Test of a cheap power bank

I got a small aluminium power bank (Figure 1) as a corporate gift. It is one of those 18650 single-cell power banks you can buy on eBay for about 5 bucks. How good are these? Do they meet their specs? Well, let's see.

Figure 1: Power bank under test with a tastefully pixelated logo of the company I got this gadget from

Experimental part
The power bank was fully charged and then a 10cm cable with stripped wires on one end and a USB-A connector on the other was used to connect the power bank to a MightyWatt R3 electronic load (24 A/3 A, 30 V/5.5 V range).
A constant current of 0.5 A was drawn from the power bank until it switched itself off. Charge was calculated by integrating the measured current over time and total delivered energy was calculated by integrating the product of measured current and voltage (i. e. power) over time. The voltage drop across the cable was compensated for by measuring the cable resistance and increasing the voltage used for calculation of power by the product of current and cable resistance.

Results and discussion
The voltage from the power bank varied between 4.85 V and 4.88 V, which means the regulator kept a very stable voltage throughout the discharge and provided voltage high enough to charge devices connected to the power bank.
The power bank shot itself down after 2 hours, 14 minutes and 45 seconds after delivering 5.33 Wh of energy and showing a useful capacity of 1.12 Ah at the output voltage, which corresponds to 1.48 Ah at the nominal 3.7 V of the internal battery, not including losses at the internal boost converter. The stated capacity was 2.2 Ah (Figure 2), which means the power bank failed to deliver what it promised and was only able to deliver about 67 % of the stated capacity. Using a ballpark figure of 90 % boost converter efficiency, the power bank delivered 75 % of the stated capacity. Considering the unit was brand new, it is a rip off.

Figure 2: Power bank specifications showing the rated capacity in wrong units (mA instead of mAh)

Cheap power bank failed to deliver what it promised on the packaging. You can expect about 2/3 of the stated capacity to be really available. That being said, it is actually usable if you take the stated capacity with a pinch of salt and you don't mind that the device does not have any indication of the remaining capacity.


Crappy USB cable under test

What is in this test (Introduction)

This is a test of power handling capability of a really cheap USB cable. How cheap? I don't know because I found this cable in my box of USB cables. I probably got this bundled with some device. What you will find inside this test? A (not so) shocking revelation that chinese manufacturers are able to make a cable with virtually no copper at all.

I wasn't really going to test the cable in the first place. I was actually going to test a cheap powerbank I got as a corporate gift. You know, one of those small powerbanks that have a single 18650 cell inside and a boost converter ending in a USB port. And for testing, I needed a cable so I could connect it to my MightyWatt R3 electronic load. So I cut the cable, saving only the USB-A connector end with a few centimetres of cable. Inside, I found four tiny conductors that had barely any metal in them.

How I tested the cable (Experimental part)

The USB-B connector was cut from the remaining part of the cable, which was subsequently used for the experiment. The length of the cable for experiment was measured using a tape measure. Both ends of the crappy cable under test (CCUT) were stripped and the foil and braided shielding was removed (Figure 1). Using the removed part with the USB-A connector, the power conductors were identified to be red (VBUS) and grey (GND). These conductors were stripped to bare copper and twisted together at one end, forming a single conductor.

Figure 1: Crappy USB cable and its internal conductors. Red wire – VBUS, gray wire – GND. Green and white wire – differential data pair.

A power bank was used as the power supply. A MightyWatt R3 electronic load (24A/3A, 30V/5.5V ranges) was used as the measurement device.

The positive terminal of the power bank was connected to one end of CCUT, the negative terminal was connected to negative (PWR-) terminal of the load. The other end of CCUT was connected to positive (PWR+) terminal of the load. The voltage-measuring (sense) terminals of the load were connected at the ends of CCUT to measure the voltage drop across it (Figure 2).

Figure 2: Measurement setup showing MightyWatt R3 electronic load, power bank and CCUT

A 30-second linear sweep from 0 to 0.5 A was applied by the load followed by 60-second constant current at 0.5 A. Voltage drop across CCUT was measured as well as the applied current. The resistance was calculated as a ratio of voltage drop and applied current at the beginning of the 60-second constant current phase. The ambient temperature was 22 °C. Approximate cable gauge was estimated from the experimental data using a table on Wikipedia.


So how bad was it? (Results and discussion)

Pretty bad…
The total resistance was 3.7 Ω and slowly increasing as the cable was heating up. The length of the cable was 1.55 m so back and forth 3.10 m. That means the specific resistance was a solid 1.19 Ω/m, which is horrible. The wire gauge was approximately 36 AWG and its cross-section around 0.0127 mm2, which can be safely rounded to zero :-) If you really put 0.5 A through this cable, you would get 1.85 V drop, which would probably cause any device supplied from that cable not working.
On the bright side, MightyWatt R3 turned out to be great at such an experiment. The accuracy within 1 % was available when the current was a meager 1/180 of the used range (Figure 3).

Figure 3: Relative measurement error as a function of current range. Accuracy better than 1 % starts at approximately 1/180 of the current range. Heating causing the increased resistance is visible as the vertical slope at the right end of the curve.


The bottom line (Conclusions)

Crappy cables are crappy. And at the USB 2.0 maximum current may be totally unusable.
May this be a cautionary tale and also an inspiration how to use an electronic load to measure stuff!