Zener Diode identification - Ultimate Speed Battery Charger
I found out that one of the SMD Zener diodes is shorted
...
Yellow SMD Zener diode is dead

Actually, no, it isn't a dead Zener diode. See below for details.

Only one letter &quot;D&quot; as a description.

Here is the relevant part of the photo you supplied:

That part you have indicated with the yellow arrow (silkscreen designator &quot;DZ2&quot;) is a &quot;zero-ohm jumper&quot;. FYI that's a &quot;0&quot; marked on the component.
So either:

The manufacturer wrongly installed a zero-ohm jumper, in place of the correct component which should be in position DZ2.

Or:

The designer decided that the component (likely Zener diode, yes) intended to be originally fitted as DZ2 is no longer required (due to cost saving, or changes to other components, or any other valid reason) and so it has been deliberately replaced with a zero-ohm jumper.
I suspect that this situation applies, since Zener diode DZ3 also seems to be in series with the photodiode in the opto-coupler. It would be reasonable to only need one of them.


You will need to reverse-engineer the circuit some more, to figure out which of those two cases applies, and to find the real fault.
But the summary is: The component fitted in the position for DZ2 is supposed to be shorted. So that is why you measure it as shorted.