Inside some cheap lithium cell chargers. With schematic.

These units are available suspiciously cheaply from eBay and are designed to charge a wide range of lithium cells using a long spring loaded battery contact. The charge current is variable according to the cell voltage, with the current progressively reducing as the cell charges. The circuitry is basically a super-minimalist flyback switcher powering a generic lithium charger chip which seems to have the useful facility of detecting the cell voltage and swapping the charge contact polarity accordingly. Rather predictably, track isolation is minimal and the transformers internal separation is an unknown variable. I'd suggest not touching the contacts of these units while they're plugged in. If you enjoy the videos on this channel you can help support it with a dollar for coffee, cookies and non-compliant chargers at   / bigclive