DISCLAIMER! Lithium-ion batteries are dangerous and can even explode under certain conditions, such as overcharge, short circuit, etc. Avoid overcharge, overdischarge, overcurrent, short circuits, overheat and mechanical damage.
I bought this power bank kit for around 20 CZK (around 0.8 USD) on Ebay. Like the previous power bank kit I reviewed, it seems to be decently built and works fairly well for such a low price, and it takes regular rechargeable unprotected 18650 lithium-ion batteries (3.6 or 3.7 volt nominal voltage). These are quite commonly used in laptop battery packs, etc.
The kit consists of a simple (in this case black) plastic case, which can be closed with a white piece of plastic (that is held in place by plastic clips - can be snapped on and carefully removed with a knife or a small flat head screwdriver).
The assembled kit (click for full resolution)
The assembled kit - another image (click for full resolution)
Opened, with a battery installed (click for full resolution)
Reverse engineered schematic (click for full resolution)
The little circuit board is based on the VD4308 chip. I haven't managed to find any datasheet for this chip. There also is a 3.3µH inductor and a few other passive components.
I measured a charging current of around 750mA. The power bank's output voltage with no load is around 5.2-5.3V and drops to around 4.8-4.9V with a 1A load, but might get even a bit lower (depending on state of charge). It heats up (can be felt, the connectors get a bit warm) under such load, but the heating isn't excessive
Using an oscilloscope, I measured the ripple to be at around 100mV peak to peak with a 10 ohm (500mA at 5V) load. The power bank's operating frequency is ~500kHz.
The powerbank terminates charge at almost exactly 4.2 volts. Short circuit protection (output goes off until load is disconnected and reconnected) and undervoltage protection both seem to be properly functional.
There are 2 indicator LEDs - a red one and a blue one. The blue one lights up when a load is connected. The red one indicates charging (blinking when charging, continuously lit when charged). The quiescent current draw with no LEDs lit is about 20uA, which is a very good value.
The power bank sometimes shuts off with low load (<50mA, e.g. only my Charger Doctor connected) for no reason and stays off until load is disconnected and reconnected, but works fine with my phone, Arduino Uno or anything else that draws more than about 50mA.
A functional single 18650 battery powered power bank kit for a low price. It's a pity it has problems with low loads.