I just got this assembled last weekend.
printables, complete with F3D file.
This is a battery adapter that will allow you to insert D cell batteries into a LiitoKala Engineer Lii-500. I included the Fusion 360 file, so you can customize it to fit other applications, but odds are it will fit most 4+ 18650 based chargers that are also compatible with Ni-MH.
It prints in two halves that I simply hot glued together around their perimeter. It's not like this part is going to see very high loads… The parts are designed to print without supports. My copies are 20% infill (you could easily go with less) and 3 walls for an overall wall thickness of 1.16. Wall thickness is something I wouldn't change, at least for the battery side so the battery terminals don't eventual fail. The batteries are a very slight interference fit to ensure a robust electrical connection.
For terminals, I used 8mm wide nickle strip. I'm sure you could use something else, but I had some on hand and it seemed like an obvious application. Connections between the two halves were made via wires and solder.
You really don't know the risks unless you open it and minimally reverse engineer it. The charger shouldn't be very complicated and it's not directly connected to mains so that is nice. Charging chips are super simple and there are not too many ways to screw those up, unless they do.
If anything at all is shady, it'll be the power brick or the way the USB plug is powered. If the brick is crap and somehow starts passing mains, I am willing to bet that the USB plug will pass the bulk of the current through you or whatever is connected to it. It's something to check, anyway.
Reverse engineering to determine safety is what UL does. If anything here, the charger needs to account for fucked batteries with fail-safes but as long as the batteries are fine, it shouldn't be a problem
(I can't disagree with you, cause you ain't wrong. ;) I do probably need to clarify my point though.)
That is exactly my point about the device not being UL rated. More than once, I have needed to add or replace poor ground connections to the chassis of some device, when applicable. If there is a failure point, it's usually where there were cost savings is involved and generally not with the charging circuit itself.
Most battery charging ICs have decent fail-safes for bad batteries. It's just economical to use the same, or similar, generic IC across hundreds of products. (The TP4056 (and clones) is a decent example of wide adoption, but it's not quite a 1:1 with this particular application. It has good trickle charging and a temperature safety, but not battery chemistry logic, that I am aware of.)
Again, it's just something to look for when inspecting rando devices. To your point, cloned charge regulators may have deleted safeties, so that is a thing.