Mastech M-830B DMM voltage reading

[Stinger]

Hello friends !

I have problem with my 24yo Mastech digital multimeter.

When i measured 12V battery of cordless drill, my Aneng AN8009 DMM read 11,81V. Mastech M-830B read 9,53V.

For tests, AA, AAA batteries and 13,5V charger read with good voltage on both DMM.

Are parts (resistors?) dead ?

Thanks for your help.

[Stinger]

No Jedi repair here ?

[MosherIV]

It is a 40year old design. They are dirt cheap. The only reason to fix one is for education.

Just search for dt830. They are various places with schematics.

Did you disturb the pot in the meter?
If you did then, you dusturbed the calibration and that is all that is wrong.

[wraper]

For tests, AA, AAA batteries and 13,5V charger read with good voltage on both DMM.

I'm not convinced there is any problem with a multimeter after reading this. Difference with one measurement could be just because it has 5 times lower input resistance than AN8009.

[Stinger]

It is a 40year old design. They are dirt cheap. The only reason to fix one is for education.

Just search for dt830. They are various places with schematics.

Did you disturb the pot in the meter?
If you did then, you dusturbed the calibration and that is all that is wrong.

Fixing for education of course.

Yes, i recalibrated it with her potentiometer (because little offset).

For tests, AA, AAA batteries and 13,5V charger read with good voltage on both DMM.

I'm not convinced there is any problem with a multimeter after reading this. Difference with one measurement could be just because it has 5 times lower input resistance than AN8009.

You mean resistance of probes ? I tested probes of AN8009 on DT830b and same wrong voltage read for the drill battery.

[iMo]

If the drill battery was empty, you could get lower reading when the input resistance of one DMM is 1Meg, and higher readings with the 10Meg inp res DMM, for example.
With fully charged battery or a voltage source with low internal impedance both meters will show the same.

[tooki]

For tests, AA, AAA batteries and 13,5V charger read with good voltage on both DMM.

I'm not convinced there is any problem with a multimeter after reading this. Difference with one measurement could be just because it has 5 times lower input resistance than AN8009.

You mean resistance of probes ? I tested probes of AN8009 on DT830b and same wrong voltage read for the drill battery.

No, the input resistance of the meter itself. It presents a small load on the battery, and if that load is different (which appears to be the case here), then it can produces different readings.

We suspect the 12V battery isn’t very full, so its voltage is quickly sagging under even a tiny load. Try this: take a 10K resistor and put it across the battery, then measure with each meter. (When paralleled with the internal resistance of the meter, the meter’s resistance is so much higher as to become virtually irrelevant here.) Most likely, the readings will now be very close. (And most likely far below 12V.)

And, as imo said, try a lab power supply. Chances are the meters will agree at all voltages.

[iMo]

In other words - the internal resistance of the source (ie a battery, PSU, charger, etc.) creates a voltage divider with the input resistance of the attached DMM (ie. 1MOhm, 10MOhm, 10GOhm).
Empty batteries have large internal resistance (ie. kiloOhms), fully charged ones have got low internal resistance (ie. Ohms).

[Stinger]

Thanks for your comments.

These 24yo 830b DMM have heavier input resistance than modern AN8009, resistance who making wrong voltage read on li-ion battery.

This hypothesis appear good looking read of alcaline batteries and DC adapter on both DMM.

[tooki]

No, the old Mastech has much lower input resistance (1MOhm), meaning that it puts a higher load on the DUT (device under test, in this case the battery you’re measuring) than the AN8009 (10-11MOhm, depending on range). That’s why the voltage is sagging when you measure it with the old Mastech.

It has nothing to do with battery chemistry, only with the fact that a dead battery will sag under load.

[Stinger]

Thank's for explanation.

[tunk]

Is the battery dead? If not, I doubt that the input resistances can explain the
difference - they equates to about 1 and 10 microA which is putting almost no
load on the battery. Also, if you set it to the 200Vdc range, what does it show?

[Stinger]

Hello,

The 12V 1,5Ah battery is new and near fully charged.

Same value are read with offset on 20V, 200V and 1000V DC range.

[tooki]

Is the battery dead? If not, I doubt that the input resistances can explain the
difference - they equates to about 1 and 10 microA which is putting almost no
load on the battery. Also, if you set it to the 200Vdc range, what does it show?

Hence why all of the discussions above about input resistance specify that we are talking about dead batteries. And indeed, for the difference the OP describes to happen, the battery would have to be really dead.

The reason we suspected a dead battery is that even a high load shouldn’t cause a fresh 12V battery to drop to below 10V. Under no load, a 12V battery should be well over 12V when fresh.

Hello,

The 12V 1,5Ah battery is new and near fully charged.

Same value are read with offset on 20V, 200V and 1000V DC range.

Stinger, you’re now saying the 12V battery is fully charged. (You never said this before, even though we suggested it was due to an empty battery.) If you fully charge the battery, does the 830B still measure it as much lower than the Aneng? When charged, they should both measure practically the same.

[Caliaxy]

Are you getting the same good AA/AAA battery readings on Mastech on both 2V and 20V scales?

[Stinger]

Stinger, you’re now saying the 12V battery is fully charged. (You never said this before, even though we suggested it was due to an empty battery.) If you fully charge the battery, does the 830B still measure it as much lower than the Aneng? When charged, they should both measure practically the same.

The drill battery are fully charged.
I measured after little drilling. 830b read 9,37V and 11,62V with AN8009.
I can test again when fully charged but i'm sure 830b read voltage of drill battery with offset.

Are you getting the same good AA/AAA battery readings on Mastech on both 2V and 20V scales?

Yes. I measured AA, AAA and DC Adapter on 20V, 200V and 1000V DC scales on Mastech.
No reading problems for 830b (and same reading results on AN8009).

[wraper]

IMHO you are not even measuring battery voltage as such. Might be it's output is disabled and you are measuring leakage. Or measure on incorrect terminals. Just measure with 2 meters simultaneously connected. So if there is any voltage sag, it will be present on both meters.

[Stinger]

IMHO you are not even measuring battery voltage as such. Might be it's output is disabled and you are measuring leakage. Or measure on incorrect terminals. Just measure with 2 meters simultaneously connected. So if there is any voltage sag, it will be present on both meters.

Hi,
I measured drill battery with 830D and AN8009 simultaneously connected.
Voltage sagging at 9,16V it on both meters.
830B single read 9,37V.
AN8009 read 11,62V.

[Caliaxy]

Well, if the voltage drops that much by increasing the load from 10M (AN8008 alone) to ~0.9M (AN8008 in parallel to 830B), there is something wrong with your battery. If my calculations are right, this equates with a battery internal resistance of about 281K, about 281000 times more than it should be. Your 830B might be good (very likely) or not, but your drill battery is very likely bad.

Why were you trying to measure the battery in the first place, is it because the drill was not working even thought the battery was fully charged?

[tunk]

Most li-ion battery packs have some internal protection circuitry - I have no idea how,
but it may be that it somehow interferes with your measurements. Could you open it and
measure directly on the cells?

[wraper]

IMHO you are not even measuring battery voltage as such. Might be it's output is disabled and you are measuring leakage. Or measure on incorrect terminals. Just measure with 2 meters simultaneously connected. So if there is any voltage sag, it will be present on both meters.

Hi,
I measured drill battery with 830D and AN8009 simultaneously connected.
Voltage sagging at 9,16V it on both meters.
830B single read 9,37V.
AN8009 read 11,62V.

Then it's obvious there is no problem with multimeter.

[Caliaxy]

Of course, the other (theoretical) possibility is that the battery is good but your 830B impedance is very low. Again, if my calculations are correct, for the figures you report, the input impedance of your 830B should be about 4.2 ohms (for a battery internal resistance of about 1 ohm). In this scenario, the current going through your instrument should be about 2.2A.

If your 830B measures other voltage sources with no issues, this scenario is very-very unlikely.

Just for fun, why don't you measure the current passing through your 830B while you measure the voltage across the battery terminals? Connect your AN8008 (on A/mA) in series with the 830B (on 20V) and with the battery and read the current on AN8008. If you measure something of the order of ~10-20 uA, the 830D meter is good, the battery is bad. If you measure something of the order of ~2A, the battery is good, the 830M meter is bad.

[wraper]

If cordless drill works, there is nothing bad. It is not a requirement for battery to be bad to get such measurement. All you need is measuring not actual battery output voltage but something else.

[Kleinstein]

Some of the tool batteries have a protective / switching inside the battery-pack.  It needs some extra signal to enable the power and the voltage read can be just through some MOSFET leakage. The lower impedance of the 830B can than make a big difference.

[magic]

Of course, the other (theoretical) possibility is that the battery is good but your 830B impedance is very low. Again, if my calculations are correct, for the figures you report, the input impedance of your 830B should be about 4.2 ohms (for a battery internal resistance of about 1 ohm). In this scenario, the current going through your instrument should be about 2.2A.

2A from a 9V battery is plenty of power, something in the DMM would be smoking if that happened