Supply a 86W CREE XLamp CXA2540 with a remote battery (18650s?)

Hi everyone,

It is my first post here
-> Here is a short presentation about myself.

Please be kind with my English and especially the technical vocabulary I will try to use correctly.
I also need to know that I have very basic knowledge of electronic; I may not know things that appears trivial to you.

Back to the topic:

I would like to use a CREE CXA2540 for photo purpose (and for the fun of throwing lumens :rock in a old quarry and I am wondering how to supply it correctly and efficiently.

I made tests at home with the following assembly:

AC-220V (HOME) ---> DC 24V via this ---> ~40V-2.1A via this boost-converter ---> LED

Here is a photo of the "thing":


I placed the LED on a radiator with thermal greace for passive dissipation and I put a strong fan on the radiator for active dissipation.
(40°C MAX with the LED on full power over time which I think is a very good job taking into account the material I have).

Note that the boost/step-up converter works as a LED driver (from what I understood), sometime acting as a voltage limitation, sometime as a current limitation. (I need to understand better how it is working). It is anyway a great peace of electronic :thumbsup:, you can set the voltage and/or the current to supply perfectly the LED and get the desired intensity.


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Nowwww, I would like to supply the LED with battery to use it remotely (puting appart the Fan which I will have to use and supply aswell in a similar way).

From what I understood from the CREE CXA2540 datasheet, giving (40V;2A) makes the LED works at ~90-95% of its max intensity, close to those 8000 lumense I then suppose?
Anyway, I took this (40V;2A) as a aim.

To supply the LED remotely, I was thinking about building a battery pack using 4 couplers with 16x18650 3000mAh that would supply my boost converter with ~30V that would itself boost the current to the 40V target. The pack would have a "capacity" of 6Ah if I am not mistaken. (which I likely can...)

Here is the circuit I have in mind:



My questions/interrogations:

1. Please tell me if I am wrong in what I say. Maybe I am dreaming, I don't know, I have basic knowledge, I am trying things here.
2. Do you think it is a good way to supply that voltage/current ? a protected ultrafire 18650 is around 2-3 bucks on ebay I think... so I would have the pack for ~32$ excluding transportation and couplers. But I do not now any alternative. I don't want to have a car battery... Weight/size is a decision factor here for me. I think i would prefer a pack that is already "done". If 18650 are good, would you advise me a reliable/good-price seller on ebay or on the candlepower marketplace ?
3. Could you help me calculate how long the battery pack would last before it gets depleated, taking into account my previous figures which are: 30V - 6Ah (battery) ---> 30V - ?A ---> 40V - 2A. I actually don't know how to calculate how much current my boost convertor will drive to the pack ? I am curious how long it would last. 1h? 30min? 5min?

Thanks A LOT in advance for your comments/help I am looking forward and happy new year everyone !

--
Richard

I know my post is (over)long but I still have hope someone will have a look at it

I don't have any experience with 18650 batteries, so I can't comment on how well suited they are.

When I do the calculations for run time, it looks like you could get a bit less than 2 hours of operation from the battery that you've sketched out.

The calculations that I ran are:
Power to the load is 40v x 2.1A, resulting in 84W
Power to the boost converter, assuming an efficiency of 90% (just a guess), is 84 watts divided by 0.9, resulting in 93 watts.
The current from the battery is 93 watts divided by 30V, resulting in 3.1A.
The battery run time is 6 amp-hours divided by 3.1A, resulting in 1.9 hours.

A lot of these numbers are not precise or are just estimates, so the run time could vary proportionally.

good luck!

whothedonk said:

a protected ultrafire 18650 is around 2-3 bucks on ebay I think...

Click to expand...

Don't do that. Any 18650 for that kind of price, especially claiming 3000mAh, will VERY likely be complete junk (eg. re-wrap of a used 1200mAh) and not capable of giving the current you require. Proper pricing will be more like say $8 for a pair of 2500mAh, going to the 3000mAh range will about double that, so may even work out cheaper and give better runtime to go with more lower capacities in parallel.

Might get more / better answers in the "Flashlight Electronics - Batteries Included" section, lots of "battery people" in there.

whothedonk said:

Hi everyone,
My questions/interrogations:

1. Please tell me if I am wrong in what I say. Maybe I am dreaming, I don't know, I have basic knowledge, I am trying things here.
2. Do you think it is a good way to supply that voltage/current ? a protected ultrafire 18650 is around 2-3 bucks on ebay I think... so I would have the pack for ~32$ excluding transportation and couplers. But I do not now any alternative. I don't want to have a car battery... Weight/size is a decision factor here for me. I think i would prefer a pack that is already "done". If 18650 are good, would you advise me a reliable/good-price seller on ebay or on the candlepower marketplace ?
3. Could you help me calculate how long the battery pack would last before it gets depleated, taking into account my previous figures which are: 30V - 6Ah (battery) ---> 30V - ?A ---> 40V - 2A. I actually don't know how to calculate how much current my boost convertor will drive to the pack ? I am curious how long it would last. 1h? 30min? 5min?

Thanks A LOT in advance for your comments/help I am looking forward and happy new year everyone !

--
Richard

Click to expand...

1. Looks ok mostly
2. Don't use the cheap e-bay X-fire stuff, it is junk and even dangerous in some cases. Going for quality 2600mAh cells will probably the cheapest option with good cells. In any case go for quality protected cells (especially with buck and boost drivers without a low voltage cut-off).
3. The calculation Steve K made is correct.

Keep in mind that the battery holders you are depicting in the OP are not suited for high (>1A) current applications, 1.5A is pushing it. I'm not sure if there are higher current battery holders, but maybe someone can send you in the right direction.
Possibly, you can find an 8 cell battery holder made by a member: FiveMega. Those are certainly high current capable.

Also: Boost drivers will drain more Amps from your batteries when the voltage drops, which is bad news when you are already using low current battery holders which drop too much voltage over the springs (they will get HOT!).