Voltmeter accuracy

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richardabeattie

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We are to measure state of charge by reading the voltage to the nearest 0.1V using digital voltmeters which usually sell for under £20. How do we know they are that accurate?

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One night next week arange to meet a load of your mates in the local pub. Ask them all to bring a meter with them and take a selection of small batteries. Then compare the readings from all the meters. In my experience they are remarkably acurate. Tell SWMBO that your are doing research. If any of your mates don't have a meter ditch them as they mustn't be boaty people and you should not be wasting your time drinking with them. IMHO. Paul

<hr width=100% size=1>there is nothing-absolutely nothing-half so much worth doing as simply messing about in boats.
 
You could easily loose 0.1 v in the test leads never mind the meter. For accurate measuring we use 4 wire connections on datron type equipment. Bit extreme that though.

To be fair, I took my cheapy meter in to work and gave it a NAMAS calibration (did a boring stint in the calibrations standards lab at work, one down from national physics), bit naughty as it would be classed as a foreigner, ah well. Anyway, it actually performed remarkably well, falling away only at the extremes. AC voltage was poorer than more expensive units, but the DC up to 200v was surprisingly good.

My meter also gave better resistance results than the £400.00 jobs we use in work.

I would be inclined to trust it, surely for state of charge you are looking for relative readings anyway, off charge, on charge, settle etc.

What is it you are trying to measure exactly

<hr width=100% size=1><font color=red>Woof</font color=red>
<A target="_blank" HREF=http://www.topcatsail.co.uk/P.html>Posting pictures tutor</A>
 
Which all assumes that if the voltmeter is to be used for measuring state of charge then, if the meter's accuracy is to be of any value, the State of Charge versus Open Circuit Voltage curve for the battery has to be known just as accurately for the particular battery being monitored. That will not be known and will, in any case, vary through the life of the battery.

I have noticed that some books show a nice linear curve for state of charge vs open cct volts, but as far as I am aware from actual battery curves that is incorrect. Even if it was correct then one would still have to accurately know both the fully charged voltage and the fully discharged voltage of accuracy of circa 0.1 v if one was to be able to draw the line accurately enough to make a voltmeter of similar accuracy of any practical sense.

So I tend to look on the advice of many that one needs an accurate voltmeter to establish state of charge as being questionable. It would seem to me that at best, State of Charge can normally only be estimated quite inaccurately using a voltmenter no matter how accurate the voltmeter is.

So, after that, in my view the accuracy of the digital voltmeter hardly matters and is likely to be amply accurate enough to produce the inaccurate results one is applying it to determining /forums/images/icons/smile.gif.

John

<hr width=100% size=1>I am the cat but I am only 6.
 
Cheap digital multimeters are quite accurate with one caveat. If the battery goes flat they can read too high before the display goes dim. That is because the voltage reference used internally falls below what it should be. So if you get queer high readings check the 9V battery. will

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Equipment likely to be more accurate than user consistency.

Internal battery voltage the most critical - worth using alkaline 9v.

In fact it's the difference between non-load and charging batteries that is more significant than the actual voltage in assessing batteries state of charge so, providing the inaccuracy is consistent, accuracy is irrelevant.

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Check the specs!

The real answer is that very few DMMs are spec'd to be accurate to 0.1V in the 12V range. A typical hobbyist meter might be 1%+- 1 digit - or up to 0.2V off at 12V. I sometimes calibrate mine against a precision voltage reference (5.000V)

However in most cases reproducibility/consistency is more important than absolute accuracy.

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Add surface charege problems to accurate indication of battery charge by voltmeter and you an see why I use a hydrometer. This means I must use wet lead acids, and I can see good reaons why certain classes of boater would not want acid flying about, but they are cheap and easier to test.

Tony brooks

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Thanks for clearing that up John ... what I wasn't sure about I now know I don't know the answer even when the answer's in front of me.

/forums/images/icons/smile.gif Jeff.

<hr width=100% size=1><A target="_blank" HREF=http://users.swing.be/FDB/centurion/index2.html>Centurion 32 Web site</A>
 
Richard is an economist - he will make sense of anything /forums/images/icons/smile.gif.

John

<hr width=100% size=1>I am the cat but I am only 6.
 
My two ST60 Multis show 0.5V plus and 0.2V plus -at least in a consistent way.
Raymarine customer support said there is no way for a user calibration.
Pity

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Assuming that the same meter is being used on the same battery, the reading is a relative difference reading and not absolute. In that instance assuming that the circuitry is stable the variance from reading to reading will be about 1% of 0.1 volt, or even 0.2 volts is the meter is a really cheapo affair... good enough however to tell you the state of the battery I would have thought

<hr width=100% size=1>Def: Yachting - a way of spending the kids inheritance
 
Shipswoofy said:
"You could easily loose 0.1 v in the test leads never mind the meter. "

Forgive me, but isn't that cobblers? Volt drop needs current and there isn't any worth speaking of. V=IR and all that.

Southy

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Makes a resistive shunt ammeter a waste of time, after all full deflection is only normally 0.075 volt, a dirty connection and you have no reading.
Also makes the makes the battery status meters a waste of time as they use resistive shunts and battery voltage to work out usage and available capacity.


Brian

<hr width=100% size=1><A target="_blank" HREF=http://link>www.kddpowercentre.com</A>
 
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