Making your solar power station

My house’s balcony does not have a power outlet, but I want to take the weather data from there to feed my Home IoT the information on the temperature, relative humidity, sun light, etc.. All these time my weather station in my balcony (recently upgraded) was living on a 8800mAh Li-ion rechargeable battery pack, and it roughly lasts a bit more than a week before a recharge is needed. Which was good but not good enough. I want a totally hands free solution, that does not require me to recharge the battery once a week just to keep things going, therefore, I look into solar panels, hoping it would bring some light to this.

A quick search on the China’s largest online shop revealed thousands of choices of solar panels, with sizes and power ranged from miniature, mW panels for watches and calculators, to large, kW panels for industrial use. After some painful filtering (they have all those data from price, size, sales volume, credibility, speed of delivery, location, etc. search and filter criteria) process, I have come to select two different panels as an experiment, and this is one of them.

The above solar panel delvers 5V at max 1A according to the data available from the seller and costs a little more than €7, not a bad bargain.

However, even with 5V regulated power from the panel, it will not be enough to use with the weather station, I don’t want it stop working when there is no sun light, for example, in the night time. Instead of powering the weather station directly, we will use it to charge up a rechargeable battery so that it will power our weather station even the sun is not there.

At €1.4 each, I bought a few of these 3.7V rechargeable Li-ion batteries, I hope they will be suitable for use with the weather station as it drew a bit more than 4mW, so it should last about 2 days without the solar panel giving off any power to charge it. More about this later.

You will need a charging circuit to properly charge the Li-ion battery. A few quick searches yield a very interesting board – an all-in-one battery charger with 5V DC-DC voltage switching regulator output board, selling only at €1.7 each, provides the necessary batter charging function, overload protection and 5V regulated output.

All the components – battery, charging circuitry board, solar panel – are in place, it is then just a matter of putting them together and place under the sun for it’s first use.

As seen from the above picture, the red, charging LED lits up when there is enough sun light around, this should supply enough charging current to keep the battery fully charged during the day time.

As mentioned earlier, the 1200mAh battery shall last for approximately 2 days when fully charged. This is because the weather station only wake up from deep sleep every 10 minutes, and spent approximately 10s to read the sensors’ reading, connect to Wi-Fi network and upload the data.

Using a bigger capacity battery might be problematic because of the relatively small charging current – 1A only under direct sun light, and on average, <100mA in a sunny day when not under direct sun light. That is, with more or less 8 hours of indirect sun light, it will never fully charge up even a 1200mAh battery during the day. However, it should be ample enough to cover the energy used by the weather station during the night time.

//Ed