Edit: so I just looked up the LiFePO4 batteries and I'm already sold. I just want to find small enough (and reliable) modules for this project now

Edit 2: something in aware of is over discharging Li-ion batteries. So, the charging circuit board I'm seeking, would have that kind of protection built in, correct?

Edit 3: looking up buck converters, would this be acceptable? What about this for a charging board? Would this board charge LiFePO4 batteries safely?

Hey, I really appreciate you taking your time to help a guy out. Your knowledge, understanding of my lack of knowledge without being a pompous ass towards me and the solid advice you've afforded me is so greatly appreciated. Thank you so much.

So for starters I know I can use rechargable NiMh batteries and whatnot but I want the experience of modifying my controllers, which are meant to only take AA batteries and want to convert to Li-ion with USB C charging, I'm not concerned with fast charging either as that will over complicate things for me since I'm just getting started. I want to get comfortable with this first and return later on when I'm more knowledgeable.

I truly do appreciate you sharing your knowledge and explaining in layman's terms about how the steel Li-ion batteries are numbered and that the soft packs probably aren't the best use in my case. I would've been pissed buying the soft ones and it being for naught years or even months later.

I watch a lot of sobering videos, I've never heard of LiFePO4 before, I am intrigued so I will definitely be looking into these. I will say that I really like the idea of getting rechargable AA sized steel clad, cause I can very easily integrate a charging circuit and USB Type-C plug into my controllers and the safety and reliability is a huge plus. Plus, I can dispose of them without having to disassemble my gamepad when they're dead and unable to take a charge. Can you tell me if the LiFePO4 batteries have the save numbering convention as the Li-ion steel clad batteries?

Since I'm most likely going the 14500 route, can you advise me the correct charging circuit and buck converter that is tiny in size (approx 53mm Long x 40mm Wide x 20mm Tall)? Since space is rather limited in my controllers, I would prefer a charging module that doesn't have a USB port on it to begin with so I can use the tiny USB Type-C modules I already have and the space is rather limited in the 4x controllers I am wanting to modify. There are other projects I'm wanting to implement a charging circuit into but there's controllers and my Wii sensor bar are the perfect projects to test the waters and get my feet wet.