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customcolor
08/21/2011, 10:06 PM
this is what i want to do...

i have a 3 watt blue cree q5 led...i want to use it as a moon light.
i would like it to run off 2 rechargeable AA batteries and come on when the lights go out.
recharge the led from a solar panel under my tank lights.
use an inferred switch from a landscape light to tun it on when the lights go out and burn till the batteries die each night.

now can this harm the led?
will it kill the batteries too being drained so much?

Gorgok
08/21/2011, 10:40 PM
Will the light even run off of 2.4V? (1.2V is what most rechargeable AA batteries are.)

heathsanders
08/21/2011, 11:14 PM
Will the light even run off of 2.4V? (1.2V is what most rechargeable AA batteries are.)

I just got a 48-LED kit from rapidled that contained a AA battery holder for two AA batteries. The Duracell batteries I'm using in it run 1.5v * 2 = 3v. They include the holder to test individual LEDs. What the op is thinking of doing should work just fine assuming two AAs run in series to get to 3v, however I would put in a current limiting resistor to dim down the brightness and allow longer output from the batteries. This resistor can be a fixed resistor or a variable resistor (potentiometer).

martinmcnally
08/21/2011, 11:16 PM
Hey customcolor, you can definitely do that. However they do power up the LEDs pretty dim. I think you would have a hard time getting enough light out of them on batteries.

We run 2 LEDs of 4AA for our testers. You can see it in operation in this short video: http://www.youtube.com/watch?v=skOYR7qywc4

Martin

kcress
08/21/2011, 11:17 PM
You will not have success in this endeavor. You would need a much a larger battery and then a charge controller.

customcolor
08/21/2011, 11:42 PM
You will not have success in this endeavor. You would need a much a larger battery and then a charge controller.
so the 3w led flashlights dont run for long?
the controller i do agree with...you can over charge in a trickle charge...i think

BeanAnimal
08/22/2011, 05:32 AM
this is what i want to do...

i have a 3 watt blue cree q5 led...i want to use it as a moon light.
i would like it to run off 2 rechargeable AA batteries and come on when the lights go out.
recharge the led from a solar panel under my tank lights.
use an inferred switch from a landscape light to tun it on when the lights go out and burn till the batteries die each night.

now can this harm the led?
will it kill the batteries too being drained so much?


A single AA NiMH battery can provide about 2.5 Watt Hours of energy at 1.2 Volts. It would take (3) AA batteries in series to provide the forward voltage to drive the LED. Those (3) batteries, fully charged would give you a little less than an hour of run time (real world maybe 30 minutes due to other losses). Real world you would need a series/parallel array of around 30 batteries.

Charging those (30) batteries is simply not going to happen with any PV array under your hood.

Lastly, any light you "steal" from the tank and direct towards a PV array, is light that does not make to to the corals, so you are not getting anything for "free".

As bothersome as it can be, the laws of physics govern everything. :)

BeanAnimal
08/22/2011, 05:48 AM
I just got a 48-LED kit from rapidled that contained a AA battery holder for two AA batteries. The Duracell batteries I'm using in it run 1.5v * 2 = 3v. Alkaline batteries are not 1.5V, an alkaline cell is 1.225V, so two of them in series are ~2.45 Volts

That is below the forward voltage of most LEDs, it would take (3) cells in series to achieve a forward voltage of enough magnitude.



They include the holder to test individual LEDs. What the op is thinking of doing should work just fine assuming two AAs run in series to get to 3v, however I would put in a current limiting resistor to dim down the brightness and allow longer output from the batteries. This resistor can be a fixed resistor or a variable resistor (potentiometer). Even at 50% drive current, the (3) batteries are going to last no longer than an hour. D cells would get the OP closer (about 10W/h) but charging them daily is simply not going to happen with a small PV array in the hood.

AquaticFins
08/22/2011, 08:19 AM
so the 3w led flashlights dont run for long?

My 5w LED flashlight averages an hour and a half of run-time on two CR123A batteries - from my understanding, they're true 3v batteries and a little more powerful than AA cells.

BeanAnimal
08/22/2011, 10:36 AM
The best CR123A batteries have somewhere in the neighborhood of 1500mAh (1.5 Amp hours).

Your 5W light is not likely consuming 5W and lasting 90 minutes... In series two 1500mAh cells would be 6V and 1500mAh. At a drive current of 1000mA, best case you would be getting 90 minutes without driver loses, and the batteries would take a beating at that high rate of discharge. I am not sure what the forward voltage of the emitters in your light is, but I suspect around 4V and a drive current somewhere in the neighborhood of 800mA if you are getting 90 minutes :)

customcolor
08/22/2011, 11:01 PM
granted i didnt want to hear what i just read but that was one thought i had about the batteries...hey...better to ask befor i spend time and change trying it out...

now what about a 1w led....is that conceivable?

WingoLED
08/23/2011, 12:57 AM
granted i didnt want to hear what i just read but that was one thought i had about the batteries...hey...better to ask befor i spend time and change trying it out...

now what about a 1w led....is that conceivable?

How about getting a led flash light from Home depot that cost like $1.5-$2.0 and then wire your charger/controller circuit to it. You got your LEDs, you got your battery holder for price of less than $2.

I did many LED installation in the wild environment using this cheapo flash light trick to illuminate the dirt trail, steps and pot holes. I use the batteries that come in the package and they lasted over to the next morning.

Or did what i used to do-take a garden light (also from Home depot), choose the white light version, remove the shell and wire the panel wire the tank light shines. Cost < $5.0. This is very dim though.

BeanAnimal
08/23/2011, 07:32 AM
Guys, you can't cheat the physics. A Watt is a Watt and you can only draw so much power from a given battery and/or PV array. If you need more Watts, you need more batteries and larger PV arrays.

customcolor
08/23/2011, 08:10 AM
I was only thinking of 1 1watt not 3. If a 3watt would last about an hour...a 1 wat would last 3 hours...right? I have a window behind the tank...maybe use that as my charging light source?

Sent from my Ally using Tapatalk

BeanAnimal
08/23/2011, 09:01 AM
First of all, we need to stop refering to LEDs in regard to "Watts" as they unit is only meaningful with regard to the general classification of an LED.

For example, the XR-E has a maximum SPEC (for blue) as follows:
Forward Voltage 3.7V
DC Current 1000mA
That equatews to 3.7 Watts, not including the losses in the driver.

If we assume a very efficient driver and ideal batteries, the load would be around 4W. At 1000mA of drive at such a high discharge rate, 1500mAh battery would be lucky to survive an hour, not the 1.5 hours calculated by Ohm's law. Ohm's law isn't broken, the battery is simply not capable of sustained high discharge current.

If we cut the drive current to 350mA @3.3V, then that same battery would likely last close to the calculated 4.25 hours. That power dissapation in the LED is about .825W and the driver (if it is efficient) maybe another .25W for a total of about 1W.

Hope that helps.