@mads0100, I think there are step-down devices, but they are more for 12v-->5v... Not sure they can limit voltage for something like 13.8-->12.5. But that's outside my area! My power supply was/is variable, so I just racheted it down and not too much later, ended up with an STT8300. The STT seems to be quite happy with the higher voltage.
My failures started when I stopped dragging marine batteries to the field and switched to a power supply. I'm embarrassed to admit it took me a couple of night to make the connection, but once I realized, I tested with the variable supply. The voltage where I started to see shutter problems was around 12.7 - 12.8.
Best,
Craig