Lithium Ion 18650 four cell series/three cell parallel charging

[AlienRelics]

I'm trying to build a large battery pack using a bunch of scavenged, repaired packs. They have a BMS in each, but that just balances the cells and protects them from over/undercharge. I'd like to put a charge controller separately on each battery, consisting of 18650 wired 3 parallel, 4 series at 16.8V max. Mostly what I keep finding when I search for BMS are not BMS, they are just over/undercharge protection circuits, not charge controllers. The charge controllers I find are multipurpose and too expensive.

I would like something that charges 16.8V so I can continue using the scavenged BMS system. I can step up the 13.4V from my van battery (through a switch), and then step down the 16.8V coming from the LiIon packs down to 13.4V again to run the things I'm running on it. Ham radio, camera system, that sort of things.

These came out of something that has a 16.8V charging system, relying entirely on the battery protection board in the batteries to balance and protect from over/undercharge. The charging system limits current and keeps it from continuously charging below the threshold current.

I suppose I could wire up a 16.8V regulated supply, then current limit it. I found some on Amazon but the advertised built-in current limit is meant only for catastrophic use, it shuts down the supply momentarily and fires it up again repeatedly, getting very hot. I'd have to add high side current sensing to each one. It'd be nicer if there were already something made for this that would perform all of the desired functions of a LiIon charger like the current reducing as it reaches full charge, and maintenance where it won't start charging again until it drops to a certain level.

My alternative is a more involved method of programming all this behavior into an AVR or something similar. Perhaps with a digital potentiometer, or use the DAC output in such a way that the AVR must enable charging current, so it defaults to no charge. I'd still need to add high-side current sensing.
https://batteryuniversity.com/article/bu-409-charging-lithium-ion

[thm_w]

"Mostly what I keep finding when I search for BMS are not BMS, they are just over/undercharge protection circuits, not charge controllers."

That is the common definition of a BMS, it is not a charge controller.
For your own safety I would recommend buying a LifePO4 12V battery, prebuilt with BMS and balancing circuitry inside. 50Ah cost about $160 or so.

You can get current limited modules for charging batteries. Search charging module or similar.

[AlienRelics]

Charging module are the secret words, apparently. With that info, I found a 16.8V 5A lithium charging module that accepts 8 to 36V and outputs 16.8V, and manages the lithium charge.
https://www.aliexpress.us/item/2251832647004249.html

Buying a brand new battery kind of defeats the purpose of recycling refurbished cells.

I see Alliexpress has a bunch of 16.8V 4S 2A chargers that accept USB-C and step it up. Not very expensive, then each 16.8V battery has its own charger, powered by a 12V to USB-C multiadapter.
https://www.aliexpress.us/item/3256804911713494.html

8 port USB charging station cigarette lighter plug.
https://www.aliexpress.us/item/3256805756081537.html

Each output from the batteries isolated from each other with zero-ohm rectifiers.
https://www.aliexpress.us/item/3256804155056051.html

Of course I may chase my tail for a while and end up deciding it is simpler to just buy that $160 battery and a charger and be done with it. Still need a charger.

I'm thinking of putting a bunch of these in the house for a powerwall. Keeping in mind I need to do all of these to be as safe as possible, and inside of some kind of fire resistant container.