This project started when I found a very cheap solar power bank on Aliexpress. What really caught my eye was the claimed capacity of the power bank, 30000mAh. This sounded too good to be true, so I ordered one in order to test the capacity.
This is what I received in the mail after a couple of weeks:
The proverb "You get what you pay for" is very true in this case. The powerbank looks poorly built and feels fragile. I wouldn't rely on this power bank when going hiking, as it would probably break if dropped once. Additionally, the front panel doesn't have any kind of protection against impact or water.
The front panel has two USB 5V-outputs for charging mobile devices, and a micro USB 5V-input for wired charging of the power bank. The two outputs are rated at 1A and 2A.
Power is turned on by a momentary push button switch on the front panel. Battery level is indicated by four blue LEDs with 25% increments.There is also a white LED flashlight on the front panel, which is activated by pressing (and holding) the on/off button.
The backside of the powerbank has the claimed specification of the power bank printed on it.
Behind the back panel I found a circuit board and three Li-Po battery packs connected in parallell. All battery packs were duct taped between the solar panel and the back panel. Not the best way to fasten Li-Polymer batteries.
One of the batteries had its specification printed on it. 3.7V, 1500mAh.
The circuit board consists of a ETA9640 charge controller with built-in boost converter, an unlabled microcontroller, SS14 barrier rectifier for overvoltage protection, and a TXY8205 MOSFET, which is possibly used for turning the solar panel on/off. Since we're dealing with Li-Po batteries, I'm happy to see they put some protection circuitry in this unit.
Only one of the batteries had its capacity printed on it, 1500mAh. Assuming all three battery packs have the same capacity,
the total capacity of this powerbank is 4500mAh, which is pretty far from the claimed capacity of 30000mAh. Next step is
to test the actual capacity of this unit. A constant current will be drawn from the unit through a resistor, and the
voltage and current will be monitored with multimeters.
I discharged the powerbank at a constant current of roughly 650mA, and after 6 hours the power bank was completely drained. The resulting capacity was calculated from the gathered data, and the true capacity is approximately 3900mAh, 13% lower than the assumption I made earlier, and 87% lower than the claimed capacity.
I found the LEDs to be a waste of power, especially with these LEDs blinking during the charging process. This could be a problem when charging the power bank in low light conditions, as the LEDs would waste some of the power produced by the solar panel. I measured the current through one of the LEDs, about 0.5mA, not so bad, but they are way brighter than necessary, so I soldered a 22k resistor in series with each LED. With two resistors in place, the difference is very clear.
The resulting current with the added resistors is roughly 150ľA through each LED.Back
last edited: 2016.02.22