batteries, get 4 raw lifepo4 cells and a bms board.
I'm having good luck with a 12vdc internal Xbox one psu (had 3 laying around from dead consoles) and a 12-5v dc to DC converter (BINZET DC Converter Step Down Regulator 5V Regulated Power Supplies Transformer Converter (5V 10A 50W) https://www.amazon.com/dp/B00J3MHTYG/ref=cm_sw_r_sms_apa_glt_i_5HK95KPDF97B3HWK43ZG?_encoding=UTF8&psc=1)
Powering everything I need in a confined space. So far there is the atomic pi, 12v PC fan, USB hub, USB hard drive, and status Leads in a sealed enclosure
BINZET DC Converter Step Down Regulator 5V Regulated Power Supplies Transformer Converter (5V 10A 50W) https://www.amazon.com/dp/B00J3MHTYG/ref=cm_sw_r_cp_api_i_2mRyFbGHH8EF2
It mentions automotive in applications. I suppose below 8v minimum input during crank it is dropping below 5v but not low enough to consider a full power off.
I used a 12-to-24v to 5vdc regulator for a project where I needed both 5v and 12v available, that allowed me to use a less expensive PSU and get both voltages. Worked a treat. I think in my case I used a laptop-style 12v 3A PSU.
This regulator is 10A at 5vdc https://www.amazon.com/SINOLLC-Converter-Regulator-Regulated-Transformer/dp/B00J3MHTYG
For a wide range of PSU options, google Acopian PSU - they sell factory-direct worldwide, the PSU's are top notch quality, and they have every variety you could possibly imagine.
Here you go, under $10 on Amazon, don't get hot at 30W, and none have ever failed, so they're bulletproof to me!
https://smile.amazon.com/gp/product/B00J3MHTYG/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Make your own then...
Old charger
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https://www.amazon.com/BINZET-Converter-Regulator-Regulated-Transformer/dp/B00J3MHTYG/
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USB port
> Where did you happen to buy that converter brick? I'd like to give it a try.
From Amazon -- This item.