I want to make sure I am being as safe as possible. I have 2 18650s wired in parallel and 3 of those pairs wired in series with a 3 cell BMS board.

I also have a constant current driver at 700 mA with a fuse that breaks with current more than 1 A, that is overall powering some decently bright LEDs.

I am making a flashlight for someone and I want to make sure I am absolutely being as safe as possible; I don't want the batteries to blow up on them or anything like that.

I know that when wiring the batteries in series/parallel it's essential to have the voltages the same to prevent backflow. If their voltage is off by 0.1 V is that still acceptable? I know the BMS baord should help prevent over-discharge and such, but is there anything more I should know to be safe? Could there be any danger with the batteries leaving them wired together constantly, even when not in use?

I apologize if my questions are dumb, I've tried to do the research to be safe; I just want to make sure that I'm not missing anything and that there are no concerns and I won't end up putting anyone at risk. I can attempt to draw the full circuit if that would make it easier to provide feedback.

  • \$\begingroup\$ Can you draw a schematic or block diagram? \$\endgroup\$
    – winny
    Jul 13, 2018 at 18:14
  • \$\begingroup\$ You are on the right track. If you connect the same technology batteries in parallel, then there is no terminal difference between them. Any balancing current is a one off and not dangerous providing they are nominally at the same state of charge. The BMS will handle differences when you connect the cell pairs in series. \$\endgroup\$ Jul 13, 2018 at 18:25
  • \$\begingroup\$ Battery University has information on Li-ion safety and charging. this is a link to one of many topics: batteryuniversity.com/learn/article/safety_concerns_with_li_ion \$\endgroup\$ Jul 13, 2018 at 20:41
  • 1
    \$\begingroup\$ I am not comfortable the way you mention having a constant current source. Charging a Li-ion requires both constant current and constant voltage charging. Li-ion is dangerous. An Li-ion charger is not a hobby project. Obviously you do not have the requite knowledge or background experience to take on such a project. An 18650 is not a Li-po. \$\endgroup\$ Jul 13, 2018 at 20:49
  • \$\begingroup\$ The constant current driver is only for powering the LED, not for charging, right? \$\endgroup\$
    – datenheim
    Dec 25, 2022 at 18:42

2 Answers 2


I am making a flashlight for someone and I want to make sure I am absolutely being as safe as possible; I don't want the batteries to blow up on them or anything like that.

Do you hate this someone or like them?

You obviously do not have the requite background to make an Li-ion charger.

If you were doing it for yourself, I'd still recommend you not do it.

But making a charger for someone else is a definite no go.

BUY A FLASHLIGHT. I highly recommend Coast flashlights sold at Home Depot.
Link: Home Depot, Coast Flashlights

If you wanted to to hurt this someone, Coast makes knives too.

If you have a strong desire to make a flashlight use NiMH. What LED driver did you plan to use?


Hey Jordan main thing that creates danger with Li-ion batteries is over stressing them, you'll be running a 2S3P pack depending on the capacity of your particular 18650's you can calculate how much power the pack can deliver using Ohm's law. a typical pack like that can deliver 2-4 Amps 700mA will be a breeze. however there are few other things which to consider for example I personally I steer away from BMS's in low consumption devices because they are just useless and consume energy balancing tiny differences between the packs. With low consumption devices you'll never be more than few mili Volts off unless you over-under charge/discharge, Stress them or some are defected.

That brings up another important factor, are these new or recycled 18650's? nothing wrong with using old ones even if they are from different manufactures as long as you measure their actual capacity and they are within 10-50mA/h. If want to make this torch safe and not going to try to boost your friends bike, car, tractor, tank... with it Lol. I would suggest scraping the pointless BMS they are overrated and only beneficial in high power consuming applications.

Search instead for something like 1-8S Lipo Battery Tester Low Voltage Buzzer Alarm they cost less than £1.00 they show you what the voltages are, buzz if Packs differ too much or are running low, you can connect them on a switch and monitor the pack with clear numbers on the display. You'll learn more about their behavior and be more confident on your next project. I'm sure your friend will appreciate the ability to see what the accrual status of charge is, you don't get that with a BMS.

That is what i prefer especially when using dodgy used 18650.

few more quick pointers

if Li-on's are getting hot when charging/discharging with rated current they most likely are developing a partial internal short which will eventually become dead short and is dangerous or fun if in a controlled environment done in the name of science :)

if you charge the cells then leave them not connected to anything for few days the voltage should not change more than 2-3mV (0.03V) if it does they have developed the same condition as above.

the last one is common sense. These batteries are NOT some unstable explosives but do pack a lot of power for their size and if abused will fail usually with a smoke show.

If you are still reading this i hope you found some useful info.

Warning machines don't have brains, use your own when interacting


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