18650 lithium ion battery is a star of rechargeable ....
but she needs a bodyguard, a BMS (battery management system)
able to cut off the current under load after the 4.2 volt (risk of explosion) and stop the discharge around 2.8 volt (risk of permanent destruction of the battery) and control the amount of available energy (amperage) during the use. I build a battery pack (3) connected in series to get 12 volt, 2A minimum (available in peak, 4 or 5 A ... depending on the batteries) The BMS also controls the state of each battery (to avoid potential differences of potential and resistence), in case that a battery becomes weak and overheating with risk of explosion, the BMS goes off. Yes, the 18650 are like princess and are very delicate in use but very effective if you take good care of it.
Step 1: Stuff
a bms 3S (3S is for 3 batteries serial connection) 70 cent on aliexpress
a led indicator of charge for BMS 3S (12 volts)
Support box for 18650 batteries wit wires(use with big wires copper if Amperage is important)
a switch (switch on/off the led indicator)
soldering material,thermoplastic protection to build the pack,enclosure about your usage,copper wires
sure also three 18650 batteries with good quality and sufficiently Amperage....(3000 mhA or above )
Step 2: Connections Wires Batteries/BMS
connect first three enclosures 18650 in serial mode with supplementary wires....(dont cut the original tiny wires)
solder the negative wire at the "0V" on the BMS
solder the positive wire at the "12.6V" on the BMS
connect the tiny wire from the middle of the first and second batterie at the "4.2V" on the BMS
connect the tiny wire from the middle of the second and thirdy batteries at the "8.4V" on the BMS
Solder two wires black and red respectively ground an positive on the BMS (power);this connection is the same
door for have energy and charge the pack batteries(not at the same time)...
if you want a led indicator of charge,connect the positive and negative of led indicator directly on negative and positive on BMS (point "0V" and point "12.6V),add a switch for commutate leds on or of
Insert batteries and ceck connectivity and all wires..... and dont worry if your new battery pack dont work,is normal......
In fact,normally,all BMS have a protection due to the high energy present on all 18650 battery and risk of damage
YOU CAN WORK witout problems(soldering and connections) and when finish connections and ceck, you only need to put power wires under charger and voilà,finish....(this process actvate the BMS)...
place in adequate enclosure to preserve batteries and electronic circuit,ready to use for all your project.....
Thank's to watch