USB battery box EPCTEK 20000mAh

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The box has a unknown number of batteries and two usb outputs.
The specifications are:
I bought the box from Ebay seller: eoutlet_uk



How does it look

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The power bank is delivered in a white cardboard box without any markings.

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It contains the power bank, a usb cable and a manual.

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The two output connectors are placed on one side.

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And the usb mini input connector on the other side.

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On top of the power bank is a switch and a stipe with 4 leds. The leds show charge status.

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The bottom has some labeling about capacity and current.

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I do not know if the box contains 18650 cells or a Lithium polymer pack, but the above picture gives an idea about size and how many 18650 could be fitted.



Measurements

I could not open the box, i.e. there is no measurements directly from the battery, only from the input and output connectors.


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The first test is a load sweep to get an idea about the performance. The two outputs looks identical and can both deliver 1.8A not the rated 2.1A.

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The output looks very fine at 0.5A with about 9000mAh capacity, this is way to low for a 20000mAh box.
Note: Specifications always list battery capacity, output capacity will about 60% compared to that, due to the voltage difference between battery (3.7 volt) and output.

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The output also look fine at 1.6A

3ohm

But when look at the output with an oscillioscope, it does not look that fine. It has 120mV rms noise and 650mVpp noise at 1.6A.

10ohm

At 0.5A the noise is down to 52mV rms and 221mVpp.

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The specifications says the box need 1A when charging, as can be seen on the curve it can use up to 2.5A and the battery capacity looks to be about 15000mAh
It takes about 11 hours to fully charge the power bank with a usb power supply that can deliver 2.5A.

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Adding some series resistance, equivalent to a long cable or a weak charger reduce the charge rate significantly and increases the charge time to 19 hours.



Conclusion

This box has a couple of smaller problems:
It cannot deliver the rated current.
Charging draws lots of current at the start, but reduces charge rate later on.
It has signigicantly less capacity than specified.
It has lot of noise/ripple in the output at higher loads.
Both outputs is coded as DCP (Why is the 2A not coded as Apple 2.1A).

None of these problems prevents the box from being used, but the total makes it considerable less than a ideal power bank.



Notes

Read more about how I test USB power supplies and chargers