How to Protect Any 18650 Cell





Introduction: How to Protect Any 18650 Cell

About: Hi, I'm Tamas (Thomas), a 19 years old Hungarian guy. My hobby started more than 10 years ago. I learn electronics, physics, programming, IoT and I'm sharing my projects with you, hope you like it!

In this instructable I'll show you a very useful life hack that is used to protect any 18650 cell. You can make it only with an old cell phone battery and with some wires. The Lithium ion batteries that are used in phones have a build in protection circuit, that automatically turns off if the voltage of the cell is smaller than 3 volts. This will save the battery from overdischarging. We can use this protection circuit for any Lithium ion cell, like a 18650. The salvaged (from laptop) and the cheap 18650s haven't a protection circuit.

Now go salvage the parts!

Step 1: Tools and Parts

*soldering iron
*wire cutter

*18650 or any Lithium ion cell that you want to protect
*some wires
*an old phone battery

Step 2: Prepare and Salvage the Circuit

Get your cutting tool then try to open up the battery. Be careful, do not short the circuit, may cause fire! Cut the input wires of the circuit then check which is the positive and the negative input. You'll see there three output pins. One is the signal pin and the others are the negaitve and the positive output. You can see the + and - marks on the battery, these marks show you which are the output pins. If you salvaged the circuit go solder to the 18650.

Step 3: Prepare Wires

Prepare four pieces of copper wire.

Step 4: Heat Up Your Soldering Iron!

Easily solder together the circuit and the battery using some wires. Be careful with the polarity and do not short the battery. Do not leave your soldering iron on the battery for much time.

Step 5: Testing

Get your multimeter and test the output voltage. If it,s Ok you're done. You can use now for any project because the cell is protected. That's all.

Hope you liked, write your opinion or other ideas! Good bye!



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    27 Discussions

    A normal mobile lithium battery protection circuit will not hold any longer to a 6000mah 18650 battery..........the chip don't allow that much current through it(the circuit will assume it as short circuit and cuts the supply)
    This can be solved by using 4 or more circuits in one cell........that is connected parallel to the battery.......and the output of each circuit can be merged together and can discharge with higher current...
    Sorry for my poor English.......hope it helps!

    4 replies

    there is no 6000mAh 18650 in the world!!
    most capacity for 18650 single cell is 3500mAh!

    i said there is no "18650" cell with 6000mah capacity.

    other size of lithium ion batteries could be in many other capacities.

    but 18650 cylindrical cell can not be more than 3500mAh

    .and there is a better way to protect a lithium 18650 or a lithium polymer battery using BMS (Battery management circuit)

    ..a BMS can protect the battery from over discharge current protection ,under voltage protection ,over voltage protection...BMS are avaliable as 1s,2s,3s and 4s at a reasonable cost. BMS can have a discharge rate of 10A or higher...Also a normal BMS is only used to protect a cell ,they cant be used to charge the cells ..they may charge but the charge current will be low as below 1amp at 3.6v nearly...u still need a battery balancer to charge multiple cell...but there are BMS with balancing but i have'nt worked with it....

    here's a link for BMS with balance charging

    they cost a bit higher compared to normal BMS

    And i cant seem to edit my previous comment so i'll just post this here..


    It is one of the best ideas on this site I have come across so far... I made it, but it does not seem to protect against anything. Charging current is not limited anywhere below 2A, and cell discharged below 2.6V. I salvaged from an old Samsung battery. I wonder what the circuit does, if not limiting charging current, and cutting off below a save voltage (2.75V lowest?)... Any idea?

    Can I use also the nokia charger, to charge the 18650 with the nokia circuit ?


    2 years ago


    Very good to know, thanks. I'm assuming the protection circuit can only handle fairly low currents?

    6 replies

    No, the circuit don't changes or regulate the current.

    actually i think it regullates
    i tried shorting a nokia batery with and without the circuit
    the spark was smaller on the one with circuit.

    it might work as a charging controller also (havent tried it yet).

    pretty easy to find just ask any mobile repair store for old batteries

    Yes, of course, when you short the circuit stops the current, but don't regulates when you use the battery normal (without shorting).

    goto Page 24

    As you can see the neg Line is interrupted, this was on all Packs i disassembled so far. Notice the 2 Fets are in one Package with 6 or 8 Pins which is bigger than the Protection Ic. The current sensing "Resistor" is simpy a trace on the PCB(wide one!) which stand for some mOhm.

    Notice you cant customize the lower and upper Voltage Limits, and you should not use the Limits on aregular Basis to shut off charge and discharge because they are for safety only.

    This long rectangular Pcb will most likrly not fit into your torchlight. If you have a source for used round ones please tell. The Cellphone ones are easy to get.

    You beter test the used ones for Funktioning, one in five(or so) i found where bad.

    Have a glas of water ready when you dissassemble.

    The cellphone batterys are a good source for thin copper sheet, you better discharge them before opening, use safety goggles and put the opened cel immidiately in water.

    Because of the Cobalt and the Solvent it is not environment friendly, the Lithium doesn't matter.

    Good luck.

    I use 2 18650 strapped with a special rubber band to the back of my old cellphone(2 parallel, original from old Laptop) im a real cheapo.

    One cellphone Battery is soldered to an old mp3 Player whose internal Battery was used up.

    2 18650 parallel with cellphone Protection circuit where soldered to an radio Headphone (original with 2 AAA in series) with 2 Si Diodes in series to have a more reasonable Voltage for the Headphone.

    1 18650 is in my Universal Medion Remote, but be Carefull, the Emitter Resistor of the IR-Led must be increased(Iblew the original one up)

    Thanks mate. I was more thinking about the maximum current you can draw through this if you have a fairly demanding use. I'm guessing mobile phones aren't really heavy users. Still good to know for torches etc. Thanks

    At least 1 Amp. I did it once and can not quite remember. But the roud protectives o 18650 have no bigger Mosfet, they are rated 8 Amps.

    Your Celphone must be able to output 2 Watt RF that account for at least 500 mA. And you should discharge Liion only at 1C rate.

    I was just wondering is it possible to solder the pos. and neg. and possibly maybe the signal straight into a functional phone battery pads ? Without dismantling the charging pcb... I have these wireless headphones that need a fresh 3.7v battery

    18650s are 3.7 volts typically? I believe

    also it should be mentioned to recycle the Nokia battery right? because they are quite dangerous without the protection circuit. Because if I did this I know that I would forget in like a year that the circuit was removed. and then boom.

    I did something like this, only I used an OKR chip to regulate the voltage and wouldn't allow the battery to fire if it would overdischarge. Nice Instructable though! :)

    3 replies

    18650 is the case size in mm. 18 around, 650 in length. The voltage depends on the chemistry. Cobalt and Maganese are 4.2V, LiFPo are 3.65V. That is the recommended charge voltage.