Rexlight 2.0 question

[piper]

Hi,

I've read here in a review of the Rexlight 2.0 that there may be a problem using a NiMH battery on the high level burning out the LED.

Since a NiMH AA is 1.2V how does this happen? If this is heat related why would this type of battery run the light hotter than any other type?

Thanks,

Piper

 

[piper]

Many views but no theories.

Anyone have any ideas?


Piper

 

[adnj]

I think that the battery was providing close to 2A. At low voltages/efficiencies, more rejected heat BUT I think that the light had run constantly with more than one battery during run-time testing.

 

[whc]

Should work fine with Alkys and Ni-Mh, though it can get pretty hot, just turn it off if the happens, and you should be fine. Only when tested to the extreme (like I did with mine), then you can burn the LED, but in normal use, I doubt it will happen.

 

[TOOCOOL]

Mine only gets warm on Alkys and Ni-Mh, even if left on for 30mins +

 

[adirondackdestroyer]

I'm running mine on a 14500, is that even worse than running it on a Alk or Nihm cell? I figured it was safe to use with a 14500 since that is what it was originally advertised for.

 

[gunga]

I think the idea is power (watts) = voltage x current.

Since the voltage of the nimh and the alkalines is so much lower than li-on, the current is much higher, hence why the light runs really hot...


EDIT: alkalines may be limited by interbal resistance, while nimh can run much higher currents (and hance heat!).

 

[HarveyRich]

Originally Posted by gunga: Since the voltage of the nimh and the alkalines is so much lower than li-on, the current is much higher, hence why the light runs really hot...

OK so far. But why then do Li-ion batteries typically generate so much more light output? The additional current you talk about under NiMH wouldn't necessarily generate more heat, would it?

 

[gunga]

I think perhaps that at higher voltage, the light circuits typically run at a higher power. So at 3.7-4.2V you have a certain power (way 3W), and at 1 - 1.7V, another power (say 1 W).

You can see this as Rex light quoted different wattages with different batteries. The currents drawn by the Rex were measured to be around 2 amps using nimh (not sure if that number makes sense at 1W tho). That is pretty massive, so yeah, all that current can cause really high heat.


EDIT: another factor is cct efficiency. If the cct is tuned to run well and cool at 3.7-4.2V, it could be running with really poor efficiency using 1.5V. That can explain how Fenix manages to squeeze so much power and efficiency at 1 - 3V in the L1D/P2D series. They run in direct drive with no modes using 3.7 RCR123s, so the cct is not really designed to work well at that voltage. It does killer with 1-2 AA and/or CR123 tho.

:rock:

Too bad, I have a Fenix and would love to run with Li-on. I guess you can't have it all.

 

[big beam]

Yes current does make heat. The driver most likely is more effiecient when the V in and V out are closer together.I think a lot of the heat from this light is from the driver.Still most of the heat running a ni mh is from the LED I'm sure.

DON

 

[uk_caver]

[removed unnecessary post]

 

[gunga]

Don,

I'm not sure that makes sense. Wouldn't the LED run hotter at a higher wattage? THe Rex is brighter (hence higher wattage) using Li-on correct?

Seems to make more sense that the heat would be generated from the driver or conversion cct since the LED runs dimmer (hence cooler) with nimh.

The LED always needs the same Vf, so the current is lower in the LED using nimh (unless I'm wrong about the Vf).

Yes current does make heat. The driver most likely is more effiecient when the V in and V out are closer together.I think a lot of the heat from this light is from the driver.Still most of the heat running a ni mh is from the LED I'm sure.

DON

 

[big beam]

No the amps are higher (current in) when you use ni mh.Because the 1.2V of the nimh has to be made up in current to run at the same watts(VxA=W)

DON :thinking:

 

[big beam]

gunga,

You may be right about the drivers heat.WHC said his rex draws 2.3 amps @ 1.2ish volts.thats 2.76 watts and if only 1 ish watt is getting to the emitter that means 1.76 is wasted in the driver.THAT"S A LOT OF HEAT.I have a light where the driver is separate from the led and heat sink.It runs at .380 with a SSC ( about 1 watt) and the emitter and heat sink makes almost no heat at all.So maybe in this light most of the heat comes from the driver with ni mh and most of the heat on li-ion is from the emitter.

JUST A GUESS ON MY PART

DON

 

[gunga]

Yes, I understand that, since that is what I said originally.

What I meant is that the LED needs a constant Vf to run, the current will affect the bightness (correct me if I'm wrong on this one but I believe it is correct). nimh produces a dimmer light, so less current to the LED, however, since the voltage is so low using nimh, more current is needed to the driver/conversion cct. Hence heat being generated by the driver, not the LED itself.

That's what I meant. I'm not quite sure if that's what you meant.


EDIT: overlapping posts! :naughty:

Yeah, you could be right about that theeory on the heat (ie Li-onheat at emitter).

Again, too bad Fenix/Rex can't have it all. It's an electrical limitation I believe. Might not actually be possible to have awesome effieciency & brightness at 0.9-4.2V. Just have to compromise...

No the amps are higher (current in) when you use ni mh.Because the 1.2V of the nimh has to be made up in current to run at the same watts(VxA=W)

DON :thinking:

 

[big beam]

YES,LIFE IS A COMPROMISE.
DON

 

[gunga]

YES, I AGREE.

Btw, why are you yelling?

:naughty:

 

[big beam]

I can only type with one finger so I might as well hold my other finger on the shift key.

DON

 

[adirondackdestroyer]

So let me get this straight... the Rexlight runs cooler (or at least is safer) on a 14500 cell? If this is the case I see no reason to run it on a 1.5v cell since it has crazy output on a 14500 and good runtime I would imagine.

 

[adnj]

Self-discharge is higher on a rechargeable than an alkaline. The Rex runs fine on a 14500.

In general, small lights shouldn't be run on high for hours at a time without holding them (or doing something else to keep it cool).