A while ago I bought one of those cheap Keweisi USB volt/current testers off Banggood for very little money (
this one).
[Short version]:Cheap, so it won't be particularly accurate, and specs reflect that:
Voltage 3-9 V, +/- 1%
Current 0-3 A, +/- 1%.
At first it seemed to work OK, but the measured values looked a bit dodgy. I decided to have a closer look if the accuracy claimed was actually met in reality.
Turns out, they don't even come close. In reality, they vary quite a lot, up to tens of percents, depending on the values measured. It's absolutely no good at measuring low current, which IMHO, you might actually want to do with a device like this.
If you're looking for a USB tester like this, don't buy this one, try your chances with one of the numerous others.
[Extended version:]Here's what I did:
I took a 5 V / 1 A USB charger and hooked it up to a Maynuo M9812 electronic DC-load through the Keweisi KWS-10VA at no load, 500 mA and 1 A.
No load:Keweisi: 5.18 V / 0.00 A
Maynuo: 5.095 V / 0.0000 A
Difference: 0.085 V / 0.00 A
With no current, there's no voltage drop across the USB wire between USB-tester and DC-load.
According to specs the Keweisi should read within 1% (max. 5.15 V in this setup), which it clearly doesn't. At least the current doesn't show any offset (but it turned out there is one of about 60 mA).
500 mA:Keweisi: 5.04 V / 0.45 A
Maynuo: 4.881 V + 46 mV (= 4.927 V) / 0.4998 A
Difference: 0.113 V / 0.050 A
The voltage drop of 46 mV across the wire is what the DC-load will read too little w.r.t. the Keweisi side of the wire. Obviously, the current is the same everywhere in the loop.
The voltage measurement error has increased a bit, but the error in the current measurement is a whopping 10%!
1 A:Keweisi: 5.01 V / 0.96 A
Maynuo: 4.781 V + 92 mV (= 4.873 V) / 1.0001 A
Difference: 0.137 V / 0.040 A
Again correction for the voltage drop across the wire, now of 92 mV.
The voltage measurement error has increased even more, but the error in the current measurement has now become a "mere" 4%, a lot better than at 500 mA.
I started to suspect that the current measurement has some sort of offset which causes a shift in the measurements that always stays the same.
Because of this I repeated the test with a lowish load of 100 mA. The Keweisi then displays only 0.04 A or 60% below actual value!
Slowly increasing the load in steps of 10 mA from 0 A leads to the Keweisi starting to display 0.01 A at about 60 mA. This confirms that there's likely a fixed offset that's quite large (~40-60 mA). At low currents this obviously has a huge impact on the measured values which gradually becomes less, relatively, as current goes up.
Verification:
The measurements made by the Maynuo M9812 DC-load were double checked with a calibrated Agilent U1232A and they were bang on. The voltage drop across the wire was measured with the same DMM.
The lower voltages at higher loads may be a result of poor regulation of the USB charger and the contact resistance of the USB socket and plug.
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