Teardown and analysis of a solar Powered USB charger

This is a "teardown" and analysis - and then a bit of modification of a 1 watt solar charger with an integrated 2.7 amp-hour 3.6 volt Lithium-Ion cell sold by Harbor Freight as Item Number 68691.

Figure 1:
1 watt solar charger with built-in
2.7 amp-hour Lithium-Ion
cell.
Click on image for a larger version.

The instructions supplied with this unit are rather sparse and only appear on the packaging material - not on any printed matter included with the unit - and, unfortunately, these instructions do not really match the solar charger itself too well.

For starters, the instructions imply that one presses and holds the button for a couple of seconds to enable/disable the USB charger, this being indicated by the illumination of a blue LED along with a beep.  In fact, this doesn't happen:  Pressing the only button on the device simply turns on/off a three-LED flashlight - and that's only if the internal cell isn't fully-discharged.  There is an LED on the side that glows red when charging, changing to green when complete, but that's not really well-documented, either.

Aside for the miserably inaccurate documentation, what about the unit itself?

As it turns out, it does work, but not in the way that is documented.

What's inside:

Popping it open, I did a bit of quick reverse-engineering of the device (See Figure 4, below):

• One section of a 4013 dual CMOS flip-flop is wired to function as the push-on/push-off control device, its output turning on a FET switch for the three white LEDs comprising the flashlight.  It should be noted that as wired, the 4013's state is completely random should power be applied.  What this means is that if the internal Li-Ion cell had been run down so far that its internal protection circuit disconnected it, the flashlight could turn itself on when charge was applied either by solar or external means!
• A small surface-mount switching controller U1, a XC6368, drives a FET switch to function as a voltage up-converter to boost the 2.7-4.2 volts from the LiIon cell to the nominal 5 volts for the USB. connection.  A self-resetting thermal fuse ultimately limits the maximum current that can be drawn by the device being charged.
• U3 is the on-board is a LiIon charge controller chip, a TP4056.  Its job is to disconnect the cell to prevent overcharging (above 4.2 volts) and to "trickle charge" the LiIon cell while its voltage is below about 2.9 volts - both important factors in terms of safety and cell longevity.  Among other things, it limits the maximum charge current - which could come from either the solar panel or the coaxial charging jack - to 0.5 amps as set by R9.
• There is an undocumented coaxial power connector next to the on/off pushbutton switch for the flashlight.  It's difficult to be certain of its precise size, but it appears that a connector with an inside diameter of 1.35mm and an outside diameter of 3.5-3.6 mm will fit.  An IEC type "C" connector is probably the closest "standard" size and the tip (inside) is positive.



Figure 2:
Charger with adapters for various types of devices
Click in image for a larger version.

• There is a mini USB female connector on the charger that mates with a short cable with a coaxial power connector on it (of different size than the one on the unit itself) to which a variety different-sized supplied adapters fit, including a mini-USB, a two different types of micro-USB, and a number of other proprietary telephone connectors.  (No "iPhone" adapter, however.)

In poking around on the board I became curious if/how the voltage booster was disabled if nothing was being charged, especially considering that the device didn't behave as the instructions indicated when it came to starting/stopping charging.  It was soon apparent that there was, in fact, no on/off switch for the 5-volt switching converter and neither did it have any means of "auto-sensing" when something was to be charged, so how much current did it pull from the battery "all of the time"?

The answer to this question is "about 1.5-1.8 milliamps" - and a significant amount of that current is solely from the voltage divider consisting of R1 and R2!

What this means is that at all times, there is a constant drain of around 1.8 milliamps being pulled from the internal cell.  Actually, while a bit annoying, this isn't really too bad since:

• At 1.8 milliamps, it would take over 60 days to run down the 2700 milliamp-hour cell.
• The above would be true only if it was always kept in complete darkness.

Over a 24 hour period, this continuously-running inverter would pull less than 50 milliamp-hours from the internal cell, an amount easily made up if the unit were stored anywhere that it received even indirect sunlight for a couple of hours per day:  Perhaps even standard room lighting would suffice to break even at this level of discharge.

Charging from an external source:

What about charging this thing from an external source rather than from the solar panel?  In reverse-engineering the circuit - and also using the TP4056 data sheet - that one could apply a 5 volt supply (say, from a USB device such as a computer or a charger) to more-quickly charge the unit's internal power cell.

There is one important "gotcha", however:

You cannot apply voltage to the mini-USB connector built into the device to which the charging cable attaches as doing so may damage the device's voltage converter!

In my opinion, the choice of these two connectors is a bit idiotic as the two connectors (the coaxial and USB) on the charger itself are backwards from what they should be as without instructions to the contrary, it would be natural for someone to plug a voltage source from a charger or computer into the mini-USB connector!

I've not timed how long it actually takes to fully charge the internal cell from a depleted state, but my calculations indicate that it could be anywhere between 4 and 12 hours, depending on the current capability of the device that is doing the charging:  Toward the "long" side of this charging time from a computer USB port and on the short side of this for a dedicated, high-current USB wall-charger.  When the LED on the unit turns green, charging is complete and it should be disconnected.

A bit more about the circuits:

Figure 3:
Diagram of the solar charger showing the charge controller and the voltage up-converter circuit.
Click on the image for a larger version.

 

I decided to draw the diagram of the circuitry while I was at it - a simple enough task since this thing is more or less a collection of rudimentary circuits lifted from the various manufacturer's data sheets!  As can be seen in Figure 4 there are a number of surface-mount components and identifying these components can be a bit tricky at times.  U3, the charge controller, is comparatively large so there is room for the part number to be stamped on it.  The two transistors, T1 and T2, are smaller and there's room for only a "marking code", and the same was true of U1.  Fortunately, I could tell by the lack of a series resistor on the outputs of the switching regulator and U2, the flip-flop that they couldn't be bipolar transistors and had to be FETs and that information along with the marking code put into a web search rapidly revealed their true identity!

Posing a bit of challenge was the identity of U1, the switching voltage controller, but this soon fell into place due to its somewhat unusual pinout, that there are only half a dozen manufacturers of 5-pin switching regulators in that particular type of SMD package, and the fact that it was really a 2.7 volt regulator reconfigured for 5 volts using R1 and R2 as a divider:  Why they didn't use the version that was pre-set internally for 5 volts is anyone's guess - maybe they just had a lot of the 2.7 volt versions around!

U1, which is apparently a Torex XC6368A271MR, is a fairly good device with low internal power consumption and designed specifically to drive an outboard switching transistor, including a built-in "soft start" circuit and operating in the 70-100 kHz frequency range. To reduce power consumption slightly, the "Vdd" pin of the regulator is connected to the voltage input rather than the output which would be at a higher voltage (about 5 volts):  Normally, such a circuit would allow the DC current to flow through the inductor and diode to "bootstrap" the operation of the circuit since U1 itself is capable of starting at below 1 volt.

Interestingly, L1's inductance value is 250 uH - a strikingly high inductance value for such a voltage up-converter.  Ideally, the inductance would be much lower for the typical POL (Point-Of-Load) voltage converter - maybe 1/3 or 1/6 of this value (perhaps 47uH) in order to maximize conversion efficiency at higher output currents, but the choice was likely made here to minimize the quiescent current of the voltage converter.  I've not measured the efficiency of this converter, but I have little doubt that it could be made to be significantly better by using a heavier-duty FET (or several FETs in parallel) and more optimal value of an inductor - not to mention an inductor with a higher current rating.  If this were done, the quiescent current would be higher, however!  As it is, the device did a fairly reasonable job of increasing the charge on my Razor phone by over 30% over the period of an hour or two:  Not terribly fast, but it did work.

It would appear that if one wanted to shave nearly 1 milliamp from the converter's quiescent current, R1 and R2 could be rescaled upwards, keeping about the same proportion and, possibly, adding a small capacitor (47pF-100pF) across R1 as compensation to keep it stable:  Why didn't they do that in the first place?

Figure 4:
Inside the solar charger - including the added on/off switch.
Click in the image for a larger version.

To the solar charger, I added an on/off circuit (and indicator) shown in the diagram above, inserted at point "X" consisting of an SPST switch, "Rz", a 10k resistor and "Dz", a high-brightness blue LED.  With the value of Rz calculated to set the LED's current at only a few hundred microamps, its added current load is rather minimal while providing an obvious indication that the inverter is powered up.

The upshot:

How well does it work overall?  It's "OK", but not really great.  There are a number of fine points throughout that incrementally reduce overall usefulness - no doubt to save a bit of cost.  One problem with some phones is that they may attempt to pull too much current from the charger, cause the voltage from the converter to drop below the device's charging threshold and then it would stop charging - only to repeat the cycle over and over.  Unfortunately, when this happens many phones' displays will light up and eat up a large portion of the charging current, the result being that charging may never progress!  If this happens to you, there's probably nothing you can do about it unless there happens to be some way to keep the display from "waking up"!

For charging audio players and most phones, it does work - although a bit slowly.  The 1 watt capacity - which is only accurate for direct sunlight on a clear-sky afternoon - is likely not enough to maintain anything but rather light usage of a telephone over the course of a day, but it is more than enough for the making of the occasional telephone call or putting enough charge on a phone to make an emergency call.

A warning:

Considering its construction - mostly of ABS plastic - one should NOT place a device like this on the dashboard of a car to charge it:  Not only is the temperature in a closed-up car going to be high enough to melt/warp its plastic case, but it will also damage/reduce the life of this - or any - Lithium-Ion cell!

(It's also worth noting that the natural tinting of almost all car windshields is going to be enough to significantly reduce the amount of light reaching the panel and thus slow its charging!)

Anyway, there you are - in case you were wondering!

[End]

This page stolen from ka7oei.blogspot.com