2 x Cree and 2 x LiIon battery: What converter?

I'm building a dual Cree light to run from a 2 x 18650 battery pack. (Is that 7.2V or 7.4V?) I would like to use a current regulator to provide dimming via a pot or UI, but...

Since the Vf of the Crees so closely matches the voltage of the battery pack, do I need a buck or boost converter? I assume a boost converter would be preferable, because the battery voltage will pretty quickly drop below 7.4V under full load (1000 ma). If I use a buck converter, the LEDs will only run in regulation for that short time, assuming I have enough extra voltage in the first place considering the efficiency of the converter.

A boost converter should give me a substantially longer period of regulation, but I imagine I will have to take care not to hook it up to fresh-off-the-charger batteries (4.1-4.2V each).

At least this is my thinking. Please correct me if I'm wrong. That said, I don't know much about boost converters. Are there any that allow dimming via a pot, UI, or multi-stage switch? Also, there is one other stipulation: The board must fit into a space about 7/8" round. I tried one of my bFlex boards and it won't fit.

I am researching, but I need a little reassurance from you guys. I will see what I can come up with.

Ikonomi, the Fatman comes in mind:

http://www.taskled.com/fatman.html

It's a 0,80" board and allows dimming via an external pot or second resistored switch.

Best regards

____
Tom

Ikonomi,

You're right about your close battery/emitter Vf problem............You need a Buck/Boost converter like this.



Not sure whether or not you can connect an external pot though.


Tim.

The Fatman sounds closest to what I need, I think... The problem I see with the GD is that the Vin limit is not high enough to run 7.2V.

I had looked at the Shark, but Wayne noted that it does not handle Vin > Vout very well and will likely burn out. I don't want that! George at TaskLED mentions the dangers of connecting a higher Vin, but I don't anticipate the difference (if any!!) to be high enough to pose a danger to my LEDs. I guess there is a 50% chance that the Vf of the LEDs will exceed my Vin right from the start anyway, right?

Thanks Tom and Tim for your help! The only problem I can see is that I probably won't have access to full dimming on fresh batteries, but that is okay. Thanks!

Timson said:

You're right about your close battery/emitter Vf problem............You need a Buck/Boost converter like this..

Click to expand...

Your GD converter will fry if you power it with 7.2v (or 8.4 w/fresh batteries)


Pablo

Any chance you could run the Li-Ion cells parallel, the FatMan should be able drive 2 in series at 1A like this.

Later
Kelly

Sway said:

Any chance you could run the Li-Ion cells parallel, the FatMan should be able drive 2 in series at 1A like this.

Later
Kelly

Click to expand...

Heh, I just had this thought not 10 seconds ago and ran to CPF to see if it was possible.

If there is no problem running two LiIons parallel, this will be the perfect solution. I don't know why I didn't think of it at first... I am kind of slow. Are there any issues with uneven current draw or anything like that?

Two 18650s in parallel will give you 3.7V nominal which will allow room for dimming. Only use protected cells in this set up because the FatMan will drain them down below what I consider safe with its 2.8V minimum operating voltage.

It is safe to use and charge 2 Li-Ion in parallel in fact it's a good way to charge more than one cell at a time they will balance out like this and each cell will have a true 4.1 or 4.2 starting voltage depending on what level you charge to. If you have the option I would default to the lower charge setting, you will get more cycles from the pack.

Folks if I'm wrong here please chime in

Later
Kelly

No problem running in parallel like this - in fact that's what I'd recommend. I've actually run a triple Cree light off a single LiIon cell (though the efficiency isn't great with all that voltage mismatch).

I'd agree with the advice about protected cells, with the proviso that you can also use external protection circuitry. I control my Fatman with a uController which also incorporates low battery warning (by flashing the light) and cutoff, hence can use unprotected cells.

I have an Ultrafire WF-139 charger on the way from AW, which has two separate charging channels. The two batteries will come off the charger with slightly different voltages, but I assume this will not pose a big problem. And yes, I am using protected cells. I am not responsible enough to use something that might blow up.

chris_m, I was reading some of your posts on mtbr.com earlier today where you were advising someone in a similar situation, albeit with a 14.x volt pack. I think I will have to install a two-stage switch with resistors to give myself a low mode. I've realized that there's no way a pot will fit in my application, unless someone makes a really teeny tiny 50k ohm pot. I have only calculated about a 2 hour runtime with 2 18650s with the Crees running at 1A, an estimate which I believe to be optimistic. I will have to look into how to wire up the Fatman with resistors... maybe George can help me with that.

The recent FluPIC sale has inspired me to consider using 3 FluPICs in my 3-LED light.
That way I can run the LEDs in parallel, and each one gets the right amount of current.
It should shut down properly if the battery gets low.

This also provides great redundancy. If a LED or regulator fails, I won't be left in the dark.

I get a tad over 2 hours running my triple at 700mA off 2 18650s, so your estimate sounds about right - I'd imagine what you gain in slightly less power output you'll lose by operating at a slightly less efficient point with the Fatman.

Pretty straightforward to wire external resistors into a Fatman - just have one switched between the "pot" connection and ground. More technical info on how this works and how to calculate what resistor values to use at http://www.taskled.com/techfatman.html

Unfortunately I am very new to this, and I'm not clear on what resistor values I need. I assumed that I needed 50k ohms of resistance to get full brightness, but now I see that the trim pot is wired in parallel and I don't know what that means for me.

If I want a high of 1000 mA and a low of 30-50 mA, how would I calculate this for external resistors? And it makes sense to wire one circuit between the pot connection and pot ground, but where do I put the other? Sorry for all the questions, but this is my first time doing this!