I'm working on a project to make my own super-bright bike taillight using an emergency flasher LED (something like this:)

LED flasher

The LED package requires a 12V input, but from what I've read from other people who've done this before, it will still work with as little as about 8V. It also uses a 12V momentary charge to switch flash patterns.

I'm planning to use 3 18650 lithium-ion batteries in series to power the light. Each battery is 3.7V, so I should get 11.1 V total, which will (supposedly) be sufficient. I'll put them in a 3-battery holder like this one:

3x 18650 holder

This should be easy enough to set up, but I'm wondering how I should go about charging the batteries. I haven't been able to find 18650 chargers specifically designed for 3 batteries. I've found them for 1, 2, and 4 batteries only. My (limited) understanding of lithium batteries is that all batteries that will be used in a single device should be charged together so that the cells can be balanced, and that if this isn't done, it poses a fire risk. I'd like to avoid having a fire erupt between my legs while barreling down a hill at 30 mph.

Would it be safe to use three 1-battery 18650 chargers, if I charge them all for the same time? Do they actually need to be balanced at all, or am I misunderstanding something? If they do all need to be charged together for safety purposes, then is there any way for me wire it up to get 12V out of 2 or 4 batteries, which I would then be able to charge normally?

  • \$\begingroup\$ Be aware that a "super bright" taillight might be illegal. You want to be visible to drivers, but not distracting. I don't know, but I'd expect there are legal limits for such things, for good reason. \$\endgroup\$ Dec 27, 2011 at 13:41
  • \$\begingroup\$ @OlinLathrop thanks for the reminder. I'm making sure to check local laws before I put it together, but so far, nothing references a maximum brightness, just a minimum distance lights must be visible from. The light model I'm looking at has a method to dim it as well, so I can turn down the brightness when I'm directly in front of a vehicle and might be irritating them. \$\endgroup\$
    – nhinkle
    Dec 30, 2011 at 2:19
  • 1
    \$\begingroup\$ +1 for a well articulated question. Was looking for something quite similar, and this question was so spot-on. \$\endgroup\$
    – bdutta74
    Jan 29, 2013 at 7:28

2 Answers 2


Read my just posted answer to this question. While not identical it covers aspects which will answer some of your questions.

3 x 18650 LiIons (or any 3 LiIons) will have a fully charged voltage of 3 x 4.2V = 12.6V and a fully discharged voltage of ABOUT 3 x 3 = 9V. How low low goes is up to you. Too low and battery dies.

Read my answer above re balancing. It is not NECESSARY as long as you are CERTAIN that no cell is ever deep discharged AND if charging in series, as long as no cell is in constant voltage tail off mode while you are attempting to inject full constant current at 1C. 'Attempting to" period may be short.

IF you charge this off the bike and if all 3 cells are isolated from the world (but connected to each other) then my answers above re charging one at a time apply. You can charge 3 at a time with 3 chargers ** as long as** all charger outputs are truly isolated.

An easy way to get 12V is to use one of the many many available switch mode power supplies. You can get 1 or 2 or 3 cell LiIon to 12V capable supplies.

An 18650 LiIon cell is has a capacity of about 2000 mAH x 3.6V nominal =~~ 7 Watt hours. IF your flasher worked at 1 Watt average and was anything like serious it would blow following motorists off the road. Depends on design. 1 Watt at 10% duty cycle = 10 Watts when one. 1 Watt at 1% duty cycle = 100 Watts when on. Properly collimated a 1 att red LED willl do a very very very very good job. So a single 18650 cell with inverter of say 7% efficient (low) will run for 7 Wh/1 Watt x 70% = 5 hours. Ample for most people.


  • OK, so some clarifying questions.
    1) how can I be certain that no cell is ever "deep discharged"?

No cell ever under 3 Volt.

  • Monitor voltage and prevent this happening


  • Never discharge beyond known capacity to ensure this is true.

Murphy says you will fail if you take the 2nd choice.

  • 2) if I'm using a COTS charger (and charging each cell separately), what do you mean by the charger outputs must be truly isolated? If I'm using a physically separate charger for each cell,

If the cells are not connected in any way this is irrelevant.
This is an issue only if the cells are connected as in a battery holder.

Outputs are fully isolated from the charger input.
If you operate 2 chargers from mains simultaneously you must get no sensible voltage readings when measuring from eg V+ out of one to V+ out of other.
If you plav a resistor from V+ out of oneto Ground out of th eother no current flows.
Well under 1 mA would be acceptable.
I'd expect good isolation. What is to be avoided is having ground out hard connected to ground pin in.

3 truly isolated chargers will work happily on 3 cells in series if there is no closed current path apart from the cell interconnections.

  • will it be safe if when they're done charging separately I put all three cells back into the same system together


  • \$\begingroup\$ Thanks for all the info. I just got home from a vacation, so it'll take me a little while to digest the info and figure it out. I'll let you know if I need any clarification, and accept the answer once I've got it down. \$\endgroup\$
    – nhinkle
    Dec 30, 2011 at 2:21
  • 1
    \$\begingroup\$ OK, so some clarifying questions. 1) how can I be certain that no cell is ever "deep discharged"? 2) if I'm using a COTS charger (and charging each cell separately), what do you mean by the charger outputs must be truly isolated? If I'm using a physically separate charger for each cell, will it be safe if when they're done charging separately I put all three cells back into the same system together? \$\endgroup\$
    – nhinkle
    Dec 30, 2011 at 21:35
  • \$\begingroup\$ Going to mark this as the accepted answer since it seems to contain all the relevant info, but I believe I'll end up using an all-in-one LiIon battery pack with a built in protection circuit and whatnot, as I believe it'll be safer overall considering the outdoor application. \$\endgroup\$
    – nhinkle
    Mar 14, 2012 at 5:07

I am installing strobe lights on my utility bicycle because I am sick-n-tired of people almost running me down just because they are half asleep in the dark days of winter. Bright lights help “blind” drivers see you. No cops ever bugged me about my tail light, and if they get on my case about the strobes, I will have to point out that it is an “emergency” when car drivers do not see me in the half daylight of the pacific northwest.

I got a white 9 watt shaped beam (to the sides) and for the front and two amber 4 watt strobes for the sides and for the rear I have a large L.E.D. truck tail light I use with a blinker (for turn signals), but I am installing a strobe flasher for the tail light. It runs on an 18650 pack of 3 cells. I will be using a 4 cell pack for the other three lights. Because the new strobes can take up to 24 volts. If the stats for your light says it will take up to 14.8v or less you must use a 3 cell pack.

If you are building a pack, and you want to use a circuit board, the electronics are so small now these protection boards have balance charge circuits built in, so it would be even safer to charge. But the new I-Max B6 will not like the confusion of more circuits.

The I-Max B6 charger (old style) charges through the protections circuits, even though the instructions say that it will not; I set mine on 3 cell 3.6 volts per cell to make the cells last longer.

They should be charged to no more than 4.2v or even 4.1v or even less if possible. It has recently been proven that the longer a battery cell holds a high voltage the faster it will degenerate. So if you want a full charge of 4.2 volts, use it right a way. If you charge them to something like 5 volts each or more, you are begging for trouble!!!

3 Cells in series can be charged faster than single cells because they have more resistance that way. I set my charger to 1 amp. So a 4 cell pack could charge faster at a higher voltage. (1/3 volt each cell) if you need the charge faster try it at a higher voltage. But if using cheap Chinese cells charge them in a steel ammo box!! If they produce enough heat to feel you may have dangerous problems. If the pack does not have the wires to the balance plugs the cells will even them self's out after a while so don't worry about that.

The new battery chemistry {LiFePO4} is safer than the older 18650 Li-ion cells. But I do not know how they would charge in an I-Max B6. NiMH is much safer but much heavier.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.