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Building a solar garden light

Posted by Matthew Little on

This blog post covers my prototyping of a simple solar garden light. I wanted to make a simple kit to convert a jam jar into a solar powered garden light.

These kind of lights are readily available, but they usually contain a nickel-metal-hydride (Ni-MH) battery (usually a 1.2V AAA sized cell). I have seen a lot of these lights which use cheap solar panels and the batteries have failed due to being left for a long time (over-winter) with a discharged cell.

I thought I could use an ultra-capacitor for energy storage, as these can be left for a long time discharged with no issues. I found some 10F (!) 2.7V ultra-capacitors and thought they would work nicely with a small solar panel.

I found the QX5252 solar lamp controller IC, which charges a battery and also provides on/off control and voltage step-up to drive some white LEDs.

I rigged up the circuit following the data sheet, but with my ultra-capacitor in place of the battery. This worked well and, after a few minutes in bright sunshine, was well charged to around 2.1V.

The energy stored in a capacitor is proportional to the capacitance and the voltage applied. The energy stored (E) = 1/2 x C x V² , so at 2 V and 10 F the unit stores around 20 joules, which is 0.0056 Wh - not very much!. Running a single LED at 10 mA takes a power of around 2.8 V x 10 mA = 0.028 W, so this capacitor would run the light for around 0.2 hours, or 12 minutes... not really enough!

It has also been noted that this is the maximum energy stored at that voltage (2 V in this case). As the circuit will not work below 1 V, then the actual energy stored is = 1/2 x C x V², where V is (2 - 1) V = 1V. So the usable energy is just 10 joules....

This can be slightly improved by using a step up circuit (such as a joule thief). There is already a step-up circuit in the IC, which works down to 0.9 V, but a joule thief could work down to around 0.6 V, but adds complexity and still we are energy limited with the capacitor.

I charged the capacitor up and had one LED on the output. This ran for around  18 minutes, which means that one LED is actually using around 5 mA.

So to increase this I either need to reduce the output power (which would mean lower brightness) or increase the capacitance.

Running three 10 F capacitors would give me around 36 minutes of LED output. Still not great & not practical...

So physics has shown that I really need to use a Ni-MH battery for this circuit! Ni-MH cells are available with capacities of around 600-800 mAh. This would give me around 0.72 Wh, which means a fully charged battery would run for 25 hours. Much more like it!

So this prototype was changed to using a 1.2V 600 mAh Ni-MH cell. Not what I wanted to do, but sometimes ideas don't translate into practice too well.


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