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I just got my first multimeter and when I measured a 12V 1.2ah lead-acid battery I get AC 28.8V and DC 13.4V. Is this normal?

In addition I tested an AA 1.5V battery and I got 2.9V (AC setting 200) and 1.58V (DC setting 20).

Is my multimeter broken?

The meter is a UniTrend UT33C and the lowest AC range is 200V.

LATE UPDATE I went to the store and tested another UT33C and it also displayed an AC voltage. I asked to try a different model (UT39A) and it correctly displayed 0VAC when I tested an AA battery, so I exchanged my multimeter for the other model. Thanks.

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1  
In AC mode, the meter should be A-coupled and when measuring DC voltages it should read zero. Why are you trying to measure DC in AC mode anyway? The DC readings look right to me. –  Armandas May 3 '12 at 12:21
    
Are you sure that you're using the correct scale? –  clabacchio May 3 '12 at 12:21
    
@Armandas Sorry I'm a novice in electronics, so I don't quite follow. Both the 12V and AA batteries are DC, so why do I get AC voltages if the multimeter is in AC mode? Should the readings be 0 VAC as you said? –  joshu May 3 '12 at 12:24
    
@clabacchio Yes, I've repeated the measurements several times. In AC mode the lowest range I can set is 200V and in DC mode I set it at 20V. –  joshu May 3 '12 at 12:25
2  
I bet there is a 'm' or even a 'u' near the AC voltage unit :) –  clabacchio May 3 '12 at 12:46

4 Answers 4

up vote 6 down vote accepted

Situation: AC meter range reads double DC value when AC range is used for DC:

I have a meter of the same brand and very similar model (UT33B as opposed toU33C) and my meter behaves exactly as reported. Details at end.

At one time it was extremely common for AC meter ranges on analog meters to read about double the correct value when DC was applied.
What you are seeing is almost certainly somebody using the old style circuit.

Battery voltages vary with state of charge and during charging and discharging. Nominal values may be quite different from actual value. eg

  • 12V Lead acid - about 10V very flat to 13.7 V under normal charge and over 14V in some situations.

  • 3.6V Lithium Ion - under 3V fully discharged and about 4.2V fully charged (per cell)

  • 1.2V NimH - About 1.45V on charge when fully charged, 1.35 V just after charging., 1.3V fully charged after a while, 1.0 - 1.3 during discharge, under 1V sometimes when fully discharged.

  • NiCd - similar to NimH

  • AA Alkaline - 1.60 to 1.65V very new. 0.9 - 1.5V when discharging.

  • AA carbon zinc or heavy duty - 1.5V new. 0.9 - 1.5 discharging.


Update - reported results independently confirmed:

I recognised the meter from the picture :-).
I bought a number of those a while ago to allow multiple simultaneous sanity check measurements on some equipment. I just now checked the AC/DC behaviour with DC in and, as expected, results are as reported. As noted above, this is consistent with what I have seen in meters in the distant past.

Applying a few semi random voltages to 200 VDC and 200 VAC ranges I get

      2.9   5.9
      6.4  13.5
     11.9  25.6
     38.1  83.4

On 500 VDC / 500 VAC ranges the last voltage gave

     38  83

Slightly more than 2:1 and increasing slightly as Vin rises.
I'll try to get around to tracing he circuit sometime soon.
(3:30am now and morning appointment so ...)

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My question was updated with the multimeter brand and model. It's a digital multimeter. –  joshu May 3 '12 at 13:20
    
Thanks Russell for taking the time to test your multimeter. Is it worth exchanging my meter for a slightly more higher end such as UT39A? –  joshu May 3 '12 at 16:42

The DC readings are perfectly normal. A 12V battery isn't a 12.00V battery.

The AC readings are odd. A multimeter will usually measure either the peak value, or, more often the average of the rectified signal, and assuming it's a sine apply a correction factor to get the RMS value. For instance for a sine the RMS value is 1.11 times the average. That means that a multimeter can only correctly measure sine shaped signals, unless it's a true RMS meter, though they are usually more expensive. In both cases the reading is way too high to explain it this way; if the 1.58V were interpreted as average it should show an RMS value of 1.75V. Even the ratio AC/DC is different for both.
Anyway, expect to get strange results if you use the meter inappropriately. Use the DC setting to measure DC, use AC setting to measure AC.

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Fair enough I understand that to measure DC you set the meter in DC mode and the same for AC. Nonetheless, from the other comments and answers as my multimeter doesn't show 0VAC I should exchange it. –  joshu May 3 '12 at 12:59
    
@yonatan - Yes, the 0V is what Olin says. But that would imply that the AC range uses a series capacitor, and I'm not sure all DMMs have that. If there's no capacitor it would give a (wrong) reading based on a DC signal which it would interpret as the average of an AC signal. –  stevenvh May 3 '12 at 13:04

So called "12V" lead-acid batteries do not hold a constant voltage. They are usually charged at a fixed voltage of 13.6. If you measure your car battery with the engine running you should get something around that.

Otherwise the battery voltage will vary depending on how full it is charged, what current is going into and out of the battery, and the temperature. 13.4 V is within the plausible limits.

Put a 1 A load on the battery and see what the voltage is. In fact it might be instructive to fully charge the battery and watch the voltage over time as it discharges. Put around a 1 A load on it and then take a reading every minute or so and plot the result. You'll likely learn something about batteries in the process. Allowing a lead-acid battery to deep discharge can damage it. Below 8 V is generally considered bad, but there is no reason to push it just for this little experiment. End the experiment when the battery level reaches 10 V. Most of the energy has already been drained by then, and that's a nice safe level that won't hurt the battery.

For extra credit, compute the current at each reading, then integrate that to get the total energy supplied by the battery. Also compare the Amp-hours you actually get versus the claimed value, then explain why your measurement is considerably less.

As for the strange AC measurement, does this persist or is it a one-time reading? I would expect even a cheap meter to settle to 0 V after a few seconds when set to AC but given a DC voltage. The initial change from the leads open to connected to the battery is a AC signal, so a strange reading for a short time is normal. I see others have already elaborated on RMS and average measurements, so I won't get into that here.

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The AC measurement persists. It does not settle to 0V regardless of how long I keep the probes on the battery. –  joshu May 3 '12 at 13:03
    
@yonatan: That is surprising. Even the really cheap meters I have seen AC couple the input in AC mode. What happens when you give it a known sine wave with 0 DC offset? I don't know what shortcut your meter is taking to pretend to make AC measurements, but that's the one case it really should produce the correct reading. If it doesn't, then your meter is broken. –  Olin Lathrop May 3 '12 at 13:28
    
hmm how would I "give it a known sine wave with 0 DC offset"? Do I need an oscilloscope because all I have in my little toolkit at the moment is my "trustworthy" multimeter ;) –  joshu May 3 '12 at 13:34

13.4V DC is within the normal float voltage range on a 12V DC lead-acid pack. 1.58V is also within a reasonable tolerance on a 1.5V cell.

As for the AC readings, I suspect your multimeter isn't performing a true RMS measurement. A higher-quality meter would likely give you more accurate results, which in this case should be 0V AC for both batteries.

To confirm whether your meter is actually broken or not, you could put it on AC and (very carefully) measure the voltage at a wall socket. Make sure your meter is rated for the wall socket voltage! Depending on where you are in the world, that voltage should be either 115 or 230 +- 20V. In the US, the voltage should be 110VAC; check this map if you live elsewhere: https://secure.wikimedia.org/wikipedia/en/wiki/File:Weltkarte_der_Netzspannungen_und_Netzfrequenzen.svg

If the meter still gives you strange readings, a broken meter is a possibility. Also, check the battery in the multimeter. Sometimes they can give strange results with low batteries.

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