Inside an Inexpensive USB "Power Bank" Charger

Update 4/10/2016: I measured the idle current from the battery when there is no load connected to the output.  When first connected, the circuit draws about 330uA (microAmperes).  This drops to about 27uA after a few seconds.  So, that means that a charged cell should last a reasonable amount of time until it's needed.

I purchased a cheap inexpensive USB "Power Bank" on eBay from an "overseas" supplier.  The total cost was $1.39 and included free shipping.  The device requires a 18650 Lithium cell which I purchased separately in a pack of two from a domestic supplier for  $5.00, for a cost of $2.50 per cell.  I had a few problems making the power bank work, but I was able to solve them with a little effort.  First, as shipped, the plastic case is closed, which meant I had to pry it open to insert the 18650 Lithium cell.  The case has molded in snap closures in 6 places, so this took some effort.  The case got a bit nicked up in the process, but nothing too serious.  Second, there was a pressed in dome on the tab that connects to the positive terminal on the battery stuck up too much to allow the cell to be inserted.  So, I removed the circuit board and used a pair of pliers to smash down the dome until the tab was flat.  This worked , in the process, the stiff wire that runs down the negative side and forms the negative terminal came unsoldered from the PCB.  But, I was able to resolver it faire easily.  The only issue was making sure the rotation of the wire was correct to allow the spring end of the wire to line up with and fit back into its slot in the case.  Here a photo that shows the power bank after applying my mods:

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Here are some photos that show the small PCB in more, if somewhat blurry detail:

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With some investigation, I was able to recreate the circuit implemented on the PCB.  It's based around a single chip that's identified as a "Hotchip" HT4921E.  I found a data sheet for the chip but, as it was written in Chinese, I has to use Google Translate to glean any details.  Form this I learned that the circuit as some type of load sensing function that turns on the 5 volt output only when it senses a sufficient load and then switches off when the load is removed.  Before this, the voltage across the output pins reads about 3.7-4 volts, which I suspect is the voltage of the lithium cell.

By trying various resistors between the +5 and Gnd on the output USB connector I determined that a 22 Ohm resistor, or about a 168 mA load seems to switch on the full 5 volt output.  I was able to verify this using an adjustable current load which showed that a load of around 150 mA was needed to switch on the 5 volt upverter.  But, interestingly, once engaged, the upverter seems to stay active as long as the load draws at least 5 mA from the output.  The active state of the output is indicated by the illumination of a Blue LED.  A micro USB jack is provided to change the battery and, when connected to a 5 volts source, the Blue and Red LEDs blink in alternation.  I presume the LEDS changes to some other state when the cell is charged.  Here's my reverse-engineered schematic of the circuit:

From inspection, it appears to be configured as a "boost" configuration switching power supply on the output, as would be expected.  I suspect that resistor R1 is used to set the charge current, but I have not confirmed this, or investigated it in more detail.