Re: Dodgy multimeter? Battery testing

[andyoxon]

Last year I picked up a LIDL Parkside Multimeter PDM300 C3, for occasional home use. 
I've noticed that this YT vid shows the unit failing to detect high overload voltages "Dangerous Parkside multimeter".  https://www.youtube.com/watch?v=QxPIRtZb8Ck
What think the panel?  AFAIA there there are no 1000V sources inside UK homes...   Clearly, it's not a 'professional' device.  Stated AC range is up to 300V, with a '300V fuse'.

Someone else (video99 on YT) recommends the unit, but says not wise that it uses the same inputs to make current measurments as high impedance measurements...

No  I didn't look at reviews before* buying... 

[Diver300]

It’s not ideal, but voltages that high are hard to find in a house, especially DC voltages. I would far rather that meter than nothing and whatever meter you have, always test your meter before assuming something is dead. You set any meter to read DC volts and it would show mains AC voltage as zero, or just not make connection with the probes, so checking that some reading can be made is important.

The same connections for current and voltage is also not the best. It is worth finding out if the meter can still read voltage with a blown current fuse.

[Feanor]

For a basic domestic multi-meter, It'll be fine.
You are not in an professional electronics maintenance lab, working on >1Kv kit.

I have several high-end Fluke meters, and I'd not want to subject any of them to extreme overload conditions either!

At my former workplace, we had special High-Voltage probes to work with the HV PSUs which were used to drive radiation detectors at voltages between 1000 - 1500v.
These were divide-by-1000 voltage probes, and were pretty expensive.
There were only 2 in the maintenance lab, and we shared them.

But the advice to check a meter is working before confirming a circuit is dead is good.

[andyoxon]

Thanks both.    Today I learnt that you can't accurately check voltage status of a Li-ion battery with a multimeter, because of the battery's 'internal resistance' characteristics.

[Diver300]

Thanks both.    Today I learnt that you can't accurately check voltage status of a Li-ion battery with a multimeter, because of the battery's 'internal resistance' characteristics.

That, IMVHO, is bollocks. The internal resistance of a Li-Ion battery is more than a million times smaller than the resistance of a multimeter* so the 1 ppm error will pale into insignificance compared to other error, such as the accuracy of the multimeter.

If there is any current being taken by other loads or current being supplied by a charger, then the internal resistance of the battery will make a difference. Also the accuracy of the multimeter can be important.

Finally, state-of-charge can’t be accurately determined from the no-load voltage as battery condition and temperature affect things as well.

*Unless the battery is tiny and knackered and the multimeter is absolutely terrible.

[grams]

IME lithium cells don't suffer from the deceptive "surface charge" problem that lead acid does, so the no load voltage is fine for giving you a rough idea of the SoC.

[andyoxon]

Thanks both.    Today I learnt that you can't accurately check voltage status of a Li-ion battery with a multimeter, because of the battery's 'internal resistance' characteristics.

That, IMVHO, is bollocks. The internal resistance of a Li-Ion battery is more than a million times smaller than the resistance of a multimeter* so the 1 ppm error will pale into insignificance compared to other error, such as the accuracy of the multimeter.

If there is any current being taken by other loads or current being supplied by a charger, then the internal resistance of the battery will make a difference. Also the accuracy of the multimeter can be important.

Finally, state-of-charge can’t be accurately determined from the no-load voltage as battery condition and temperature affect things as well.

*Unless the battery is tiny and knackered and the multimeter is absolutely terrible.

Ok good to know...    I'll file that 'what I thought I'd learnt earlier about testing li-ion batteries' as... 'don't necessarily believe everything you read on t'interweb'.

[Pickled Onion]

Internal resistance comes into play with the old zinc carbon/zinc chloride non-rechargeable cells. The open-circuit voltage will be typically 1.5 V or more even on a near-dead cell. The way to check these is by adding a load (such as a small resistor) and measuring across that, or most easily just measuring short-circuit current directly for a few seconds with a multimeter set to amps. This works because of the high internal resistance. On a good cell it will stay constant, if it's near-dead the current will start high and drop quickly. DO NOT **EVER** try this with Li Ion cells or any other rechargeable, as pointed out above they have a very low internal resistance and the short-circuit current will be very high. However the discharge characteristics of those mean that the open-circuit voltage drops with use so that can be a good indication of health.

You definitely can check the charge state of Lead Acid batteries with the open-circuit voltage, this is standard practice. The voltage gives a good indication of the level of charge, as long as it's not immediately after charging. It won't tell you if the battery's worn out though, where it can show a healthy open-circuit voltage but that drops quickly under load.

[andyoxon]

Thanks.

'parently Li-ion batteries have a different 'discharge curve'* to say, NiMH batteries.  So that for a li-ion battery, if said battery has been e.g. 70% used/discharged, the measured no-load voltage may actually be very similar to a nearly fully charged no-load voltage, and that there is only a significant drop in voltage measured on a multimeter when the li-ion battery has say <10% charge remaining.  Might this explain why I don't seem much voltage diffs between my li-ion batteries, because none of them are close to exhausted?  And so, using a multimeter to check li-ion batteries is of limited use because there's not much diffs in V between e.g. 70% full and 30% full?

* https://www.batterypowertips.com/how-to-read-battery-discharge-curves-faq/

[grams]

The 0.2C line on the third graph on that page matches my experience of measuring no load Li Ion cells.

It should be easy to tell the difference between a fully charged cell and a 60% depleted one.

It may not be so easy to accurately tell where a cell is during the 20-80% depleted range.

[Kim]

AIUI the 1000V rating on test equipment basically means "this is designed to survive the spiciest transient you're likely to encounter on 'low voltage'^[1] installations unless something Extremely Wrong happens in the distribution network".

Meanwhile a 300V rating means "if you connect this to the mains, it may result in incorrect readings, magic smoke release and even DETH".  Note that 240V AC peaks at around 339V, and if you don't understand why, you shouldn't IMHO be connecting multimeters to mains wiring.

On that basis, I wouldn't get too upset that a '300V' rated meter fails to give a sensible indication of overload voltages above that rating, on the basis that it's only really suitable for continuity testing, battery checking, hobby electronics sort of things.

A sensible rule of thumb is you want a proper Cat III rated meter from a reputable manufacturer for working on live mains.  It's about how it fails when exposed to high voltages as much as whether it can measure them.


_{[1] In the distribution sense, so 240V/415V AC mains.}

[Feanor]

I think the OP mentioned that the meter's AC range goes up to 300v AC, and that will presumably mean RMS.
Which suggests that it's probably OK for measuring normal domestic mains voltages, assuming no horble fault conditions.

It does seem a reasonable criticism that the meter reports Overload conditions as Zero, rather that !!! OL !!!, but that's going to be a bit of an edge case.

For occasional domestic use, I'm sure it's fine.

[andyoxon]

AIUI the 1000V rating on test equipment basically means "this is designed to survive the spiciest transient you're likely to encounter on 'low voltage'^[1] installations unless something Extremely Wrong happens in the distribution network".

Meanwhile a 300V rating means "if you connect this to the mains, it may result in incorrect readings, magic smoke release and even DETH".  Note that 240V AC peaks at around 339V, and if you don't understand why, you shouldn't IMHO be connecting multimeters to mains wiring.

On that basis, I wouldn't get too upset that a '300V' rated meter fails to give a sensible indication of overload voltages above that rating, on the basis that it's only really suitable for continuity testing, battery checking, hobby electronics sort of things.

A sensible rule of thumb is you want a proper Cat III rated meter from a reputable manufacturer for working on live mains.  It's about how it fails when exposed to high voltages as much as whether it can measure them.


_{[1] In the distribution sense, so 240V/415V AC mains.}

Thx.   Something like...  pure peak sine wave V of 339V, being 1.414 x the RMS effective (heating power as DC) V. ?    Not planning to poke about in the live mains - probably stick to measuring batteries etc...  I did once remove & replace a dead capacitor (known issue) in a DVD recorder unit, to get the freeview tuner working again, oh yes, and Etrex circuit board surgery.

[T42]

I have one of those el cheapo multimeters. It had been giving strange readings from my bike batteries for the last year, until last week I twisted the selector dial to the next range down, noticed and clicked it a notch back up again. Since then it's been perfect.  Now one swallow doth not a summer make, but I'd guess that the selector dials might be among the dodgiest components of such gadgets.

[Gattopardo]

Last year I picked up a LIDL Parkside Multimeter PDM300 C3, for occasional home use. 
I've noticed that this YT vid shows the unit failing to detect high overload voltages "Dangerous Parkside multimeter".  https://www.youtube.com/watch?v=QxPIRtZb8Ck
What think the panel?  AFAIA there there are no 1000V sources inside UK homes...   Clearly, it's not a 'professional' device.  Stated AC range is up to 300V, with a '300V fuse'.

Someone else (video99 on YT) recommends the unit, but says not wise that it uses the same inputs to make current measurments as high impedance measurements...

No  I didn't look at reviews before* buying... 

Have used similar and cheap one for checking voltage, cable continuity on earths and high resistance of cables.

I have one of those el cheapo multimeters. It had been giving strange readings from my bike batteries for the last year, until last week I twisted the selector dial to the next range down, noticed and clicked it a notch back up again. Since then it's been perfect.  Now one swallow doth not a summer make, but I'd guess that the selector dials might be among the dodgiest components of such gadgets.

You see that the inside clicky thing is just a ball bearing and moves a contact on the dial on the circuit board.