Let's say I somehow got transported to medieval times. I have my phone, laptop or other devices with rechargeable batteries, but I forgot to take my solar chargers. Luckily, my every device is powered by standard micro USB cable, which I happen to have with me. I know that USB uses 5V direct current, from 0.5A to 2A.
To create current, I can use magnets and copper wires, which are luckily sold by local merchants (wires) and magicians (magnets). This will, however, give me alternating current. Using some diodes I could convert it, but semiconductors aren't readily available in my local medieval shop.
Next problem is, I need very specific and stable voltage. Definition of ampere won't help me unless I get two wires of infinite length.
So question is: How can I create a stable current source to charge my electronic devices without exploding the batteries?
This is related to question asked in I was thrown into the middle ages, how do I power my time machine? , but answers there don't seem to say much about voltages used. Phone batteries might need pretty specific voltage, and I'm mainly interested how I can either check the voltage of my source or build one having stable 5V (or some other that won't destroy my batteries).