# How to reduce voltage for step up converter input?

I'm planning to do something you will probably advise me not to - diy a sub $50 lifepo4 charger. So far (I think) I've got it figured out - I'll use boost converters in parallel, which should work if they have current limiting capabilities. The ones I got were advertised as 600W modules, but I see they have a 10A fuse on the input, so that holds true only when the input is 60V.. I think. My battery is 36V nominal, so to get the maximum out of my boost converters I need to supply their inputs with a voltage as close as possible to the lowest charging voltage. I plan to set my BMS to cut off the load when a cell reaches 2.8V, so lowest possible battery voltage will be (12x2.8) 33.6V. The charger will need to be able to provide 40A of charging current, or about 1500W. In order to do it with the "600W" boost modules, I figured I'd need 4. If each can take 10A with input voltage around 30V, then that'd be close enough. The modules cost$10 each, so in order to meet my sub \$50 criteria, I need an ultra cheap power supply solution. I can get used PC PSUs for pretty much next to nothing locally and after some digging around I believe they can be modified slightly and connected in series to produce higher dc voltage.

Finally, on to the question. If I connect 3 PSU's 12V rails in series I'll get 36V. The problem is that this is greater than the theoretical lowest battery voltage of 33.6V.

Would it be possible to use forward biased diodes in series to reduce the voltage?

I have a bunch of IRLZ44N mosfets laying around from old projects and what not, which the datasheet says have a diode voltage drop of 1.3V. Is it possible to use their internal diodes, as in would they be able to handle 20A, with heatsinks of course, and possibly active cooling.

simulate this circuit – Schematic created using CircuitLab

I could use the third PSU's 5V rail to make (12+12+5) 29V, but at 20A 3 volts are 60W, so I'll take that if I can. I'm not at all concerned about efficiency only about supplying the boost converter with as much power as possible.

Also, are there any better methods of reducing the voltage in this setup?

• Sounds like a nice way to set your house on fire Dec 21, 2023 at 0:12