Which voltage display to trust

gicamuci

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

706jim

How different are the voltages? I'd say trust the FM80 values. There's a post on the Outback forum where I was asked to check voltages. A high quality analogue gauge showed the same values as the FM80 displayed.

BB.

Calibrator sources are not cheap $1,000+++ is not uncommon (random website):

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

Segal's law is an adage that states:
A man with a watch knows what time it is. A man with two watches is never sure.

-Bill 

RCinFLA

I built calibrator with Maxim MAX6126-2.048v and MAX6126-5.0v reference IC's.  They are about the best you can get at +/-0.02% initial accuracy.

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.

wellbuilt

 I would use the fm80 display mine was very close to battery voltage with my test meter .
 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 . 

   I tweek the absorb time and voltage setting to keep the sg up.
 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

wellbuilt

 O ya, just pick a meter that is closest to the fm80 🤷‍♂️

gicamuci

Thanks for all the ideas.

I finally decided to go with the SG and then tweak the voltage and absorb time, as wellbuilt suggested.

706jim

Reminds me of my indoor outdoor thermometer(s).
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!