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So I wanted to make a hand-cranked battery charger for my power bank, and I figured that if I connect a generator to a lithium battery I had lying around, I could use the output of that lithium battery to charge the battery bank, that way the first battery would act as a voltage stabilizer for the bank. (I broke the last battery bank by trying to connect it directly, that's how I got the spare battery) I figured that 4.2 volts is close enough to 5 volts that I can plug it directly into USB in. Is this stupid?

I cannot afford a buck boost converter yet.

Schematic of my abomination

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    \$\begingroup\$ Your power bank will most likely not accept 4.2 V as input and won’t charge. \$\endgroup\$
    – winny
    Commented Oct 20, 2021 at 14:12
  • \$\begingroup\$ Re, "I broke the last battery bank..." Have you measured the open-circuit voltage when you crank your generator? I have one in my junk box--army surplus from a WWII era field telephone--that puts out around 90 volts. The switch-mode power supply on the input side of a "USB power bank" probably would not like that. \$\endgroup\$
    – Solomon Slow
    Commented Oct 20, 2021 at 14:44

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Yes that will not work as expected.

Batteries need regulated current (CC mode) then a fixed limit voltage (CV mode = est. 4.1 to 4.25) until the current decays enough to call it fully charged.

So using a LM317 voltage regulator or as a current limiter would work better or at least a power resistor and a DMM to monitor voltage and current.

As always, do not overcharge or undercharge.

Define your crank generator characteristics for a better question and answer. ( open circuit voltage (at nominal RPM) and short circuit current (brief test), show details if possible.

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