Which monitor is lying to me?!

[JayGuevara]

Hi Guys,

Chinese diesel heater (display screen) wired direct to battery.
EPever BN series 40 amp
MT50 display monitor
3 month old Leoch AGM GEL 130ah battery (never dips below displayed 12.7v)
550 watts solar.

Why the discrepancy in the voltage displays, and which is correct?

Before you answer 'those diesel heaters are junk, it must e the heater display' - it's very efficient on diesel and cost £89 ($110 ish).
...and the temp display is on parity with my thermometer adjacent.

Thanks,
Jay

 

[snoobler]

All of them are "lying" to you. None of them are "correct." Maybe.

Something discharging will show a lower voltage.

Something charging will show a higher voltage.

A battery monitor that simply measures the open circuit voltage (or a simple multimeter) will give you the most accurate voltage. A calibrated instrument gives the most accurate reading.

You have not listed anything that suggests you have purchased a high quality calibrated voltage sensing device, though I would expect the charge controller's to be the most likely to be accurate due to the critical nature of charging voltage. I would not expect it to agree with a calibrated voltmeter except when it's not charging, and nothing is pulling current from the load ports (if equipped).

The only one that has the potential to be correct most of the time is the battery itself. If it is simply measuring its own voltage independent of load, it's likely correct. However, that again depends on the accuracy of its own meter.

The price nor the correlation of temperature in no way implies the heater's voltmeter is good. Just because your engine oil level is perfectly at the full mark, it doesn't mean you don't need to check your tire pressure too.

 

[JayGuevara]

My 'cheap' multimeter matched the mppt...

 

[snoobler]

How much current was flowing through the MPPT charge wires when you took your reading? Did you take the reading at the battery or at the MPPT terminals?