C rating too high for DD?

Flashlight Modding and DIY Parts
1

Today, I got a cylindical LiPo that measures roughly 15mm x 60mm (15600) so it can be easily modded to fit 18650 lights. It’s a 1200mAh cell with a 15c rating so it can handle 18 amps continuous discharge and (probably) 36 amps burst. I was planning on running an XM-L2 DD with it but now I think it’s a bad idea. With a full charge, I don’t think there will be that immediate voltage sag and the LED will just burn out. Any solutions or thoughts on this? Will using thinner wires to limit current just fry the wires?

If I understand c rating correctly, at 4.2v, my cell won’t have enough internal resistance to limit current to a safe level on it’s own. Kinda the opposite problem others are complaining about with a single 18650 DD XM-L2.

2

You could try to put a resistor in series with the emitter. The circuit would look something like (not showing tailcap switch):

So there’s an 1.2V drop (4.2V - 3V) across the resistor, and you want, say 3 amps through the emitter, so:

R = V/I = 1.2V / 3 amps = .4 ohms

Power would be V x I = 1.2V x 3 amp = 3.6 watts

So you’d need a .4 ohm resistor that’s rated for at least 4 watts.

You can adjust the calculations above based on how much current you want to have go through the emitter, e.g., for 4 amps, you’d use:

1.2V / 4 amps = .3 ohm resistor

etc.

3

You might be ok as the XM-L2 doesn’t pull current from the cell as easily as the XM-L. Vinh over on cpf did some tests on the XM-L2 even using 26650 direct drive, and the current measured didn’t go much past 4.5A if I remember.
It seems that using a multi cell light(parallel configuration) gets by this problem, allowing more current to the LED.

4

I think that the point that the OP was wondering/worrying about is that he has a battery that can supposedly put out 18 amps. If it can do that, he probably doesn’t want to go “bare” DD. I’d suggest that, for safety (not killing the emitter), starting with a reasonable resistor in series, then add more resistors (or use less ohm resistors) and measure the current.

But, that’s just my opinion.

5

If? Turnigy is a well-known, respected brand in the RC world and 15c continuous is nothing compared to the high discharge capabilities of today’s LiPos. There’s usually a much higher (double?) burst rate listed on label which mine doesn’t have but it’ll do 15c easily. It should go higher but internal heat builds quickly so typically, short spurts to 32 amps wouldn’t be unusual.

Another advantage is 5c (and sometimes higher) charge rates are attainable without degrading the cell.

6

The amp draw in DD will depend on where the voltage stabilizes under load and what the LED's Vf curve looks like. Theoretical max discharge current of the cell has nothing to do with it.

edit: Extrapolating from Match's Vf graph I'd guess if this World's Most Amazing Cell Ever In The History Of The Universe could maintain the full 4.2 volts at an infinite discharge current, it'd run a XML2 at around 7.5 amps.

7

The battery being so small, with an XM-L2 attached will cause voltage sag. That sag will cause the led to only draw as much as it wants at said forward voltage. LEDs are cool that they wont self detonate as long as the voltage supply is acceptable. With unlimited current available the led will only draw power related to the voltage it sees!

I am not all to familiar with LiPo so I cant say if sag would be significant but with good heatsinking and a copper star you should cover all bases.

8

Thanks comfychair, that’s the kind of answer I was hoping for. I’m no electronics wiz and don’t fully understand how that works so I’ll take your word for it.

9

No please don't just take my word for it, do it yourself. You may learn something you haven't even thought to ask yet.

I have a fairly beefy DC power supply, official specs on it are 21A@12v, 17A@15v, so around 250 watts. When it's turned down to 3 volts, if it scales linearly that should mean it'd be capable of around 80 amps. But as long as it's limited to 3 volts output, an XML will only draw around 1 amp with nothing else in the circuit to limit the current.

10

I meant I trust you know what you’re talking about, especially since you have a good reputation on this forum. So, that 7.5 amp figure coming from you won’t be questioned. I read that it was the inherent resistances (including the cell’s IR) in the circuit that was regulating the current with DD and assumed that without them, the LED wouldn’t survive. I think I understand the amps part but it’s voltage not dropping to a safe level that concerned me.

I only have a crappy MM so there’s not much experimenting I can do but I’m learning a quite a bit from others here. Financially, I can’t afford many failures. I’ll look for the info you referred to and try to make some sense of it for now. Flashlights certainly aren’t the simple devices they used to be.