20 August 2014

Solar lamp experiments

My family owns a solar lamp bought at the popular Swedish furniture shop. Given the amount of light it can produce, all night long, I had to take some measurements to understand how much power the same hardware could deliver to a stand-alone circuit.

What am I thinking of? Something like an off-grid sensor that stores data for large amounts of time before any human intervention.

One doubt was: does the cover influence the amount of light reaching solar panels? If so, how much?

The twin solar panel is used to charge a 1.2 V NiMH battery. I decided to measure the current flow through the battery. The experiment was carried on with the help of my daughter: connect crocodile clips, read the ammeter, put the cover on, ...

The discharge current is just above 16 mA, and the lamp with 4x 3 mm white LEDs is very bright. There is a nice step-up circuit in there. That's 19mW out. A fully charged 2000 mAh battery could keep the light on almost 100 hours.

Having never measured a live solar system, I did interesting discoveries.
Without the cover, in full sunshine and oriented right at the Sun, the circuit pushes 100 mA into the battery (120 mW). Put the cover and current goes down to 80 mA. I didn't expect those small solar panels could produce so much current! But that's the optimum condition and requires something to track the Sun: could it be energetically worth?

What happens on a cloudy day? Recharges at 5 mA without cover, down to 3.7 mA with the lamp cover. That's 6 mW instant power, a huge difference from a bright day!

Another interesting discovery about the lamp was that it keeps charging even if the switch is OFF, meaning that you can let the battery top-up when you don't need the light at night.

In the next episode. Considering a 70% charging efficiency for NiMH technology, how much power can the stand-alone device drain for prolonged use? Would it be worth to track the Sun?