Originally Posted by coopec43
But what about the USB(?) socket? Will that always be 5V if it was wired for 12V or 24V. (I'm a bit puzzled)
I think your question is a good one, especially given that the price
of such devices is low, the reputation (or even name and location) of the manufacturer is often unclear, and the specification/description of the internals often not supplied with the device.
The solution is obvious: a USB Type A socket is large enough so standard multimeter probes can enter and be put to the contacts.
Some other of the USB sockets require finer probes.
The pinout for USB A (and the other varieties) are public (see e.g. https://en.wikipedia.org/wiki/USB
So as a matter of course (or at least when in doubt) measure the voltage across pins 1 and 4. Your meter should read 5.0VDC plus or minus 5% (so 4.75V - 5.25V)
For just supplying 5VDC for charging
, pins 2 and 3 in a USB Type A socket are likely tied to ground (or not connected at all?).