Design considerations
The arduino analogue inputs will measure any voltage between 0v and 5v dividing the result into 1024 steps.
currents
The TI chip takes the shunt input of 50mV maximum and multiplies it by 100 to make 5volts.
So the Alternator shunt using a 200A murata shunt and a uni-direction chip, should measure 200/1024 = 0.2 Amps per step, so we can measure the Alternator to 200mA resolution.
The battery shunt is using a 300A shunt but a bi-direction chip. So the 300A is divided into 512 steps in each direction. Which is 580mA resolution.
voltages
Battery voltages are in the range of 9 volts for a nearly dead battery to 15.3 if running an equalisation charge. But nearly all charging voltages are quoted to at least one decimal place , such as 14.2 to 14.8 So taking an accurate measurement to AT LEAST that precision is important.
So if we were to simply assume a maximum voltage of 16v and step that down to 5v through a simple divider, we would get 16 /1024 which is 15.4mV per step ~ around 6 steps per 100mV – not very good. But what if we half the range and shift it, so that the 5 volt arduino input is spread across the 8 volt range of 8-16 volts. Then we get 8/1024 which is 8mV per step or 12 steps per 100mV. We can do this with a Schottky diode.
Interface unit
All external wires and shunts connect to the Interface unit, nothing connects directly to the Display unit. The Interface unit contains the electronics for voltage sensing, the electronics for current sensing is in each shunt, so the current sensors just pass straight through the Interface unit.
The arduino module takes a 5v supply, but can be plugged into anything up to 12 volts safely. But a narrowboat produces a very noisy supply anywhere between 9 and 15 volts, so we cannot use that directly.
One key function of the Interface unit is to be the 0 volt reference point. A critical rule in precision analogue design is to only have ONE Zero volt reference. Which means that power for the Arduino and the display must comes from an isolated power chip in the interface unit.