Originally Posted by Daubsy
Interesting. So a shunt is not needed for incoming amps from the PV? Does "cumulative amp hours" refer to battery capacity?
Thinking out loud - as opposed to knowing all the facts - I would tend to agree with others that are saying a shunt is not needed for charging amps, and that "cumulative amp hours" is only measuring incoming amp hours.
My reasoning: The charge controller probably has no idea how big the batteries are, so it can't tell the user how many amp hours remain in the battery at any given time.
However, a number of popular battery monitors (Trimetric has been mentioned) are designed to measure both the incoming and outgoing amps and they will know the battery size because you tell it. Those will require a shunt in the location indicated in the diagram. Unfortunately you can't just go out and buy a shunt at random. It has to be matched to the device in question, so while the advice to buy a 500 amp shunt may be correct for one brand, it may not work correctly with another brand.
A bit of information for anyone who doesn't know how the shunt works: It is a well calibrated, very small resistance, mounted in a big chunk of metal, designed to handle hundreds of amps without causing more than a tiny voltage drop. This small voltage drop is what the monitor measures and converts the number to Amps using the equation Voltage = Current X Resistance.