USB tester, meter, or just a number display?

I often see people measuring the capacity of smartphone batteries, power banks, and similar devices with USB testers in forum comments and reviews. Once, I tried to explain why this kind of measurement makes no sense, but today I’ll approach it from another angle and test the testers themselves.


I can already hear the comments: “There are already tons of reviews of this tester. Why do we need another one?”
Yes, there really are a lot of reviews, because this model is quite popular—probably one of the most popular ones. But in this case, the tester review is only a side issue.

First, this review includes a comparison test of 15 units. But the point isn’t to show how good or bad the tester is; it’s to explain why this device can only be used to get a rough idea of current and voltage, let alone capacity.
This review is my one big answer to the question: should you trust the results from tests using “doctors” or not?

But let’s take things in order. I ordered 15 of these testers. At the time, they cost about 2.8 dollars each. Shipping was extra, so I’ve given the price based on the total order cost.
A little while later, I picked up a package at the post office, with a bunch of little bags inside.


Inside the bags were 15 USB testers. Everything matched the order, no complaints.


Specifications claimed by the manufacturer.
Model - KWS-V20
Voltage - 4-20 V (measurement accuracy +/-1%)
Current - 0-3 A (measurement accuracy +/-1%)
Timer - 0-99 hours
Capacity - 0-99999 mAh

Since this is partly a review of the tester itself, I’ll take a closer look at one unit. The others are identical, so I won’t take every one apart :)
1. The design is pretty familiar: a USB plug on one end and a socket for connecting a load on the other.
2. The bottom of the case is matte, so you can’t really see anything through it.
3. The case is made of two halves held together by four clips.
4. The clips are very tight, so taking it apart is awkward—especially if you want to do it without damaging anything.


Inside, we find a fairly neat circuit board with an LCD and connectors. There are newer models with OLED displays, but I haven’t come across one yet.


All the other components are on the back of the board: the controller, the display driver IC, the shunt, and the voltage regulator.
The board itself looks nice, though in a few places it’s marred by traces of flux that wasn’t cleaned off.


1. The “heart” of the device is an 8s003f3p6 microcontroller from STMicroelectronics. It’s an 8-bit microcontroller with a 10-bit ADC.
The voltage regulator is located next to it.

2. Since the display has a large number of pins, an LCD controller, HT1621, is used to help the microcontroller drive it.
Near the output connector there’s a 50 mΩ resistor that serves as a current shunt. The shunt resistance is fairly high: at 2 A, about 0.1 V will drop across it. That can be significant at 5 V, and since the tester’s maximum current is 3 A, the drop can reach 0.15 V, not counting the voltage drop across the connectors and circuit board traces.
This value was chosen because there’s no shunt signal amplifier on the board, so the microcontroller measures it directly.



The display and controls are extremely simple.
The measured voltage and test time are shown at the top of the display.
At the bottom are the measured current and the calculated capacity in mAh that has “passed” through the tester since the last reset.


I hadn’t really paid attention to this before, but it turns out the tester doesn’t start counting from zero. Or, more precisely, it doesn’t start at the very lowest measured current.
1. I connect a wireless keyboard. The current draw is 110 mA, then drops to 50 mA after a while, but the timer and, consequently, the mAh counter don’t move.
2. We connect a phone. The current is 600 mA, the timer starts, and after a few minutes some mAh have “accumulated.”


I got curious about the current at which the timer starts “ticking.” It’s easy to find out: gradually increase the current and watch the timer’s separator dots. As soon as they start blinking, the count has begun.
In my case, counting started at 260 mA.


But the tester showed 220 mA at that point, so I can’t say exactly what threshold it’s set to. If we go by the measured value, it’s 260; if we go by the value “programmed” in the settings, it’s 220. And since there’s also measurement error, it could be 200 or 250.


For the tests that follow, I put together a simple test setup consisting of a 5 V power supply and an electronic load.


My USB extension cable had a large voltage drop, so in the end I connected the USB testers directly to the power supply.


Since current measurement accuracy is the main factor affecting the measured capacity, I decided to check that parameter in particular. I loaded the device with currents from 100 mA to 3 A, in 100 mA increments up to 1 A and 200 mA increments up to 3 A.
I used the most accurate unit for the test. You can see that its readings are low at first, then high; the point where the readings are most accurate is around 1.6–1.8 A. Later, you’ll understand what I meant by “most accurate.”

But the biggest problem is the nonlinearity. If the ammeter simply read high or low, that could be fixed by adjusting the shunt resistance. With this kind of nonlinearity, though, the only way to fix it is through software calibration. I have no idea how that’s done, or whether it’s done at all.


At a load current of 2.5 A, the device doesn’t get very hot. The shunt heats up the most, and since it’s located near the output connector, some of the heat is dissipated through it.


I tested 15 meters. Each one went through three stages: voltage measurement accuracy (with no load), accuracy at 1 A, and accuracy at 2 A. I chose these values because they’re the most common, for example when charging a smartphone or tablet.
I put the voltage measurements together in one group photo, since on average they all read almost the same: 5.19–5.27 V. That’s a wide spread, but most readings are around 5.22–5.24 V.


Now for the most interesting part: checking current measurement accuracy.
The photos alternate between the units tested at 1 A and 2 A.
To save you squinting at the readings, here’s the short version: at 1 A, the readings range from 0.95 to 1.13 A; at 2 A, from 1.97 to 2.38 A.

This brings me back to “most accurate.” The tester used in the linearity test above gave the best results. As you can imagine, the others are even less accurate.



You’re probably wondering why you should take my word for it. Maybe your Chinese electronic load isn’t working properly.
Fair enough—that’s a valid question, so here’s a comparison with a calibrated multimeter.
The power supply voltage with no load is 5.19 V. As a reminder, most of the testers in this test read 5.22–5.24 V, somewhat higher than the actual value. But since these readings aren’t used to measure capacity, I don’t pay much attention to them.


And now, a comparison of the actual set load current (1 A and 2 A) with the reading from the worst unit. The results speak for themselves.


But I’ve saved the “juiciest” test for last. As I said, these testers are often used to measure the capacity of mobile device batteries. Almost every time, I point out that this is the wrong way to do it: for an accurate test, the battery needs to be connected directly. In one of my reviews, I even did a comparison test. If you’re curious, you can read it here; here I’ll just include a few pictures from it.
Capacity measured by the “doctor”: 2900 mAh


Now let’s connect the battery to the electronic load.


We get 2319 mAh—a substantial difference. That difference can also be almost zero, depending on the tester and the smartphone/tablet.



The tester’s own capacity calculation accuracy also matters. People have tried to convince me that their tester is accurate, but this time I decided to show why I don’t trust these kinds of “measurements.”

So, as they say, “watch closely.”
We take four testers, connect them one after another, reset them, and apply a 2 A load. The first is the tester with the most accurate readings; the other three were picked at random from the pile.
Of course, the load current drawn by the tester itself has an effect, since it has at least a backlight. But the tester’s power consumption is low, so we can ignore it.
But even without doing that, anyone who knows even a little physics can easily see that the first tester should show the highest reading, and the last one the lowest (and the closest to the actual value). That’s clearly not what we see in the photo.

So the testers came up with readings ranging from 1222 to 1333 mAh.


That would be fine—the spread is only 111 mAh, which might even be acceptable for half an hour, i.e. 222 mAh in reality, since you really have to calculate it over a full hour.
Except for one small detail: only 1002 actually passed. I took the screenshot about 5 seconds after the photo, but the timer had already jumped from 29 minutes to 30, as you can see in the screenshot.

So in fact, the last USB tester counted 1283 when the actual figure was 1002 (and even a little less). I used the last tester’s reading because the others don’t affect it.
So, using a tester like this to measure a battery’s capacity can easily give you 3840 mAh instead of 3000—and that’s without even considering the flawed measurement principle behind these “measuring instruments.”


Of course, you may not find this test all that convincing, and you’re probably wondering what math has to do with any of this. Let me explain.
Below is a photo from another test. I didn’t think to take a photo at the time, so I had to pull a few frames from the video.
I ran another test with a load current of 2 A. I set the electronic load’s time limit to 1 hour. As soon as it had counted down that hour—and therefore reached 2000 mAh—it shut off. Those are the readings you can see on its display.
As it warmed up, the USB tester started overestimating the measured current even more, starting at 2.04 A and ending at 2.14 A instead of 2.0 A.
Though that’s not even the scary part. It would have counted 2100 instead of 2000—not just a 1% error, of course, but still tolerable.
But at the end of the test, the display showed 2499. And this is where “Chinese math” comes in. As we know, capacity in mAh is the current in mA delivered over 1 hour. Sounds logical enough.
I got a current of 2100 mA for 1 hour (actually 59 min 25 sec), so by all logic it should have displayed 2100. But how did the Chinese tester get 2100 × 1 = 2500 ???? And this was one of the most accurate units I picked out of 15.


I almost forgot one more thing. The situation is even worse when someone tries to estimate the capacity of the batteries inside a power bank using a tester like this. You really need a drink to make sense of it, but I’ll try to explain anyway.
I saw this picture in the comments quite recently, so I’ll use it to explain. A friend called me not long ago with a similar question after someone else had asked him about it, and I ended up having to explain the whole thing verbally.
You can imagine any other load instead of a smartphone; in this case, it makes no difference.

The problem is that a power bank usually contains a step-up (sometimes step-down) converter. Because of it, the current from the batteries is not the same as the output current.
Let’s say the battery voltage is 4 V, while the output is the standard 5 V. But as you know, energy can’t come from nowhere, so the current before the converter will be higher than after it. With 4 V and 5 V, the difference is a factor of 1.25. In other words, the current from the batteries will be at least 1.25 times higher than the output current—and that’s without accounting for the converter’s efficiency, which is nowhere near 100%.

You connect your tester to the power bank’s output, and it counts, say, 3000 mAh. Multiply that by 1.25 and you get 3750 mAh.
That would be fine, except there are two factors that make any attempt to measure capacity at the output meaningless:
1. The battery voltage changes during the test, so the conversion factor changes too. For example, at 3 V (discharged batteries) it will be 1.66, while at 4 V it will be 1.25.
2. The converter’s efficiency is not only hard to predict (especially when you don’t know what components are inside), but also changes depending on the load current and battery voltage. In other words, efficiency could be anywhere from 95% to 60%—quite a difference, right?

So what does that mean? We have a known capacity at the output, which we need to multiply by an unknown number between 1.25 and 1.66, and then by an unknown efficiency of 60–95%. What do we end up with? I think something close to the weather on Mars on November 20, 2025, at half past one in the afternoon. That’s why capacity must be measured by connecting directly to the battery. And let's not forget that some chargers (Opus, for example) tend to read a little high, so a proper test is a bit more complicated than simply putting a battery in the charger and pressing a button—not to mention the “doctors.”


And here's my favorite photo :)
These testers are from a different seller, but the overall picture is about the same. By the way, you can estimate the testers' current draw from the first and last readings.


I really hope I've clearly demonstrated why USB testers are only suitable for a rough estimate of current and capacity, and not for accurate measurements.
That said, these testers are very convenient in their own right and let you quickly check current, voltage, and capacity, so there's not much to criticize—they're “doing the best they can.”
So how can you use them?
1. For a quick estimate of the parameters listed above—not very accurate, but convenient.
2. For comparative tests. You can get a reasonably accurate idea that, for example, one device has 1.2 times the capacity of another, while another has 1.5 times less. In other words, relative rather than absolute measurements.
3. Recalibrate them to get fairly accurate results. But because of the “skew,” this can only be done for one measured current value. For example, if the readings are 0.9/2.1 A instead of 1/2 A, calibrating at 2 A will give us 0.8/2 A, while calibrating at 1 A could give us 1/2.2 A.
4. Try using more expensive models, but be sure to check them first—and check the “math” in particular, not just the current.

I don't want to generalize about all testers; there may be models that measure correctly, and I'm almost certain there are. But this case shows why I recommend treating measurements from these devices with caution.

Addendum. Not everyone may have spotted the problem. Let me sum it up in one sentence:
The tester showed 2.1 A instead of 2.0 A. But the key error—and the one highlighted in the title—is that the tester calculated the result incorrectly: 2.1 A over one hour gives 2100 mAh, but the tester showed 2499 mAh instead, even though the laws of mathematics and physics say it should have shown 2100 mAh.

That's all. I hope this information turns out to be useful.