My Old Power Bank

I was given a power bank quite some time ago that I’ve used sporadically when I’m away from home to charge my devices. Recently when I went to charge it I found that the connector was intermittant and about to fail. So I decided to take it appart and see if it is repairable.

Taking it appart was fairly easy, I just peeled off the upper label, removed 4 screws and it came appart. Here’s a photo of the board.

The PCB

The board is of a fairly simple design, but there are some notable deficiencies. Firstly you’ll notice the lack of any bulk storage capacitors, but just a few small surface mount ones. The main inductor is the only component on the other side of board, and it is pretty small. So I suspected that this power bank might have severe problems with ripple on the output. I decided to investigate, so I got out my oscilloscope.

Ready to measure

Basically I hooked up a load to the power bank, my phone and my tablet in turn, and used my oscilloscope to measure the waveform on the output pins of the power bank. Here’s a screen grab from the scope showing the waveform with my phone on charge.

Yikes! The ripple is really quite bad and appears to contain higher and lower frequency components. The most disturbing thing is the range of the voltage. 5.84V is greater than the usually 10% tollerance accepted, which could damage the charging circuit of a phone. This is likely because the board lacks any bulk storage capacitors.

Charging my tablet which imposes a larger load only seems to make things much worse with the ripple magnitude and frequency increasing. The voltage peaks at 6.4V which is way to high, understandably I’ve taken this power bank out of service because of how bad this is. I was going to repair the charging problem which turned out to be a faulty input socket, but I guess that’s no longer necessary.

I wondered how the unit would go with some bulk capacitance added, I noted that the Arduino that I have has exactly such capacitance on it’s input circuitry, so I tried connecting it and measuring the ripple. I didn’t get a screen capture, but the ripple became almost flat and settled at a peak of 5.13V. If they had added the bulk capacitance needed, I’m sure the ripple would have been reduced to usable levels.

I’m certainly no expert when it comes to electronics, but I can tell when something is not good. If you’re more interested in power banks I’d suggest you look into reviews and tests of them on Gough Lui’s blog/website. He performs much more detailed tests and has tested a number of power banks over time.