Expected led run time (if possible)

Hello everyone, i have an issue thats been bothering me for some time.. I hate that my gun safe has no lighting, and would like to remedy that.
I currently have a no/nc magnetic switch, and a few leds available. My thoughts were that i want 6-3v leds in my safe. Prolly 2 for the ammo shelf, and 4 for the gun area.

Now im no means a professional, so im here, asking professionals!
The leds i have are what i believe to be 4) XR-E (2600k w.white) and 2)XP-E (450nm blue). If i recall correctly, these leds operate around 3.5v-3.7v full load.what i plan is.. let me just draw it instead..


You will notice i dont have resistors in series. Since im under powering all leds do i even need them? (led series wants 11.1v but only getting 9.4v)

I am worried about current draw from 9v, and longevity of battery. What is a safe discharge rate for a 9v alkaline?
Im gonna take a wild guess at amp draw and say 3A? maybe 500ma per led?

I will only prolly be in the safe once a week for maybe a min or two to gather guns and ammo, and return guns after shooting.
Approximate runtime? maybe 1 month? 1 year?

If you expect under 6 months runtime, ill just purchace 3 18650 and run them in series. That way theyll be rechargable, and compatible with my other flashlights. but id rather not spend much if 9v will hold out like a smoke detector lol

Thanks in advance!!!

Haven't considered 9Vers for quite some time, so the following is just infering from stuff I've briefly found (primarily this site and this site)
- typical alkaline 9V is apparently 400-600mAh; so even if 3Amp draw where possible, you'd theoretically only be getting maybe 12mins maximum.
- sounds like a 9V batt is hard-pressed spitting out 500mA, let alone 3Amp.
- seems they are useable to about 6V, then they are completely useless (probably doesn't enter into the equation here)

Even with a simple straight circuit, I think the consensus would be to always use a resistor in there to set the maximum current.

My brain hasn't engaged for the day yet (if ever?) so I can't think of all the implications of the few alternatives I could think of, so I'll leave them be for the moment. But some vague thoughts were 4x AA rechargeables (4.8V, 2200mAh or so, cheap), a single 18650 with all LEDs in parallel (thinking there's issues with that though)...

RE: smoke detectors, they spend the vast majority of their time in sleep mode, and the LED blink is a split second long, with a 20mA or so LED, so uses sweet bugger all power for the "heartbeat"

The LED's should draw about 100-120mA per string direct-driven by a 9V battery per Cree's own data, so ideally you'd get 4-5 hours' runtime. But that's before the battery's voltage drops; you could end up in a situation where the string with the higher combined Vf drops while the other string runs for a while. I suspect your actual runtime would be considerably less since a 9V battery consists of 6 cells with dramatic voltage drop across their operating lifetime and high-power LED's do have minimum current requirements (I recall that they stop lighting up around 20mA or thereabouts).

Second the suggestion about using 4x AA batteries with all LED's wired in parallel. Using the correct resistors, you could reduce draw to around 100mA per LED and see about 3-4 hours' combined runtime (assuming >2000mAH cells) - probably more as the cell voltages dropped and the resistor chops out even more drive current. Using hybrid NiMH cells, you'd probably run the batteries flat long before they self-discharged.

Thanks for the responses.

idleprocess said:

Second the suggestion about using 4x AA batteries with all LED's wired in parallel. Using the correct resistors, you could reduce draw to around 100mA per LED and see about 3-4 hours' combined run time (assuming >2000mAH cells) - probably more as the cell voltages dropped and the resistor chops out even more drive current.

Click to expand...

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That sounds like the way to go for me. I can live with 3-4 hour run time for sure. Lets not forget, they will only be on maybe 3 min a week, so 4hour run time = just over a year before a recharge is needed

So if i understand correctly, i put 4xAA in series for 4.8V, wire every led in parallel with a resistor.

http://led.linear1.org/led.wiz says i need a 15 ohm 1/2 watt resistor on each led wired in parallel.

I input this info
input voltage - 4.8v
diode forward voltage - 3.7
diode forward current - 100 (leds are rated for 1A max, i assume 100mA is bright enough to read ammo boxes?)
# of leds in array - 6

• each 15 ohm resistor dissipates 150 mW
• the wizard thinks 1/2W resistors are needed for your application
• together, all resistors dissipate 900 mW
• together, the diodes dissipate 2100 mW
• total power dissipated by the array is 3000 mW
• the array draws current of 600 mA from the source.

Is this right? i mean the resistors are eating up almost half as much battery as the leds? Is this the correct equation for the estimated 3-4 hour run time?

Also, is 100mA going to cut it? I dont need the sun in there, just enough light to differentiate ammo boxes and such

You will have to experiment with LED brightness to see what works best in your application. I suspect that you'll find 100mA (likely spitting out more than a third of the LED's faceplate lumens; I seem to recall XR-E's & XP-E's are binned At 350mA) to be rather bright in the confines of a gun safe and will want a lower drive current.

Given the low level of overall current, you might be able to get away with a single 2W resistor for the entire parallel set. Bear in mind that your LED Vf's will need to be very close at your intended drive current (test for this) and you do risk Vf change over time (potentially leading to uneven current distribution as some Vf's rise and some fall).

ok so i started playing around with different voltages and leds in series and got myself stumped with this setup

Ex: 2)18650s in series for 7.4v, two 3W cree XR-E(Rated at 750mA, 1A max, see below), one carbon film 3.3 1/2watt 5% resistor.(its the only resistors ive got to play with)

• Min. lumens 80.6 @ 350mA
• Pre-soldered star pcb
• 1000mA max. drive current
• 2600K-3700K color temperature

When i power up array, current starts at 450, slowly creeps up to 500mA in 1 min time. BUT after 1 min, the resistors so hot that i cant "hold" it for more than 2 seconds.. i like the light output, but dont like the resistor temp, so i tried to add an additional resistor the the series. So now i have this

Batteries + > led> led> 3.3 resistor> 3.3 resistor> battery -

I now have a current draw of 280mA and creeps to 300mA but no hardly no heat. Im ok with this brightness/temps.
The magic question here-> Is this setup ok? Expected runtime with 2 of these setups in parallel on 2 18650s? (after placing a series at 280mA in gun safe i think 4 leds will be just fine instead of 6)

Btw, my results are from a $20 ebay 0-1A DC analog meter that Im running in series with everything. Do the batteries/leds see this meter as a load? i mean after i remove meter from series, will the leds run harder?

Thanks Again!!

I'm a lazy *******, so I just bought a "PIR LED LIGHT" from evilbay. Runs on 2 x AA eneloops, and according to the box it uses 70uA in standby mode, and 70mA with all 8 5mm LEDs lit up. Provides plenty of light for the cupboard it is in, no wiring needed (just peel of the backing of the double-sided tape and stick it in place). Waving a hand in its general direction is enough to trigger it (sometimes just opening the door is sufficent), and it switches off automatically.

Not as much fun as making your own, but effective enough for my needs.

True, I've got a nice one I got for something like $6 shipped, use 4x rechargeable AAAs from hobbyking. Used to use it in the hallway for visitors getting up in the middle of the night, but the bloody cat kept setting it off.

Also picked up a $3 "camping lantern" recently, 4x 5mm white LEDs powered by 2x AAs, it's suprising just how much light you can get out of a cheapie 5mm LED with a diffuser. Would be well and truly usable for camping.