safe amps for single 18650?

Flashlight Modding and DIY Parts
1

Hi All

I am slowly gathering the parts for a couple of lights… I’m still a total newb at this and am experimenting so please take this into consideration…

I have a couple of builds (C8 and TF-R2) that I am experimenting with. Both lights will have emitters on copper (SinkPad on the C8 and incoming new Noctigon in the TF-R2). Both are XM-L2s.

The most I have been able to get out of a light with the replacements/upgrades that I have access to, or are able to perform, is 2.9A on the new Qlite Rev A with dedomed XM-L2 (not that the dedome has any effect on amp level).

It seems that if my MM is correct the Qlite is slightly under-rated, or rather with my current upgrades (thicker wires to the emitter, copper braid on the springs), that all of my other lights are (significantly in some cases) underdriven.

I would like to build a light that is 3+ amps. Given the reports that the FT v10 drivers are overrated I was thinking to use them to get my 3+ amps (4A version for the c8 and 5A version for the TF-R2).

Because the the drivers are rated 2.7-4.2V input (single 18650 range) I would assume that the drivers are regulated to a safe discharge rate, even though they are capable of theoretically driving at up to 5A?

Is there anything I should watch out for? What if I an actually pull 5A out of the driver? (Besides a complete melt down). I am more concerned about battery safety. I am using protected Pannys and Sanyos. I am not overly concerned about optimal battery/driver combination because I am looking for the drivers to partially under-perform….

I also have a bunch of Qlite Rev As incoming with a pack of 7135s for experimentation with building up 3+ Amps.

Is there anything I should consider when trying this?

Again, my main concern is that perhaps the battery is drained too quickly?

Any suggestions (including this is not a good idea) would be greatly appreciated!

I’m still a total newb at this… I only just started experimenting with this stuff a couple of weeks ago when the first of my parts started to arrive….

Thanks!

:slight_smile:

2

I think max discharge for a standard Li-Ion (not IMR or LiPo4) is generally 2C (2 x capacity in Ah). So a 3100mAh Panasonic should be rated up to 6.2A. I found this info here and have heard it passed around before. I've seen my 2600mAh Tenergy protected 18650's push up to about 5A in direct drive.

-Garry

3

The cell's voltage sag under load will also limit the amount of current that can pass through the LED... at a certain point, you'll have to go to multiple cells in parallel, not really to increase the total current available, but to spread the load among more than one cell so that the amount of voltage sag in total is reduced. That's where the increased current will come from, by keeping the voltage up, rather than just raw current capacity of the input power source.

At high enough currents, even an infinite number of cells in parallel with zero sag under load won't have a high enough voltage to allow the LED to run at the desired current. At that point you have to go to multiple cells in series and use a buck driver, as even two cells completely flat (3v each, 6v total) will be higher than a XML/XML2's Vf even at insane currents that would make use in a handheld flashlight completely impractical as it would melt the flesh off your hand.

4

Thanks guys for the clarification! I had read about the 2C ‘limit’ for cells but wasn’t entirely sure what the correlation was to discharge rate, but that helps to clarify.

In terms of voltage sag, I am correct in understanding that if the draw becomes too high for the cell the protection circuit will be tripped? If this is the case, I am guessing voltage sag will vary from cell to cell… and circuit protection will be determined by the voltage level that the factory sets in protected cells? (e.g. TF16340swill not work on my BLF mini02 when fresh? … battery has to drop down to just under 4v before high comes on… In this case multiple 16340s in parallel would allow the light to be driven to its full potential as the voltage sag would be lowered/divided amongst the cells, allowing the max current through without tripping the protection circuit?)

If I understand correctly, according to Match’s vF ‘incorrect’ graph (for reference, not actual working values) I would need a battery that is at least ‘2600 mah’ to drive a light at 4A, not taking into consideration voltage sag and ‘real world’ considerations….

So…. Using Panny 3100s I ‘should’ be able to drive a light at 5A, but not for a long time of course ;)… I think?

I realise that for the size of light these levels are not very functional considering they are single cell lights… I just want to see what the target 3+ and 4+ A light ‘looks like’… Just don’t want to blow myself up… :wink:

5

You’re not likely to get 5 amps out of a single 18650. The problem is the voltage of the battery sags under load to below the Vf of the led. This is entirely different from the protection circuit.

6

ah… ok… and the load is determined by the number of amps the driver is pushing for?

7

Yes, but as the driver takes more amps from the battery to supply power to the LED, the voltage of the battery drops. When the voltage of the battery falls under load to the front voltage of the led the driver can no longer increase the amperage. That’s why it is much easier to overdrive the led with two batteries in series. Even two exhausted li-ion cells in series will stay well above the led’s Vf. This is what comfychair was talking about.

8

Theoretical thought experiment time.

Take a DC power supply, and connect it to a typical XML2. Now this XML2 is bolted to a diamond heatsink the mass of the sun. And the DC power supply is capable of delivering fifty billion amps, but at no more than 3.6 volts. You switch the power supply on, and only 4 amps flows through the circuit. Because that particular LED has a Vf of 3.6v at 4A. No more than 4 amps will ever flow through that circuit as long as the voltage stays at 3.6v.

9

The protection circuit is meant to be used as a short circuit protection. Most of the time the protection trip current is set a certain percentage over the 2c rating for discharge. Sometimes protection circuits current are set way over the 2c rating. Depends on the particular pcb used by the seller. Member HKJ has a excellent graph from his test on 18650’s here.
http://lygte-info.dk/review/batteries2012/Common18650Summary%20UK.html
Go almost to the bottom of the page, look under Current.