In this article, I will tell and show you how to use 18650 batteries in your projects.
Why exactly they? Having a size slightly more than a AA battery, it has about 2500 mAh capacity, the possibility of multiple charging and a couple of amperes of current. You can buy them by reference. In addition to the usual, there are high-current (20 amps), but they cost more.
The first thing to notice is polarity. On most batteries, it is not written where plus and minus. Plus it is necessary to determine the notch in the photo.
Polarity found. Now we need to somehow fix them. In no case can not just take and solder the wiring to the poles - we ruin the power source. Available options are:
Case (the simplest). It is worth a penny here.
Printed case (if there are a printer and an original project)
Homemade case (when there is nothing, but necessary)
If you really want to solder the wires or you need to fasten the battery from the batteries, you will need spot welding (here you also need the equipment and straight arms).
Here we must understand that soldering the wires to the battery itself will take a few seconds and the high temperature will, at best, reduce the capacity of the battery. And quickly welding, we reduce the risk of problems. And then you can solder the wire to the tape and insulate.
Here we have how to fix them. But you can not just insert the batteries in the circuit, because of the limits of the battery from 2.7 to 4.2 volts. If you charge them to a voltage greater than 4.2V, they can catch fire, when recharging, too. We need a small fee that will protect us from fire. In the picture - TP4056 costs about 0.3$ and safely charges your batteries.
And the second version of the board can control the discharge-charge up to 4.2V and prevent it from discharging below 2.7V.
In the picture: on the left, black and red wires - charging, on the right, black and red - battery, yellow and blue - load.
On the Internet, there are schemes for connecting several batteries for simultaneous charging from one source. The example below is in the picture, there is also a cool instruction manual on Instructables.
If you need more capacity and more current, you can connect them in parallel.
If you want to connect the batteries in series, then you need to take care of using the same batteries and the balancer board, which will protect them from overdischarge and recharge. Previously, they were expensive, but now $ 2 in China.
If you need a voltage that is not a multiple of 3.7 volts, you can use a boost converter. Applying voltage to the input put the multimeter on the output and turn the potentiometer to the desired value.
To create power banks there are special modules. For example, in the picture below. It has two USB outputs for connecting devices and one micro USB input for charging.
There is also a screen with an indication of the charge level and load on the outputs. The cost is about 2$. You can buy here.
I hope the material was useful. Write questions in the comments, I will try to answer all.