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 temperature, relative humidity, sun light, etc.. All these times 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 enough but not super duper good. I want a totally hands free solution that does not require me to recharge the battery once per week just to keep things going, therefore, I looked into solar panels, hoping it would bring some light (pun indeed) to this.
A quick search on the China’s biggest 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 search and filter criteria on price, size, sales volume, credibility, speed of delivery, location, etc. Will need some getting used to) 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 to stop working when there is no sun light, for example, in the night time. Instead of powering the weather station directly, I'll use it to charge up a rechargeable battery so that it will continue to power our weather station even when 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 circuitry to properly charge the Li-ion battery. It won't do anyone's good to just connect directly the solar panel output to the Li-ion battery as some of the articles on the Internet showed, this is really a very bad and unsafe practice. A few quick searches on the same Chinese web site yielded 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, providing the necessary battery charging function, overload protection and 5V regulated output.
There are of course the more traditional ones where the battery charging circuit and the 5V DC-DC converter are separate and require you to solder them together using two wires. Those are, however, cheaper but the price difference is only so much, so why go for the less convenient way?
With this charging/discharge board in place, all the components – battery, charging/discharging circuitry board, solar panel, 5V DC-DC regulator – 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 lit 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 then go back to deep sleep.
However, in reality, there could be days without sun shine, or so little sun light that the charging current will just ticker. Therefore, a better solution is still needed.
Using a Li-ion battery bigger capacity might be problematic with this setup because of the relatively small charging current from the solar panel – 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. It is ample enough to cover the energy used by the weather station during the night time in good days but I believe a bigger solar panel is required as well as a bigger capacity battery. Or to further reduce the power consumption of the weather station at the same time.
I will share it here if I did come up with a better solution.
//Ed
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