Which voltage display to trust

So I have an Outback FM80, a Tacklife CM05 Clamp Meter, a DROK Battery Monitor Voltage Amp Meter and some old (~ 1995) digital V,A,ohm meter.
The problem I have is that at any given time, if I measure the batteries voltage, they all display different values.
Which one should I trust ?
Buying another meter will just add to my unknown...
Thanks,
Alex
5.1kW, 12 × 250W + 4 x 275W + 3 x 330W (driving the pool pump MPPT controller or charging batteries on cloudy days) solar panels , FM80 MPPT CC, WavePower 6kW pure sine inverter, (16 x Trojan 105Plus - replaced) 16 x Dyno D90 in 8s2p strings 420Ah @ 48V battery bank.
Champion Dual Fuel 7.2KW gasoline/6.5KW propane gen for emergencies only.
Champion Dual Fuel 7.2KW gasoline/6.5KW propane gen for emergencies only.
Comments
https://www.transcat.com/products/calibration-instruments/electrical-calibrators
Some simple DIY voltage source projects:
https://www.nutsvolts.com/magazine/article/build_a_01_accurate_voltage_reference
May be close enough for your needs--Plus you can use the "stable source" to check multiple meters for accuracy.
Example of a calibration servcies company price list ($50-$65 for simple DMMs):
https://www.custom-cal.com/TypeInfo.aspx?kn=156&srv=Multimeter_DMM_Calibration_Repair
You could just use your car battery (or battery bank without loads/charging for stable voltage) to compare meters and see how far appart they are. In general, digital meters seem to be "accurate" down to the last digit +/- a few samples on the last digit at room temperature (the little I have checked).
Note that with current flowing, the voltage on the Outback battery terminals will be different than the battery bank bus voltage (cable resistance and voltage drop).
Make sure you have good battery(ies) in your meters... Have seen a few DMMs with "dead batteries" act very squirrelly. It was interesting... Measuring the same voltage at different times, repeatable numbers--Just wrong. Check a higher or lower voltage bus, and again repeatable numbers, but wrong a different amount again).
Or buy one "good meter" (Fluke or other decent brand) for your calibrated unit. And check the other meters are close (or not).
https://en.wikipedia.org/wiki/Segal's_law
A man with a watch knows what time it is. A man with two watches is never sure.
If you want accuracy you should measure battery voltage with no current in or out. Lead acid need 15-30 mins of no load resting to reach equlibrium. LFP need 2-3 minutes of resting.
With varying load/charge current the batteries' voltage will be jumping around.
When monitoring voltage live you also have to be sure the meter sampling has filter to average out 120 Hz ripple voltage caused by sinewave inverter ripple current and battery/cabling impedance. Ripple current pk-pk level will be over twice the average DC current and greater if inverter load has poorer power factor. Most monitors have low pass filter below 2 Hz corner freq to average out the ripple current.
If you are looking for cheapest meter with pretty good accuracy I would recommend the ANENG AN870. Should be able to fine one less then $40. Next up is UNI-T UT61E at about $65, Forget the newer UT61E+, little additional benefit for more money. I have two of each for kick around meters. They all do better that 0.05% accuracy.
Then I recalibrating the settings in the mate to match as close as I could get it .
I set up the charging and then tested the sg over a few days and it was all working .
Summer time gets less voltage and longer absorb time
winter get more voltage with less absorb time, I don’t think the exact voltage is that important
as long as it’s consistent
Champion Dual Fuel 7.2KW gasoline/6.5KW propane gen for emergencies only.
Both read from the same external sender and the temperatures differ by about three degrees.
As Confucius said "Man with two watches never knows correct time".
So true!