SM8124 Battery Impedance Meter

[Terabyte2007]

I was poking around eBay a couple weeks ago, looking for a battery impedance meter. I came across some extremely expensive ones, that I could not justify the cost. I then I saw this one, directly out of China for around 40 US dollars. The specifications seemed pretty good as far as chinese specs go, but I was willing to give it a try. I placed the order and received the unit about 2 weeks later. To my surprise, the unit is built fairly well, it has an outer soft rubber protective boot, and it uses a 4-wire lead kelvin type probe. Instead of clips, it uses a pinned tip which appears to be gold plated and fairly thick, around 6-8 mm. The leads are solid and heavy, they seem to be nice quality. The unit runs off a 9-Volt battery and can measure battery voltage from 0 to 100V and impedances 0-20 Ohms. Here are the documented specifications:

Voltage Measurement Range:

0 - 1.9V - Resolution 1mV
2V - 19.9V - Resolution 10mV
20V - 100V - Resolution 100mV

Power +/- (1%+1)

Internal Resistance Measurement Range:

0 - 199.9 mOhm - Resolution .1 mOhm
200 mOhm - 1999.9 mOhm - 1 mOhm
2 Ohm - 20 Ohm - 10 mOhm

Power +/- (3%+1)

Testing batteries which have known resistance, the unit seemed bang on. I don’t think the meter has any real over voltage protection if you were to measure over 100V because in the manual they warn several times about over-voltage damage and the need to place the lead polarity correctly. I also did notice that the lead tips are made with a quad spiked top arrangement which means that you need to place the tips flat to get the 4-wire kelvin reading which is not very hard to do on most batteries.

I must say, this is not a bad unit for the cost, It performs well, seems pretty accurate and appears to have a decent build quality. I think it’s a great addition to my bench and allows me to not only check battery voltages accurately but also test the internal health (Meaning Impedance) of the said battery.

eBay Link: http://www.ebay.com/itm/220588452030?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

[The Electrician]

Hook it up to a resistor of several ohms and have a look across the probes with a scope to see what it's doing.  Is it applying a sine wave?  If so, what frequency?

[HighVoltage]

$ 39 including free shipping to the USA
Euro 63 ($ 87) including free shipping to Germany
You have to wonder, how they come up with these prices
But the unit looks interesting

[kripton2035]

I bought the same for $30 + $12 shipping to france a year ago.
make some search in ebay and be patient you will find one around $40 shipped to europe.

[Terabyte2007]

Hook it up to a resistor of several ohms and have a look across the probes with a scope to see what it's doing.  Is it applying a sine wave?  If so, what frequency?

I believe in the specs they quoted 1kHz +/-10%. I'll test that today and see what the waveform looks like and the frequency.

[Porcine Porcupine]

Is anyone familiar with adjusting the calibration on these?

I just got one, and it was off 0.5 V at 12 V and read around 2 milliohms with the probes shorted together.

There are two potentiometers on the PCB. I can adjust the left potentiometer (VR2) until voltage readings are accurate. The left potentiometer also seems to slightly affect impedance measurements. The right potentiometer (VR1) seems to have no effect on voltage readings, but allows a small impedance adjustment of a milliohm or two. I can't seem to get it to show under about 1 milliohm with the leads shorted together.

There's a 2-kilohm resistor (R7) in series with the right potentiometer I tried shorting, but it didn't seem to make much difference in the range of impedance the potentiometer is able to adjust.

In case, you're wondering, I got this primarily for testing large flooded lead-acid batteries. They typically measure around 5 milliohms when new, so being off 1 milliohm is a big deal. I guess I could just subtract the offset, but I'd like to get it working right if possible.

[texaspyro]

Be careful with these... there is no reverse polarity protection.

[HKJ]

I have one of these, but it is dead. I do not know why (I have not looked inside it).
But somebody gave me a new one: http://www.lygte-info.dk/review/InternalResistanceMeterYR1030%20UK.html
And it looks considerable better (I have only used it for a few weeks, my opinion may change later).

[flywheelz]

I just got one, and it was off 0.5 V at 12 V and read around 2 milliohms with the probes shorted together.

Hi, I also bought the SM8124. Mine also shows around 2mOhms with shorted leads on the 200m range.

What I found out is my meter has dodgy switches.  If I press on the range switch then the 2m offset disappear. The function switch is also dodgy because sometimes symbols disappear from LCD and wiggling the switch they reappear.

Check it out.

[flywheelz]

So I took my meter apart and cleaned it really good from old flux residue then added some Rail Zip to the switch none gold plated contacts. Both switches no longer act dodgy  Pressing or wiggling them now have no affect. 

After all this I was hoping on lowest resistance scale it would zero out with leads shorted but I still get 2milliohms.  I guess I will have to always subtract 2 from result.

[kripton2035]

shouldn't be too hard to reverse the schematic it seems ?

[flywheelz]

Looks like the 2 to 3 milliohms offset is a common thing with these testers. I also noticed it has temperature drift.

[DBoulanger]

I have to admit, I wasn't too impressed about the soldering job.  Far from professional.

I just checked on Banggood and the SM8124 is sold out.  They now have the SM8124A which at first glance has pretty much the same specs.  The only obvious difference is the probes which have 2 pins each instead of "concatenated" ones.

It is around $64 USD.

I guess you have to have a need for this type of meter.  When someone has only few batteries, it might be rather difficult to justify.

[kripton2035]

you can still find the SM8124 on ebay, at a better price
https://www.ebay.com/itm/Portable-Battery-Internal-Resistance-Meter-Voltage-Tester-Voltmeter-SM8124/162812601314?

[DBoulanger]

Hi,

Thank you for the link.

I had to share what I found on that listing, I thought it was too good to let it go without any notice.

We pretty much all know that the main purpose of this meter is to measure battery impedance, right ?  Well, I guess the person who took the pictures for the listing wasn't aware of this.

Just for the fun of it, here are 3 pictures that were part of the listing however 2 of them raised some concerns.

AA Battery : That one is pretty much OK.  The voltage is within a good range but if you're like me, I would have been more interested at seeing the battery's ESR.

CellPhone Battery : This one is rather convincing.  The battery is REALLY dead !!!  Expecting something in the 3.7V range or so, I'm pretty sure we can call it DEAD while showing 1mv !  What bugs me though, is the fact that the meter still in the 2V range selection.  Was the user expecting such a low voltage to leave it on the 2V scale ?  Once again, no ESR.

9V Battery : Once again, the photographer had pessimistic anticipations.  He left the unit on the 2V scale and astonishingly the reading is within the range with a whopping 1.233V !!!  With that voltage who cares about the ESR, right ?

Either he had a very bad battery inventory or his supplier pulled a quick one on him !

So globally it is amusing at times to see how some sellers may have rather limited marketing skills.

I'm not perfect but I think that with very minimal effort I would have done better.  I'm sure a lot of people would have too.

[Mosaic]

There is a precision opamp https://www.analog.com/media/en/technical-documentation/data-sheets/AD8221.pdf
It outputs a 1Khz square wave based on its 2 inputs of a 1Khz sine and a halfwave 1Khz sine.. Same sine that appears at the red input lead.
This Sine wave is reduced significantly when the input leads (black & red) are shorted to evaluate  impedance.

I note that soldering a shorting wire across the PCB input lead joint reduces my Meter's zeroing error from 1.5mΩ to 0.5mΩ. So some benefit can be had by improving the gauge of the input lead wiring.
I suspect full zeroing can be achieved by augmenting the leads PCB input traces with solder tinning or a copper wire.  I have yet to try this.

I have a failed SM8124 which I will disassemble first before tampering with the working unit.

[Maximus Panin]

I'm also looking for my first tester. I choose between YR1035+, SM8124A, Volcraft BT-501 - which is better ?

[Maximus Panin]

YAOREA-YR2050

[kripton2035]

YR2050 is a DC milliohm meter. it cannot measure battery internal resistance. you need an 1KHz AC ohmmeter for that.

there is also the RC3563 that seems a good candidate. (better than the SM8124 IMHO)
https://www.aliexpress.com/premium/rc3563.html