Let's begin with the introduction of a rechargeable battery [1]. It's a type of electrical battery (storage battery, secondary cell, or accumulator) which can be charged, discharged into a load and recharged many times, as opposed to a disposable or primary battery (which is supplied fully charged and discarded after use). It's composed of one or more electrochemical cells. The term "accumulator" is used as it accumulates and stores energy through a reversible electrochemical reaction. Rechargeable batteries are produced in many different shapes and sizes, ranging from button cells to megawatt systems connected to stabilise an electrical distribution network. Several different combinations of electrode materials and electrolytes are used, including:

Rechargeable batteries typically initially cost more than disposable batteries, but they have a much lower total cost of ownership and environmental impact, as they can be recharged inexpensively many times before they need to be replaced. Some rechargeable battery types are available in the same sizes and voltages as disposable types, and can be used interchangeably with them.

Nowadays, there are many stable rechargeable batteries (e.g. 18650) that you can buy for your own IoT project, especially if you are thinking of having the device placed somewhere remotely. These devices have important parameters to be fulfilled:

Following these parameters, rechargeable batteries are a quite important element in IoT device design, since it is the "food" for device operations. Thanks to Jennifer Jarett from Top Ten Reviews [2], I will name several benefits that may turn your attention toward rechargeable batteries:

Having listed different benefits and important parameters of rechargeable batteries, now we can turn to a short brief of solar panels, the remote type of charger for IoT devices. 

Many scientists, such as N. Tesla, A. Einstein, stated that there is incessant amount of energy in the universe for all our applications. I will bring an attention herein on solar panels, which are a low-cost charger solution to power your IoT devices for years with a single battery. As I've mentioned the important parameters for rechargeable batteries in the previous chapter, let's name these for solar panels:

Today, we can see many devices equipped with some form of photo-voltaic (PV) panels, i.e. solar panels. Even if a manufacturer state that his device may last around 3 years with their supposed work regime, that can be shorter due to the measurement frequency which directly impacts the battery life. Who would use "only" the regime that the manufacturer set up for projected applications? Especially in IoT world, people like to play with different measurement intervals, data formats, device stability, etc. So, if there is no place for more batteries inside of the device, changing the battery would require too much cost and time. Therefore, adding a solar panel on top of the existing solution is a great way to go! 

Since I have built the project using MKRFox 1200 and temperature/humidity sensor, all powered on 2x AA batteries, it couldn't endure more than 10 days, although the sleep mode of MCU was also programmed. For me, even using rechargeable batteries, the current solution meant that I should take off the box from the post/wall, open it, remove the batteries, charge them, put them back and you know the rest. This isn't a convenient undertake to perform, right? Therefore, I took a simple battery charger with JST connector that will regulate the input and output voltage to charge the battery. 

For my case, i have used:

After connecting components into the circuit, I was ready to test this new solution. And it showed promising results! Putting the solar panel on direct sunlight (semi-cloudy day) gave me between 5 - 6 V of input voltage that will go to the charger regulator. Also, on the same sunlight, I got somewhere around 120 mA of current that will charge the battery. Let me simplify this: if I get around 100 mA of current during the day, and my battery (which is 5200 mAh) is now drained to 4000 mAh, I'll need around 12 hours to fully charge the battery. Of course, that is not possible within one day, but if the solar panel has a good sunlight for the whole week, I don't have to worry that my battery will be drained.
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It is vivid that when solar panel is exposed to the direct sunlight, the charger is ready to charge Li-Ion battery (green LED is ON), but when there is no good amount of sunlight, charger is in idle mode (red LED is ON). Lastly, when there is a low amount of sunlight or not at all, charger goes to shutdown mode (LED is OFF).

I believe there is a good amount of information within this article why you should consider implementing solar panel and rechargeable battery in your IoT device from the very start. Firstly, it will save you time and money within tricky embedding into the existing solution (which always have some hidden issues) and will prolong your device operation life cycle significantly and secondly, you'll have a device that is a good showcase of your idea!
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Think about the environment, as well. Using energy harvesting solutions such as this one is a vital sign that you are holding the ground about climate changes and maybe trying to make some impact? I am fully into that and will be more than happy if you prototype your devices thinking about others and their well-being. At the end of the day, it will be "reimbursed" back to you in some other form. ;)