Hi, I am quite new to using batteries to power my Arduino, and I hope you can help me.
So, I have this project that requires small and lightweight components, but I am having trouble looking for a very small battery. Then I found this thing (https://www.aliexpress.com/i/4000977457385.html) which is used for TWS earbuds.
Can I use this to power my pro micro by connecting them to a series and use a charging module to charge them?
Thanks!
Simple answer is NO
It's rated for 25mA-hours. I don't know how much current the Pro Micro uses but from what I remember my Arduino Uno measured about 60mA so that battery wouldn't last a half-hour (with the Arduino).
And if you're powering anything else you'll be consuming more current.
And it gets a little "fuzzy" because I the mA-hours are rated for something like 75% of the rated voltage. The 3.7V batterwould be about 2.8V and I'm not sure if your Arduino will run with 2.8V.
,,,And I don't fully-trust stuff sold by AliExpress, or eBay, etc. 
I use lithium batteries to power Arduino all the time. It would be possible to use these, but it would take some extra work.
First off, you would need a couple of these wired together in parallel in order to provide sufficient current. For reference, an Arduino nano has a maximum current draw of around 200mA, so to provide the maximum amount of current that you would need to power that for one hour would require (8) of these batteries wired together in parallel. If your particular Arduino isn't using a ton of I/O and doesn't need to power any other circuit components then you may be able to get away with something as small as just one or two of these in parallel; it really depends on what you need to power. So you should draw out the circuit and determine how much total current you will need, figure out how long you need the circuit to run, then calculate how many mAh you need. Each of these batteries is about 25 mAh, meaning that it can provide 25 mA for approximately 1 hour. If you connect two of them in parallel then it can provide 50 mA for one hour, or 25 mA for two hours.
Next, all lithium batteries provide an output voltage between 4.2V MAX to 2.8V MIN. You will need to boost the battery voltage up to 5 VDC. There are some pretty simple chips that allow you to do this, for example https://www.digikey.com/en/products/detail/richtek-usa-inc/RT9361AGE/2547144 will boost the lithium battery voltage up to a regulated 5V. However, it is a surface mount part which may be hard to hand solder, and it will also require a couple of extra capacitors to make the chip work.
So, you can make these batteries work, but you will probably end up connecting somewhere between 4-8 batteries together in parallel. You will also need to include some additional components to get a regulated 5V output. My advice would be to find a battery that provides more mAh in a single cell, rather than using many of these tiny cells together. Just get a single battery that provides something like 400-500 mAh and then add the additional circuitry to boost the voltage. Don't forget to design a battery charger in to the circuit as well so that you can recharge the battery. Hope this helps!
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