cree Q5 @ 1200ma with 2 x 18650's (Tiablo A9 mod)

I'm looking to drive a Cree XRE Q5 at 1200ma from two 18650's in series. I'm happy to use a resistor instead of a regulated circuit as runtime & high output will still be quite good.

The highest input voltage to the resistor would be 8.4v with fully charged batterys

What size and type resistor should I buy?

thanks

Re: cree Q5 @ 1200ma with 2 x 18650's

lightforce2 said:

I'm looking to drive a Cree XRE Q5 at 1200ma from two 18650's in series. I'm happy to use a resistor instead of a regulated circuit as runtime & high output will still be quite good.

The highest input voltage to the resistor would be 8.4v with fully charged batterys

What size and type resistor should I buy?

thanks

Click to expand...

Try this tool for calculating a resistor:
http://led.linear1.org/led.wiz
Otherwise, PM Darkzero for more help
He is pretty good with that stuff.
I would help ya more, but i have too many
things going on in my head to do math:thinking::drunk:


Robert M.

Re: cree Q5 @ 1200ma with 2 x 18650's

LED Pro says for 3.85 volts at the LED, 1,200 ma and 8.4 volt battery gives a resistor of 3.79 ohms and 5.46 watts. OK you have to guess at the LED's forward voltage and the Li-Ion's would be putting out less than 8.4 volts. But that aside, 3.85 volts @ 1,200 ma is only 4.62 watts. which means 54% of the power is being dissipated as heat through the resistor. Not a very efficient setup.

Now two 18650's in parallel is 0.29 ohms and a 0.42 watt resistor. That's only 8% of the power being used by the resistor. Considering the fact that there is some voltage drop in the batteries, you could probably direct drive an LED with 2x18650's in parallel. But you really would be better off with a good buck regulator in the series arrangement.

Re: cree Q5 @ 1200ma with 2 x 18650's

Wow, you'd need a 10W resistor and a way to dissipate the heat it makes. :candle:

Re: cree Q5 @ 1200ma with 2 x 18650's

As Al says you would be better running the 18650 in parallel and using a 7135 based driver.

I ran a Q5 off a 18650 with a 1400ma driver. It didnt run at 1400 ma very long but it ran at 900ma+ for over 140 minutes. 1200ma is 33% more current but not 33% brighter. It is a whole lot more efficient and easier though.

Re: cree Q5 @ 1200ma with 2 x 18650's

I'd go for a buck driver or a resistor with the batteries wired in parallel, much more efficient then a resistor with 2 18650s wired in series.

Re: cree Q5 @ 1200ma with 2 x 18650's

Thanks for the advice about using resistors, I will have to re-consider this as I can now see its going to be quite inefficient.

A parrallel setup is not an option, I should have explained more about this project -

I have an old Tiablo A9 without a driver, and a new Tiablo A9 coming in the post. I need 2 seperate light options, one with a Cree red XR-C and the other with a Cree XR-E Q5, plus I require full regulation with 18650's and long runtime. I can achieve this easily as I will have 2 LED/driver units

This is my solution - I have cut down a spare tail cap to use as a treaded socket and compression fitted it permantly to one battery tube, the socket then screws onto the the other battery tube, thus making a 2 x 18650 Tiablo A9 (fully regulated)

Also I can unscrew a battery tube & replace with the tail cap to revert it back to the standard A9 if ever needed. I can also change over the LED/driver units to select the red or white option]

As an option I could use the red XR-C and with one 18650, the red XR-c will have a lower forward.
I think I should go to the trouble of fitting an aftermarket driver in the 'empty' LED unit so that both LED units have their own circuit.

Here's another option I though of this morning, & I think it's qiute interesting -
I have a few spare Dereelight pills, I think I can machine out the Tiablo LED housing to fit in a Dereelight pill, it may need a brass shim to ensure good thermal & elictrical contact and should'nt be too hard to make, any thoughts??

I'll post some pics once I decide which way to go & have made a start

thanks