Higher voltage = Lower amperage.
If anything, a high voltage situation would lower actual amps across the fuse, as I recall most of the ATC type fuses are rated at 14.4V. Only thing that might happen as you pointed out, is a high resistance short that needs the extra volts to occur. But the starter-generator has a max capacity of only 20A at 14.4V at full speed...at idle, it might be able to only push 2A at charging voltage. Any draw over that would be coming off the battery, and at battery voltage.
Since the lights are most likely 55w, they draw a total of 9.16A running at 12V. At 15V, they draw 7.33A.
I recommend 15A fuses minimum, as the wiring you have going to the lights should support at least 200W at battery voltage. If you choose to rewire, I would recommend (at minimum) a wire size 14AWG with a 15A fuse. I personally use only 12AWG on my cart, and run 20A fuses.
2002 Gas Club Car
6" BU spindle lift
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