Running a 4x18650(in series) light directly from a 12v source

I jumped in on the tm16gt group buy and it got me thinking of whether it or similar lights can be run from a 12v source. I believe the batteries are arranged in series so the light should be able to handle 12v. How would I go about connecting the leads from a 12v source to the tm16gt without frying it? Has anyone does this with similar lights?


You can connect lithium-ion batteries in series, but without any protection and balancing circuit it will be very dangerous.

You need something like this, but for 4 lithium-ion cells in series (this one is for 3 cells).

I’m sorry I may have worded my question incorrectly. I was inquiring about how to use a light, such as the tm16, which is setup to use 4 18650 in series from the factory, and thereby capable of receiving 16.8 volts, to run from a 12 volt source such as a 12 volt outlet in a car.

The voltage may or may not be high enough to run 4x XML but in any case most stock drivers cannot handle the voltage spikes caused by vehicles. DIWdiver on another forum made drivers to run from 12v auto systems. I’m sure there must be other members that have done similar and may chime in.

That’s a problem I’m running into as well. I’ve seen people on different forms connect non automotive drivers and haven’t heard of issues with that but there is a big chance of failure from what I’ve heard. I ordered some tvs diodes from digikey for one of my boost drivers but haven’t had a chance to test it yet. I’m going to place it on the leads going to the driver to shunt the transient voltage spikes. None of that has been tested yet and no one I’ve talked to has experience with this sort of thing. So there’s no guarantee that it’s going to work

12v from a car battery.

Depending on the conditiom of the battery/alternator it’s possible to get anything from under 12v to over 13v, if you end up getting an average of say 12v with 4 leds/emitters that’s only only to give you 3v across each one.

3v is low, those member’s more familiar with leds/emitters can give you a better idea on what’s possible but I’m sure three would work, (four?)

From my understanding, 12v is when the battery is flat. When the engine is running 14v or more is normal.

~13.8v when charging and when fully charged.
12v is the nominal voltage of a lead acid battery (same as 3.7v is the nominal for liion ) 10 volts is considered flat.
As said before the voltage/current in a cars system is rough with spikes being common, anything you buy or build for car use has to be built to withstand this rough treatment.

Cheers David

Thanks for all of the responses.

So, if I understand correctly, you guys are saying that voltage fluctuations within the 12-15v range are still rough on the driver despite the driver being able to handle 16.8v from the 18650s? So its not necessarily even the peak voltage which can cause issues but the fluctuations as well? Or is it that the voltage may speak above 16.8v and cause damage?

I will add that I previously tried to direct drive 4 xml2 in series from a tractor battery(tractor on) and the first led to receive the positive lead was fried. In fact the led was so hot that it actually flowed and slightly shifted off of it is pad(pcb?). I have yet to determine whether led internals were damaged or if the shifting on the pcb simply shorted the led out. Either way, it was not good. This happened twice, and both times the first led in the series quit working after a little while.

Have pics of the setup but can’t get them to show up off of photobucket.

It is.

Useful info.

A good way to check if your battery is on it’s way out, possibly not charging is to try and put the electric window’s up. They’re a high drain on the battery, even with a good battery and you try 2 window’s you’ll notice the difference and they’ll be noticeably sluggish.

Here’s a piece from The Little Fuse Website ….

Independent studies by the Society of Automotive Engineers (SAE) have shown that voltage spikes from 25V to 125V can easily be generated [1], and they may last anywhere from 40ms to 400ms.