Solar Rechargeable Pocket Flashlight Teardown

I got this little key ring torch from somewhere I can't remember. I think it cost me a USD or two. It worked well as a torch for a while, but eventually gave up the ghost. It is supposedly solar rechargeable, so lets open it up and see what is on the inside.

The device was glued together, so I couldn't save the enclosure. The inside revealed a very simple circuit. I thought shell was rather well engineered, to good tolerances.

There isn't too much to the electronics, there is a coin cell battery, three LEDs (which I know to be white), a button, and a diode. It is a single sided PCB that is glued a soldered together with a small solar cell panel. There is a mix of through-hole and surface mount components. (The PCB is even has a green solder stop and a white silk screen!)

Lets take a closer look at the components, starting with the battery. I measured the  voltage to 104.5 mV, or 0.1045 V, which means this battery is pretty much dead. The battery was shrink wrapped with solder tabs. Removing the solder tabs and the wrap revealed that this was a Lithium Ion Battery of LIR2032, with a nominal voltage of 3.6V. LIR2032 refers to the battery type, in this case a Lithium Ion Rechargeable and 2032 is its Height and Diameter. So I guess the torch could be rechargeable? Lets dig deeper.

I quickly inspected the (assumed) diode, and measured a forward bias voltage drop of 0.635 V. The diode is there to prevent reverse current into the solar panel, when the panel is not illuminated. Reverse current would damage the panel.

Taking a closer look at the little solar panel, I measured the open circuit voltage to be in the range of 6V under a good desk lamp. I measured the current to be 0.6 mA. The voltage was measured over the now removed battery tabs, and the current across the button. Since the voltage was measured across the solder tabs of the battery, the true voltage of the solar panel is a little higher due to the voltage drop across the diode.

The little button is a surface mount device. Nothing special about that one.

The LEDs mounted on the PCB are of thought hole type, and are white. They are connected in parallel. A typical white LED consumes 20 mA of current at 3.5 V.

Bringing it all together in a nice schematic was simple enough, because of the (extremely) low component count and single sided PCB.

Now, there aren't current limiting resistors for the LEDs, which I guess is to save on component and manufacturing cost. The battery voltage is pretty well matched, but still an LED does not behave according to Ohm's law, so the LEDs could burn out by taking in to much current. (On the other hand, there is a diode to protect the solar panel, So I guess some considerations were made?)

But what really killed this device was the charging circuit. Charging a lithium ion battery requires tight control of the current flowing into the battery, the charging voltage, and the battery temperature. The curve below is for a standard charging characteristics of a LIR2032 battery (0.5 CmA are for normal charge). I guess the solar panel is somewhat within the range, having a current of 0.6 mA and an open circuit voltage of 6 V. There is also no under-voltage or over-voltage protection, meaning you can drain or over-charge the battery to destruction.

In conclusion, it was a mechanical nicely designed little torch. The solar rechargeable aspect of the device is however just a gimmick. I don't think the electronics would do the battery any good, and maybe the thing would have lasted longer had the battery not been connected to the solar panel, and used a standard non-rechargeable battery. Still, it is impressive to get these components (solar panel, rechargeable battery, 3x white LEDs) for 1-2 USD...