Just got some Cree XR-E stars off Ebay

Got my first Cree XR-E today, stars. I bought 4 of them from Luxon Canada. Anybody know them?

Unfortunately they are only P2 bin. Are they still bright enough to be worth using for mods instead of 3 watt Luxeons or 1 watt Luxeons?

For example, would P2 bin still be significantly brighter to put in an Elly or another idea, in a 3D MagLED?

I hope I didn't screw up by buying P2 bin, I got all excited reading about Cree XR-E that I didn't realize different bins control the brightness. I hope I didn't buy some Cree that are no brighter than a Luxeon...

If all else fails I can always use them for one of my many in-place lighting project ideas, for example, been wanting to mod the dome light of my car for a long time...

Question, not to get too far off topic, but if I got some kind of driver and ran some Cree stars for my dome light, how many do you think I would need to have good brightness like the stock dome light? Also, if I used something like 2 Crees, how long would you guess I could leave them on without draining my car battery too low to start the engine? I have an Optima red top battery that is rated 800 cold cranking amps, 55 amp-hours, does either of those stats help to calculate?

Hellbore said:

Got my first Cree XR-E today, stars. I bought 4 of them from Luxon Canada. Anybody know them?

Unfortunately they are only P2 bin. Are they still bright enough to be worth using for mods instead of 3 watt Luxeons or 1 watt Luxeons?

For example, would P2 bin still be significantly brighter to put in an Elly or another idea, in a 3D MagLED?

I hope I didn't screw up by buying P2 bin, I got all excited reading about Cree XR-E that I didn't realize different bins control the brightness. I hope I didn't buy some Cree that are no brighter than a Luxeon...

If all else fails I can always use them for one of my many in-place lighting project ideas, for example, been wanting to mod the dome light of my car for a long time...

Question, not to get too far off topic, but if I got some kind of driver and ran some Cree stars for my dome light, how many do you think I would need to have good brightness like the stock dome light? Also, if I used something like 2 Crees, how long would you guess I could leave them on without draining my car battery too low to start the engine? I have an Optima red top battery that is rated 800 cold cranking amps, 55 amp-hours, does either of those stats help to calculate?

Click to expand...

You'll need to know the reserve capacity of the batt and the current draw of the driver you're going to use. That's about it.

here are some numbers taken from the cree xr-e data sheet:

Group Code Minimum Luminous
Flux @350 mA (lm) Maximum Luminous Flux (lm) @ 350 mA
N4 62.0-67.2
P2 67.2-73.9
P3 73.9-80.6
P4 80.6-87.4
Q2 87.4-93.9
Q3 93.9-100.4

The 55 amp-hours is a good indicator of how long you can leave the "lights on", however the measuring method used affects this (in fact you'll probably get a "better-than" result due to the very low draw of the LEDs, I don't know how car batteries are tested (well, I have known, but don't remember the parameters specifically), anyhow, but the 55Ah rating is probably based on expected "average" draw in a car environment (maybe 10A constant draw for lights on or something).
Okay, regarding brightness, the Cree's are roughly 70lm/watt (average total output), compared to around 18lm/watt (really good incandescent/halogen outputs, figure more like 12lm/watt for a dome light bulb is realistic). So if you can get the bulb's current wattage, (figure 4 watts maybe for a dome light? I have no clue...)... then take that figure (4 watts in my example here), times 12 divided by 70 = around 0.7 watt of Cree power to equal a 4 watt dome light bulb (if 2 bulbs, double it obviously).
All that's highly variable, and adapting the Cree to the housing (or making a custom one) can affect all this a lot, but it gives you the general idea....
so, now let's say you decide/calculate you need 2x1-watt Cree's to replace the current bulbs, a typical Cree voltage may be around 3.3volts, and you need 2 watts total, so you'll need I=P/e (current = watts/voltage), so your expected current is going to be 2/3.3 = .6 amps. You can tweak the numbers, go serial, parallel, etc with your driver, but the total figures should add up the same.
SO now with all this, 0.6amps of draw on the battery at 55Ah = 55/0.6=91.7 hours of battery life with dome lights on, or nearly 4 days. But remember that's conservative since all batteries have better lives when running well under there "spec" ratings, as would be the case here, so you'd probably get 5-7 days.
All this ignores circuit losses for your driver circuit, housing losses for the current bulbs, etc, but hopefully it's enough to get you started on the "real" calculations.

Very informative, thanks!

With my current incandescent bulb, if someone (not me of course) accidentally leaves the dome light on tonight, my battery is toast by the time I notice it tomorrow morning

But it sounds like with the LED dome light, I would simply notice in the morning, hey someone left the dome light on all night! And start my engine and drive to work

Yikes, that sounds to me like you might have a bad battery, I have a dual-dome light on my little MX-3, with a smallish battery (not a fancy Optima that should have far better capacity, mine is around 40Ah I think) and I have come out in the morning with the dome lights on (door didn't close properly, or was looking for something and forgot to turn it off) with no difficulties at all. Even if you have 2-20 watt bulbs (like I said, I have no idea what typical dome lights are. but that sounds excessive), that'd be 40/12V= 3.3A of draw, 55Ah/3.3A = 16 hours of runtime if your battery is nominal. If it's dying overnight to the point it won't start, there may be something else going on.

Oh actually, I didn't mention, the incident with my battery dying happened before I got the Optima. My old battery was pretty weak I guess.

I'd say go ahead and put it in your Ellys... cheap Crees for cheap lights. It will still be significantly brighter. Never seen a MagLED so...

As soon as I get my Elly i'm throwing a cree in it. My first mod so it won't be too expensive if I smoke it

Check out this thread for good elly/mod info
https://www.candlepowerforums.com/threads/141874

Christexan said:

Yikes, that sounds to me like you might have a bad battery, I have a dual-dome light on my little MX-3, with a smallish battery (not a fancy Optima that should have far better capacity, mine is around 40Ah I think) and I have come out in the morning with the dome lights on (door didn't close properly, or was looking for something and forgot to turn it off) with no difficulties at all. Even if you have 2-20 watt bulbs (like I said, I have no idea what typical dome lights are. but that sounds excessive), that'd be 40/12V= 3.3A of draw, 55Ah/3.3A = 16 hours of runtime if your battery is nominal. If it's dying overnight to the point it won't start, there may be something else going on.

Click to expand...

From what I understand any modern car battery, Optima or otherwise, will be ruined if a draw is on it past a certain voltage, even if it's brand new. Oh, you can jump start it when that dim dome light reminds you "doh!" you screwed up and you can use it to get back and forth to work for a few months or more, but it's a lose/lose proposition when it keeps needing more jump starts.

So yeah that battery was toast. I've ruined a couple of optimas over the years with big (like 5 amps with fan cooling big) car stereos in my Corvette, first a yellow top then a blue top even. "Deep cycle" doesn't mean "abuse me, I can take it" just that they see you're admitting to past abuse when you buy it so they give you a shorter warranty. At least it seems that way.

Deep cycle means that the battery has fewer, more physically robust plates.
This trades off discharge rate for better being able to take cyclic usage.
Car batteries _hate_ being cycled more than a few percent, and may rapidly die, due to the thin plates buckling under charge.