CC and remote Voltmeters different?

In general, make sure your batteries are "charged/full" for DVMs... Seen some strange behaviors with weak batteries (consistent, but wrong, readings--If I recall correctly, saw "added voltages" such as 4x 12 volt batteries in series did not add up individual battery voltages (I.e. 12 volts * 4 added up to something other than 48 volts, etc.).

You can find precision voltage sources at Amazon/etc...

Some of the higher end charge controllers have a programmable offset to "re-calibrate".

Is the remote getting its battery voltage via its own battery wiring, or is it communicating with the charge controller?

If there each has their own "sense" leads/connections... There is always the possibility of there is current flowing, that the voltage readings would be different because of voltage drop across wiring/connections. I.e., the voltage measured at the charge controller would be different vs battery bus (if there is "relatively" high current flow).

-Bill

https://en.wikipedia.org/wiki/Segal's_law

The saying was coined by the San Diego Union on September 20, 1930: "Confusion.—Retail jewelers assert that every man should carry two watches. But a man with one watch knows what time it is, and a man with two watches could never be sure." Later this was — mistakenly — attributed to Lee Segall of KIXL, then to be misquoted again by Arthur Bloch as "Segal's Law".[2]

As Dave says... Critical systems should have multiple sensors for critical systems... Typically 3 at least. Otherwise, for systems that are not critical, then a "sensor failure" should not cause catastrophic failures.

A recent example is the Boeing 737 Max that used angle of attack sensors to "manual override" pilot controls (push nose down if plan is close to stalling)--The major issues was the failure of one sensor (showing high angle of attack near stall conditions) would trigger the MCAS (override computer) to push the nose down even if the second sensor indicated normal flight conditions. And the pilots could not apply enough control pressure to "over power" the safety system (2 planes crashed).

The MCAS was created because less than engineering solutions to create a more fuel efficient version of the plan required larger diameter engines--Force the new engines to be mounted higher and further forward--Which can tend to push the nose up and create stall conditions. And any mention of the MCAS and how to override was removed from manuals. And as I recall, there was an optional "red MCAS failure light" (extra cost option for the aircraft).... All's of this was so pilots could fly old and new version of 737's without additional training (save crew training costs). (As I recall, the pilot could pull the circuit breaker for the MCAS and turn off the servo/controller override and allow pilots to regain control/cancel MCAS override of controls).

https://www.icheme.org/media/19013/lpb287_pg24.pdf

And, of course, Boeing never tested a sensor failure of what was know to be (relatively) failure prone AOA sensors (bird strikes, icing, maintenance issues, etc.).

-Bill