Originally Posted by ciaranp
if i put a multimeter on it what should the voltage be 13.8 or 12v?
A 13.8 V regulated supply should be putting out 13.8 V. Allowing a reasonable error for both the multimeter and the power supply that could maybe show as high as 14 V.
12.0 V would essentially be a flat battery
12.7 V would be a full charged battery.
13.8 V is probably what your electrics sit at when the engine is running with a fully charged battery (keeping them 'topped up')
14.4 V is when batteries start to get a bit unhappy.
So electrics designed for a 12 V system actually need to be able to cope with something like the 12-14.4 V range of input power, and most will do wider than that, e.g. The spec for the A65
says is requires a supply voltage of: 10.7 - 18.0 V d.c.
I can't see any reason why a supposedly regulated 13.8 V power supply should be delivering > 18V unless it is faulty / badly made. Your plotter will be drawing something like 0.5 Amps - which seems like the sort of load (neither low nor high) a device clearly intended for running car type electrics should be able to support.