YR1030, a milliohm meter for measuring low resistance and the internal resistance of batteries

I've posted quite a few battery reviews, and people often asked why I don't measure the batteries' internal resistance. My answer was usually the same: the instruments I had couldn't measure this parameter accurately, so there was no point in taking measurements. But fairly recently, I got hold of a specialized instrument designed specifically for this kind of measurement.
This device belongs to a very specialized class of measuring instruments, though I can imagine it being useful for more than just working with batteries.


I ordered the instrument from Taobao, from the store of a well-known Chinese—I'm not even sure what to call him, so let's say customizer—100 MHz. In fact, it didn't really matter where I ordered it from; at the time, he simply had the lowest price, and he also had low-resistance resistors.

First, what is a battery's internal resistance? I won't go into too much detail, and I'll try to explain it roughly, but hopefully clearly.
Imagine an ideal battery with no self-discharge and no internal resistance—a sort of "spherical horse in a vacuum." This ideal cell is inside your battery, but there are also two resistors inside it: one called Rs, connected in series with the battery, and another, Rp, connected in parallel. In this model:
Rs is the resistance known as the internal series resistance. It determines how much current the battery can deliver.
Rp is the resistance that discharges your battery while it sits on the shelf.

In reality, it's all a bit more complicated than this simple pair of resistors, since a battery is a chemical cell, but this is more than enough to get the general idea.

The second diagram on the right also shows parasitic resistances outside the battery, such as the contacts in the holder, which increase the series resistance, and, for example, your circuit, which may have a small resistance and also discharge the battery.
To be fair, the same thing happens with capacitors, for example, and this parameter is called ESR (Equivalent Series Resistance). Even an ordinary inductor can be considered to have this parameter, due to the winding's DC resistance.

With external components, we can improve things—for example, by using better-quality holders, or even soldering wires directly to the battery, cleaning the circuit board, or using lower-power components to reduce leakage. But with internal parameters, we can only influence them indirectly, for example by changing the temperature. As the temperature rises, both resistances decrease. So, to minimize a battery's self-discharge, it's stored in a cool place; to reduce its internal resistance, it's used "warm."


So what does all this look like in real life, rather than in virtual circuit diagrams?
Let's take a couple of batteries, the US18650VTC4 and the LGDBHG21865 (better known as the chocolate batteries).


Since internal resistance is an important parameter, it's almost always specified in the datasheet. For the first battery, for example, it's 12 mΩ (0.012 Ω).


For the second, it's up to 17 mΩ.


In practice, internal resistance is what causes a battery to heat up, which becomes noticeable when it's operating under a high current.
For example, 12 mΩ at 15 A gives a voltage drop of 0.18 V. Multiply 0.18 by 15, and you get 2.7 W of heat.
It's even worse for the second battery: 17 × 15 = 0.255 V, and 0.255 × 15 = 3.825 W.

Of course, this is all very rough and simplified, but the photo below gives a clear example: after a full discharge at 15 A, the first battery is at 70 degrees, while the second is almost at 80. Besides the temperature, there's also a greater voltage drop under load, which can be critical for high-power devices such as e-cigarettes, power tools, and various quadcopters, RC cars, and so on.



There are various tools you can use to measure this parameter, but the best approach is to use a specialized instrument. In this review, I'll try to explain why. But first, let's get to the review itself.

My order arrived in separate bags: the instrument was in one, and the resistors in the other, since they were ordered as a separate lot.
So, I have:
The instrument
Test leads for the instrument
Test resistors.

The seller offers lots of options for additional accessories. I chose the instrument + probes option, whose price is shown in the title, as well as a set of resistors.


I ordered the resistors to check the accuracy of the instrument, both the one under review and others I have on hand. The seller charges $1.64 for this set ($1.48 when I ordered), which is a pretty good deal.
Resistor values
1. 1 mΩ 1%
2. 2.2 mΩ 0.5%
3. 10 mΩ 0.5%

The resistors have a four-wire connection, are rated for up to 10 W, and can be mounted on a heatsink.


And since these are branded resistors made by Isabellenhutte, I was able to find their datasheet, which lists both the resistor specifications and their internal construction. The datasheet says that resistors are also available with 0.1% accuracy, but I only have 0.5% and 1.0% versions, which is still pretty good, especially for such low resistance values.


The kit included four-wire test leads. In fact, almost all instruments of this kind use a four-wire connection for the component being measured.


Here I'll quote my own explanation of four-wire connections from another review.

When measuring resistance the usual way (and, by the way, not just resistance), a parasitic factor like the test lead wires can have a significant effect. I think many people know that few multimeters read 0 when the probes are shorted and the meter is set to its lowest range. The display usually shows some value around 0.05–0.5 Ω, which is the parasitic resistance.
Sometimes you can compensate for it by enabling the relative measurement function (Rel), but this isn't always convenient, and it doesn't always work correctly.

As for measuring the internal resistance of batteries, the vast majority of popular chargers such as Opus, LiitoKala, iMAX, and others use a two-wire connection. My electronic load, which I use to test batteries, has a four-wire connection, but the wires join near the alligator clips, and the battery is connected at just two points. Even if I modified the battery holder to use a four-wire connection, it wouldn't make much difference anyway, since practically all these devices measure resistance using DC.


The principle of resistance measurement is quite simple. Connect the component to a current source and measure the voltage across it. But since the wires have resistance, the result will be the sum of the component's actual resistance and the wire resistance.
When the resistance is high, this usually doesn't matter much. But when we're talking about values of 1–10 Ω or less, the problem becomes very noticeable.
To solve this problem, the current path through the component is separated from the measurement circuit.


In practice, it looks roughly like this, as shown in the diagram.


However, when measuring the internal resistance of batteries—and capacitors, for that matter—testing with DC is not appropriate. That's because chemistry is involved, as are the processes that take place during charging and discharging.
That's why battery internal resistance is conventionally measured at 1 kHz, although some batteries also specify resistance under DC conditions. The screenshot shows that the value can differ significantly (the upper value is for AC, the lower for DC). And while a four-wire setup can still be modified, building an instrument that measures using AC is somewhat more complicated. This principle is used in proper battery testers and capacitor ESR meters, as well as in LCR meters in general.

That, essentially, is the answer to why I don't measure this—and don't recommend that others do it—with common consumer-grade devices that have neither a four-wire connection nor an AC measurement mode.



The probes consist of four spring-loaded contacts inserted into metal tubes. They're comfortable to hold; the wires are a little short, but quite flexible. They connect to the instrument via a USB connector.
The package also included four spare contacts. The part that connects to the component under test is shaped like a flower, so it grips the component lead quite well and doesn't slip off.

Using a USB connector may seem like a questionable choice, but personally, I find it more than convenient. Besides making a solid connection, it's also easy to repair.


I have almost no complaints about the device's exterior: it's a neat little gray box.


The button labels are in English and Chinese. There are only four buttons, though, so it's hard to get confused.


The basic specifications are printed on the bottom of the case. The full specs are as follows:
Resistance measurement
Range: 20 mΩ, resolution: 0.01 mΩ, accuracy: 0.7% + 7 digits (with the ZR function enabled)
Range: 200 mΩ, resolution: 0.1 mΩ, accuracy: 0.5% + 5 digits
Range: 2 Ω, resolution: 1 mΩ, accuracy: 0.5% + 5 digits
Range: 20 Ω, resolution: 10 mΩ, accuracy: 0.5% + 5 digits
Range: 200 Ω, resolution: 0.1 Ω, accuracy: 0.6% + 5 digits

Voltage measurement
Range: 2 V, resolution: 0.001 V, accuracy: 0.8% + 5 digits
Range: 20 V, resolution: 0.01 V, accuracy: 0.8% + 5 digits
Range: 28 V, resolution: 0.1 V, accuracy: 0.8% + 5 digits

One end has the probe connector and a microUSB port for charging the device's battery. When I took the photo, I noticed that the labels were upside down. Then I thought about it and realized it makes sense: when you plug in the connectors, you hold the device with the screen facing you, and the labels are the right way up. I've more often seen it done the other way around :)


The Power button has several functions:
1. Turns the device on.
2. Press and hold to turn it off. A Yes/No prompt appears; “Yes” is on the left, so it's the middle button.
3. Press briefly to open the settings menu; press again to exit the menu.

A short press also turns on the backlight for about 10–15 seconds. The backlight can turn on automatically when voltage appears at the device's input, i.e. when you connect a battery.


On the right are two buttons: Range R and Range U. The first switches the resistance measurement range (auto, 20 mΩ, 200 mΩ, 2 Ω, 20 Ω, 200 Ω); the second selects the voltage measurement range (auto, 2 V, 20 V, 28 V).
I used the device in auto mode the whole time. The ranges switch quickly, and I found no problems. A couple of times it didn't switch at the very edge of a range, but that's due to the automation's hysteresis.


1, 2. The middle button is labeled HoldZEROR. A short press holds the reading; a long press disables the automatic zero-hold function. ZEROR is enabled by default (ZR on the screen), and I took all my measurements in this mode. You can also turn it on or off manually.
3, 4. The menu: it's both hell and heaven at once. I'll try to explain.
Hell: everything is in Chinese, and as far as I can tell, most units come with Chinese menus.
Heaven: if the device is working and set up properly, you don't need to do anything here. Everything works fine out of the box.


I found an English version of the menu online. It shows the following options:
1. Normal mode
2. Component sorting mode.
3. Backlight duration, 5–60 seconds
4. Time before the device turns off and enters power-saving mode
Auto power-off: 5–60 minutes
Power-saving mode: 5–30 minutes
5. Battery sorting mode settings
A - RaUxa (set value)
B - RbUxb (set value)
C - RcUxc (set value)
6. Number of batteries in sorting mode
7. Calibration
8. Restore factory settings
9. Battery charging current: 200/400 mA, 200 mA by default. The last option isn't available in the older version of the device, though I don't think it's particularly necessary.


Here are the same menus in Chinese, in the same order as above:
2. Component sorting mode.
3. Backlight duration, 5–60 seconds
4. Time before the device turns off and enters power-saving mode
Auto power-off: 5–60 minutes
Power-saving mode: 5–30 minutes
5. 6 Battery sorting mode settings
A - RaUxa (set value)
B - RbUxb (set value)
C - RcUxc (set value)
7. Number of batteries in sorting mode
8. Calibration
9. Restore factory settings
10. Battery charging current: 200/400 mA, 200 mA by default. The last item is missing from the old version of the device, although I don't think it's particularly useful anyway.

A quick note on navigating the menu, which I find extremely awkward.
Power — selects the desired menu item; it also exits a menu item without saving the settings.
Hold — saves the setting and exits the submenu.
Range R — increases the selected setting; this button also moves up through the menu items.
Range U — navigates through the menu and its submenus, but only downward.

If you're not sure about the selected setting, press Power. To save it, press Hold. To change a setting, use Range R; to select which setting to change, use Range U. To exit the menu, select item 1 and press Power.



Power-saving mode; press the Power button to exit.


There are four screws on the bottom of the case, so the device is very easy to take apart. The display had stuck to the front panel, though, and I barely managed to pry it off.




Before the tests, a few words about some quirks
1. The screen simply shows an overload when the probes are open.
2. If you connect the probes to each other, it displays 0
3,4. But what surprised me was that when I tried to measure the resistance of my skin, it showed nonsense. I later realized that this makes sense: the meter uses a four-wire measurement, so the corresponding probes also need to be connected to each other.


First, I decided to check the shunts. Although, strictly speaking, this doesn't matter much, since the result depends on the accuracy of the shunt itself and the combined measurement errors of both multimeters.
I used two multimeters:
UT61E in current measurement mode
UT181A in voltage measurement mode.

Perhaps I should have connected them the other way around, but I didn't run that experiment.

The test was performed at two reference current values, 1 and 5 A. The measurements showed that:
The 1 mΩ shunt measures 0.997 mΩ and 1.0008 mΩ
The 2.2 mΩ shunt measures 2.206 mΩ and 2.2076 mΩ
The 10 mΩ shunt measures 10.021 mΩ and 10.0214 mΩ.
The readings at 1 and 5 A differ slightly, most likely because the ammeter shunt heats up. There were also small fluctuations in the voltmeter's last digit, around +/- 2 counts, but in any case, the readings match the stated values.


1. I also connected the meter under review to resistors using a four-wire setup.
2, 3, 4. The results are excellent. At first, the meter shows a lower value, but after a couple of seconds it settles on the displayed reading. The value is very stable, with the last digit occasionally jumping by one count.


Next, I decided to compare it with my RLC meter, but got some rather strange results.
1. Zeroing by connecting the probes through a piece of copper wire.
2, 3. The 1 and 2.2 mΩ resistors are spot on
4. The 10 mΩ resistor read 9.1–9.2 instead of 10
5, 6. Just out of curiosity, I tried some ordinary 5% resistors rated at 0.1 and 0.22 Ω. The results were more or less reasonable, which points to linearity issues specifically at the low end of the range.


I took the same 0.1 and 0.22 Ω resistors and checked them with the meter under review. It showed resistance slightly higher than the RLC meter.


Next, I decided to experiment with my previous meter. First, I tried zeroing it by connecting the probes directly. Now the results were reversed: the 1 and 2.2 mΩ resistors read high, while the others were almost identical to the meter under review.
My RLC meter is specified at 0.5% in its basic configuration and 0.3% after additional calibration. At 0.5% and a 1.5 Ω range, the error would be +- 0.75 mΩ. You could say the results are roughly the same in both cases, but they're right at the edge of the range, and somehow it still doesn't feel right. It seems that for measuring low resistances, you need to use one zeroing method, and for resistances of 5 mΩ and above, another. :(


The measurements above were taken at 1 kHz, just like with the meter under review. But after I switched the RLC meter to 100 Hz, the results improved noticeably. Overall, I think I need to investigate further, since the RLC meter has additional settings and there may be a way to adjust its linearity.


After that, I decided to check a few more resistors:
1. 0.47 Ω 1%
2. 5.1 Ω 1%
3. 9.76 Ω 2%
4. 75 Ω 1%


I didn't independently verify the 9.76 and 75 Ω resistors, but I did check the 0.1 and 0.22 Ω resistors shown earlier, as well as the 0.47 and 5.1 Ω resistors, using the same method as for the shunts.
The results were:
The 0.1 Ω resistor actually measures 0.09817 Ω
0.22 Ω — 0.21721 Ω
0.47 Ω — 0.47054 Ω
5.1 Ω — 5.105 Ω.


And the results obtained with the meter under review are, in my opinion, pretty good.


Since the meter is designed for use with batteries, it can measure voltage as well as internal resistance. Maximum input voltage is 28 V, and it's best not to exceed it. The polarity can be either way, though—the voltage will simply be displayed with a minus sign.

During the test, I compared the voltmeter readings with a more accurate instrument. The results were excellent, although in almost all the tests the meter read 1 digit high, which is perfectly normal for digital instruments.


I also ran an additional test using a capacitor and the three shunts shown at the very beginning of the review.
First, I measured the capacitor's internal resistance, then connected the shunts in series with it and checked how much the measured result differed from the calculated value.


1. Capacitor ESR: 30.1 mΩ
2. Capacitor + 1 mΩ resistor: measured 31.4, calculated 31.1
3. Capacitor + 2.2 mΩ resistor: measured 33.2, calculated 32.1
4. Capacitor + 10 mΩ resistor: measured 40.7, calculated 40.1

The results are very good. The test with the 2.2 mΩ resistor was a bit off, but I think that error is acceptable.


And, of course, batteries. First, I tested a two-year-old battery whose datasheet specifies a resistance of 12 mΩ.
2. Fully charged: 12.46 mΩ.
3. Discharged: 12.68 mΩ
4. Here's an example of how internal resistance increases at low temperatures. The discharged battery was cooled to about -20 degrees. As a result, its resistance increased by almost 1.6 times.


Here's a test of batteries in the “give them to your enemy” category.


1, 2. Yellow, charged and discharged.
3, 4. Blue, charged and discharged.

As you can see, these are absolutely terrible. If you put one of these batteries in a power bank, its high internal resistance will make the power bank shut off before it has even used up the battery's already tiny capacity.
The photo shows the battery voltage after discharging in the power bank.


And here's a resistance measurement of a lithium iron battery. The resistance is quite high, partly because the battery is small. The smaller the battery, the smaller the electrode area and the higher the resistance. That said, resistance can also vary between batteries of the same form factor: “high-current” batteries have low resistance, while “high-capacity” ones have higher resistance but greater capacity.

1. Resistance at room temperature: 114.4 mΩ
2. Resistance at -20 degrees: 140.9 mΩ, or 1.23 times higher than at +25.

The US18650VTC4 shown above had a difference of almost 1.6 times, but I can say that if you put a load on the battery, self-heating quickly brings its resistance back to normal.


As a little extra, here are the waveforms at the instrument's probes.
1. Current source output only.
2. The probe pairs are connected. Since this is the main mode of operation, all the following waveforms were captured with the probe pairs connected.


Waveforms in different operating modes.
Auto, 20 mΩ, 200 mΩ, 2 Ω, 20 Ω, 200 Ω.


And finally, a little bonus. While looking for resistors to test, I came across a strip of 0.1 Ω resistors—or at least that's what their color code says: brown, black, silver, gold = 0.1 Ω, 5%.

But here's the interesting part: out of 6 pieces, only 1 (one) had a resistance of about 0.1 Ω, while the other five measured 0.224 Ω! I even checked them on the same strip, as you can clearly see in the photo.
I bought the resistors from a trusted seller in a brick-and-mortar store. I did once come across incorrect markings a while ago, but in that case all the resistors on the strip were marked incorrectly. But this...



Video version of the review


To sum up, I definitely liked the instrument, thanks above all to its high accuracy and ease of use. It can be used to measure the internal resistance of batteries, check capacitor ESR, and—also very importantly—measure very low resistance values.
The only real downside is that the menu is entirely in Chinese. This will be especially inconvenient if you need the sorting function, unfortunately :( In normal use, you don't need to navigate the menu; everything works “as is” and quite well.

I was a little disappointed by the difficulties I ran into when using it with my RLC meter. I need to investigate why this happens. As it turned out, it only just manages to squeeze within the specified 0.5%, but the two different setups produce an offset in opposite directions. At 100 Hz, however, the readings are correct.



That’s all from me. I hope you found the review useful, and I’ll be glad to answer any questions or hear your suggestions for tests.