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.


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