Update 08/01/2016: If you’re looking for full-size multimeters with USB support. There’s a new article on budget DMMs. Of which 2 DMMs have USB data connection support.

The USB port is one of those things in the 21st century that provides the necessary “lifeblood” (electricity) to charge and re-charge all of our fancy cellphones and gadgets. Yet rarely in this process is there a good way to know that things are going well in the power transfer process. Maybe you’ve experienced plugging in your phone to charge and yet for some reason it doesn’t appear to take a charge or perhaps it’s just charging too slowly. And how does one figure out what to do about this? Surely, there is a little gizmo that can help us out here!

And sure enough, there is. In fact, there’s more than just one. There’s a whole bunch of them. They go by the names “charger doctor”, “USB detector”, “USB meter”, and even “USB power monitor”. At the end of the day, they’re mostly just digital multimeters designed for the USB port, but what they do is insightful!

And the best part is that they sell for under $15.

Meet the devices

The devices reviewed here are as follows:

Adafruit Charger Doctor

A few months ago while browsing the ever-awesome Adafruit website, I stumbled upon the “Charger Doctor”. A simple LED read-out displays the voltage and the current drawn from the USB port. The meter displays the voltage for ~1 second and then the amperage for ~1 second and alternates between the two. The port also allows for USB 2.0 data pass-through.

Bestope USB Detector

The Bestope USB Detector appears to be produced by a variety of manufacturers. Or perhaps one company makes all of them and a few different companies buy them and slap their name on it and call it something like “USB Detector” (which is an odd name for a digital USB multimeter.) Fortunately, that’s where the silliness ends.

The design is actually pretty solid. Like the Charger Doctor, the USB Detector has an LED display which has a dual read-out. The top display (in red) shows the voltage, while the bottom display (in blue) shows the current draw.

And to make things better, it has a short flexible cable and two USB output ports, one of which passes the USB 2.0 data through while the other is purely a charging port.

Drok OLED USB Meter

The Drok OLED USB meter is quite a bit different than the rest of the pack. It forgoes the simple 7-segment LED style display for a blue dot-matrix style organic LED (OLED) screen. When the device is first plugged in, the Drok logo appears and then quickly initializes in under a second.

The USB meter displays 4 parameters:

Voltage (V)

Current (I) in Amps

Total Power (P) in Watts

Current drawn (C) in milliAmp-hours

Like the other devices, it also allows for USB 2.0 data pass-through.

There is also a button on the backside of the Drok USB meter which (when pressed by a thin pin or toothpick) will conveniently rotate the display 180 degrees.

PortaPow USB Power monitor v2

The PortaPow USB Power monitor (version 2) is a giant compared to the other devices. Like the USB detector, it has a flexible cord which cleverly flips back onto the device securely when not in use.

When plugged in, the blue back-lighting comes on for about 5 seconds. On the dot-matrix LCD display is the name PORTAPOW at the top, followed by the voltage, current (in amps), and instantaneous power draw (in watts).

Like the others, it allows for USB 2.0 data pass-through.

Examples of using the USB meters

In the process of reviewing what these USB meters were good for, the following situations were encountered:

The faulty USB power jack

In this scenario, the USB power port wasn’t supplying enough voltage. As shown in the screenshot, the output is 4.65v. This is not inside the voltage range of valid USB voltage output (which is typically 4.75 to 5.25v). After wiggling the USB power jack, the voltage bounced back up to 5.05v and the USB device started charging again. Needless to say, the USB power jack was disposed of for being unreliable.

The cellphone that won’t charge or charges too slowly

This is somewhat less common these days, but the experience is that phone’s charge light comes on, but doesn’t seem to be charging very quickly. With the USB meter connected, it’s easy to see that the line voltage is fine, but the USB power supply only provided a maximum of 500mA (0.50A). This makes it easy to realize that the solution is to use a power port that can provide a full 1000mA (1.0A) or more.

In another scenario, the phone wasn’t really drawing any current (less than 0.1A.) The solution turned out to be shutting down the phone, taking out the battery, waiting 10 seconds, reseating the battery, and plugging it back in. The phone started taking a charge at 0.8A again.

Measuring Raspberry Pi or Arduino power use

If you’ve ever used a Raspberry Pi, you know that it runs right off a USB port. With the latest models, the RPi will display a rainbow icon in the upper-right corner of the screen letting you know that your power supply is not really able to supply enough current. This was something experienced when trying to run the Raspberry Pi 2 directly from the USB port on a monitor. It was only able to draw 0.05A which was surprisingly enough to run the Raspberry Pi, but very slowly and unreliably.

With a USB meter, it was trivial to see how much current was being drawn and with an appropriate power supply (shown above drawing 190mA @ 5v), it was easy to measure how much power the Raspberry Pi or something else (like an Arduino) would use.

Determining the power capacity of your portable battery pack

Perhaps you have a portable battery pack like the one shown here. Or maybe you just want to know how much capacity your phone can take. The box will tell you the rated mAh of the battery inside, but perhaps you wish to know how much power the pack can really supply. This is where the Drok meter excels!

It can measure the amount of current consumed over time for a battery pack (measured in milliamp-hours.) For a cell phone, it can show you how much current was drawn to charge it to 100%. With this knowledge, you can determine what percentage of the original capacity of the battery it is still capable of and if you need to replace your cellphone’s battery.

In this test scenario, a Li-ion battery pack with a claimed 4000 mAh 3.7v battery was selected. This equates to approx 14.8 watt-hours. Plugging in the Drok meter and draining the battery pack fully, it measured a total mAh usage of 2266 @ 5v which equals 11.3 watt-hours. About 25% less than advertised.

This result can be interpreted as:

The battery pack starting to lose its capacity (which happens to Li-ion batteries over time.)

The DC-DC boost conversion circuit is less than efficient (75% isn’t bad, but it could be better.)

The Drok meter stated exactly how much charge could be expected out of the battery pack.

Testing the USB meters

Voltage accuracy test

The first test was determining the accuracy of the USB meters. All devices were individually connected to a 5v rail and voltages recorded. Then a digital multi-meter was put on the USB voltage rail as a comparison.

While conducting the tests, the PortaPow exhibited a voltage droop of 0.03v when the LED backlight was turned ON. When the backlight is OFF, there is no voltage droop. The other 3 USB meters did not have this problem.

Result: All meters were within +/- 0.5% deviation from the correct voltage. The Adafruit and the PortaPow were about 0.01v below the digital multi-meter, while the Drok was 0.02v below and the Bestope measured 0.03v below the digital multi-meter which measured exactly 5.00v.

Voltage range test

For this test, a Female USB A port was connected to a benchtop power supply and each device was tested at 3.5v, 5 v, and their highest rate voltage which was 7v for the Adafruit and the PortaPow, 10v for the Bestope, and 13v for the Drok.

All devices worked at 3.5v except for the Drok which would attempt to start up and then fail. (At 3.7v the Drok worked fine.) The Adafruit Charger Doctor worked down to 3.0v, but started giving voltage readings that were off by 10%. The Bestope managed to operate all the way down to 2.5v, but like the Adafruit, it too started giving “high” readings that disagreed greatly from the digital multi-meter.

At 7v, the Drok and the Adafruit device were pretty much spot-on reading 6.98-ish volts, while the Bestope read a highly accurate 6.96v and the PortaPow was giving a low reading of 6.61v (off by ~6%. Yikes!)

At 10v, only the Drok and Bestope were tested since they were only rated for such a voltage. The Drok and the Bestope both reported within a few 1/100ths of a volt of 10v with the Drok being slightly more accurate. In testing the Bestope was able to operate at 10.5v, but the read out stays at 9.99v.

The Drok does indeed go up to 13v, though interestingly enough, as soon as it goes over 13.0v by 1/100th of a volt, the read out shows “—-“. So clearly the Drok is able to go a little over 13v, but the manufacturer chose to inform the user that this was the top end of its range. Not surprisingly, the Drok was fairly accurate showing a reading of 13.0v when at 13.03v.

Also, the Drok has a clever little over/under voltage warning system. When the voltage read is below 4.7v or above 5.3v, a little blinking up or down arrow appears to inform that user that the voltage is outside the safe USB voltage range. This was a really nice touch as none of the other devices had such a feature.

Result: All USB meters were able to operate up to their specification.

NOTE! These USB power testers can operate outside of normal USB voltage range, but they DO NOT perform voltage correction to the proper 5v of a USB rail. Thus, if the USB meter reads more than 5.25v, it can be potentially hazardous to your device if it doesn’t have internal circuitry to handle such a voltage.

Current draw test

This test was conducted by connecting up enough resistive load to draw 0.5A, 1.25A, 2.5A, 3.0A, and 3.5A @ 5 volts. The loads used here were a combination of 4-ohm and 10-ohm resistors which are rated to safely dissipate at least 50w of power.

Resistance Adafruit Bestope Drok PortaPow 10 Ohms 0.42A @ 4.62v 0.49A @ 4.67v 0.46A @ 4.75v 0.42A @ 4.71v 4 Ohms 1.0A @ 4.38v 1.04A @ 4.33v 0.98A @ 4.49v 0.99A @ 4.42v 2 Ohms 1.85A @ 4.11v 1.83A @ 3.92v 1.87A @ 4.16v 1.81A @ 4.15v 1.67 Ohms 2.15A @ 4.05v 2.12A @ 3.81v 2.18A @ 4.08v 2.14A @ 4.02v 1.43 Ohms 2.4A @ 3.93v 2.38A @ 3.70v 2.46A @ 3.97v 2.42A @ 3.95v

As shown in the table, the voltage starts dropping away from 5 volts. Here it shows that 4-ohms draws 1.0A where 1.25A was expected. This was initially a curiosity.

After examining the USB cabling being used, it became apparent that the small gauge wire being used for power (26 or 28 AWG) was causing voltage droop. This voltage drop calculator confirmed the suspicion.

When all the resistors were connected in parallel to create a 1.43-ohm circuit, each device was drawing as much as 2.4A @ 3.9v.

Result: All meters were able to handle up to 2.4A of current. None of them felt even a little warm to the touch.

Technical Review

Conclusion

The Best all-round USB meter is the Drok OLED ($15). Without a doubt, this is the best meter of the bunch. It also costs the most, too. It slips right in-line with your USB port and offers a very clear and detailed read-out of voltage, current, power, and mAh. It tolerated the widest voltage range (3.5v – 13.0v) and it also appears to be highly accurate as well.

Drok also offers a USB meter with the OLED unit in-line with a longer extension cable and a micro-USB port. (not reviewed here.) Presumably, it performs the same as the stand-alone unit.

The Bestope USB Detector is a good alternative ($10) because it is cheaper, has a short flexible cord, and has two USB ports for supplying power. Like the Drok, it has a wide voltage range (3.2v – 10.0v) that it can operate in and was tested as accurate through that range. And while it doesn’t have the power and mAh measurement, it does give a good, clear reading on both voltage and current.

And the rest…

The Adafruit Charger Doctor is still a decent little USB meter for the price ($7.50), but the alternating display of voltage and current is bothersome to many. Maybe you’re okay with this, though.

The PortaPow USB power monitor v2 ($12) is also a neat device. It appears to be highly accurate, yet the voltage droop when the back-light is turned on is disappointing. It’s also kinda bulky and the lack of mAh measurement is a missed opportunity compared to the Drok. Voltage Range testing also showed that its voltage measurement was off by more than 5% when significantly outside of 5v.

PortaPow also makes a much more accurate (and more expensive) USB power meter that has its own internal battery. Unfortunately, it costs $45. It seems to have more measurement functions, but at this price, it doesn’t seem worth it.