Diy buck converter using atx psu for parts

I have several high quality atx psu’s which are partly broken and want to salvage parts from them. Between all of them I have enough working parts to make some working units but I’d rather use the parts for something else. I have a whole bunch of 18650 batteries that I want to combine into one massive battery. But the issue is they are different brands and capacities. So the plan is to make an overly complicated battery discharger and charging circuit that can charge and discharge them all safely. I’ve organized the batteries by brand age data sheets and capacities into groups which can safely be mixed and ones which cannot. I plan on using an arduino to charge each battery with custom settings individually for safety and then using relays discharging each group one after another to get one big battery by mixing all these batteries in a safe manner. I know this is complicated and It would be easier to just buy new batteries and proper bms circuits but I’m doing it because I enjoy all the extra work. Anyway to do this I’m gonna need a whole bunch of voltage/current regulating circuits and current measuring circuits. I’ve got most of this sorted out already and have the sufficient battery chemistry knowledge and programming skills to do it I just don’t Know how to make buck converters. So that’s what this question is about. I don’t want to just buy them I want to make them from scrap parts without buying anything. I want to make it using switch mode voltage regulation. I can use an arduino to provide a variable pwm signal I just need help with the rest of the circuit. I can easily find some online but they all require specific components which I would have to buy and wait to be delivered as I have no where near by where I could order them. What components might I be able to find in an atx psu to do this and how would I connect them together? I’m not asking for an answers too specific any advice can help. I’ll be continuing my own research on the side while O wait for people to answer this question and trying to get a list of all the specific parts in each psu. Just don’t try to change my mind about not buying anything and doing this with what I have lol. It’ll be hard but I know it’s possible. Also if it’s helpful the specs are as follows. Input must be 12 or 5 volts. The input is very steady so don’t have to worry about that. Must be able to handle a current of at least a few amps the higher the better but I can always just make several of them and attach them together for more amperage. The output voltage should be variable but specifically I need about 4.2v pretty much all lithium batteries with a nominal voltage of 3.6v charge around 4.15-4.25 volts that’s why I need that voltage range as an output. Even just links to useful resources would be appreciated. I don’t mind a lot of reading :). Pretty much all of the buck converters and the battery circuit will hopefully be powered by one Arduino but if there isn’t enough pins for all that I do have multiple. Just makes the coding a bit more complicated and adds a bit of extra cost. I have one of the large arduino though with like 50 plus pins. I forget what it’s called. I also may just resort to raspberry pi’s if the coding ends up not fitting in the memory of the arduino a since I have a feeling it’ll be a very large program relatively speaking. Also I don’t mind straight up making parts from copper wire ect. Just nothin that involves advanced chemistry equipment and machines. I’m trying to save money lol so If i need a factory it’s off the table.

Touch here for the full post on the Arduino Apprentices tumblr

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