Run time for leds

You could just use one of the rapid table calculators (google) and figure out the time yourself.

A lot will, of course, depend on the efficiency of the current limiter to power the LEDs. The formulas will only give you run-time with 100% efficient current limiting.

I did not see any calculation that determines run time.

I have run leds for 45 minutes and remaining voltage is 11.93 volts.

Andy

There you go then. Just measure the run time.

There's nothing that will give you approximate run times based on the information you provide. You can get a theoretical run time from knowing the watts you are dissipating in the leds and dividing that into the watt-hours you can expect from the battery, the voltage and current of the source determine the watt-hours it can sustain.

Take the number of LEDs multiply them times the watts each for total power in watts. 5*8=40

Take the capacity of the battery in amp hours and multiply that times the voltage and that gives you watt-hours. (3,000 mah = 3 amp hours) You have the three - 3.7V batteries wired in series for a voltage of

11.1 (not 12V) 11.1 * 3 = 33 watt-hours

Then divide the total watts the leds want into the watt hours the battery can provide. 33/40=.825 hours

BUT:

Presumably, you have some means of controlling the current the LEDs can pull? What do you have and how are the leds arranged - series/parallel? A single white led will need up to 3.5 volts across it with current limited to about .7 amps.

That (series parallel, and current limiting, and efficiency of the limiter) is what makes answering your question impossible with the data you supply.

That and actual real battery voltages and LED voltage drop etc.. I'm only working with nominal values here.

There is no means of controlling what the leds pull.

They are wired in parallel.

Thanks for your help.

Engineers would design-in some means of current control. If you are just wiring a bunch of LEDs in parallel and putting them across a battery, the only thing keeping the LEDs from burning out is the internal resistance of the power source.

If you haven't burned any out... to date.... you will at some point if/when you increase the battery pack capacity.

A lot of LED flashlights work the way you are suggesting. They take three AA or AAA alkaline cells (4.5V nominal) then wire the LEDs in parallel. The LEDs pull enough current and the battery can only supply so much current that it works, and is cheap.

But if that same bunch of LEDs was wired to a 4.5 volt power supply that could supply unlimited current while maintaining 4.5V those LEDs would be giving off smoke as well as light for a short while.

You may get away with what you are doing, but it isn't good design practice - particularly in using the lithium batteries in series for

11.1V (nominal). If the batteries aren't protected internally from over-current failure they may explode.

If they are protected, and that is what is keeping the thing from blowing up, you or someone replacing the batteries in the future may put in some unprotected ones and it would be unsafe.

If they are all in parallel, and the LED needs 3.5V, and your battery is 3.7V, why wire them for 11.1V? Might be safer (if not safe) to just wire the parallel LEDs to batteries (that are charged and balanced) in parallel.

The LEDs themselves need some minimum voltage to light at all. A white LED is really a blue led with phosphors that give off red and green colors so it appears white to human eyes.

If you used a prism or diffraction grating, to look at the color you'd see the primary bands of light broken out separately (not a continuous spectrum from red to violet) A CD ROM makes a good diffraction grating... tilt it to break out the spectrum.

The reason the LEDs change color slightly when the battery runs down is because there isn't enough light to excite all the phosphors equally, so they usually pick up a green tint (green phosphors being more energetic than red as a general rule)

Take care and be careful with what you are doing. Empirical discovery and experimentation is fun and helpful, but it helps to understand enough to stay safe.

Leds need 12 volts. They are made for cars. Like indicator lights.

Andy

They probably expect 13.2v-14.0v. If I did the mat correctly, 3x3.7v is 11 .1v. Depending on the internal design of the lamp, this my not be enough t o produce any light, or it might glow dimly down to 3v.

If I really cared, I would do the experiment and see how long the LEDs stay lit. At $6 each, this experiment costs less than $20 and you will get a v alid answer applicable to your situation instead of guesses and pontificati on based on incomplete information and a poorly formulated question.

There are some LEDs that work on 12V but they are specially made to do so. Bare LEDs (which is what I assumed you mean since you didn't specify or include a link) work on a voltage specific to a color.

A 12V circuit for white LEDs may include three LEDs in series along with a dropping resistor to limit current. Most small indicator LEDs are 5 mm in diameter and run on 20 milliamperes.

But if a LED is made for 12V (or even 120V) there's more to the package than a single led, there's a circuit to limit current built in.

Gives the following Color-voltages for LEDs

Infra-red 1.2V Red 1.8V Amber 2.0V Yellow 2.2V Green 3.5V Blue 3.6V White 4.0V

Their white LED is probably an ultraviolet or near ultraviolet one with phosphors. (similar to how fluorescent lights get white light)

Post a link to a data sheet for the LEDs you have.

Here is a link to Cree high power LEDs:

Notice the typical currents and voltages? What you get should be more or less those values, but it isn't carved in stone.

Cree is setting some of the standards and at the forefront of LED development. The values they give are based on keeping the LED cool enough to function. They would be "de-rated" at higher ambient temperatures. (that's the chart marked "thermal design.")

Electrical Characteristics chart shows the LED lighting on ~2.76 V and

100 milliamps to ~3.76 V and 1,000 milliamps. Higher current would burn it out or shorten its life.

LEDs don't typically burn out when they are used correctly, but they do have a "half-life" and get dim with age, and get dimmer faster at high currents.

I recently built some plant grow-light bars. I'm using ten one watt LEDs in series per bar (~3V each) and have a current limiter for 300 milliamps. At an ambient temperature of 25 C the LED's were reaching

50 C (mounted to an aluminum strip 1.5" X 18" X 1/8") I added some tiny fans to keep the LEDs ~5 C higher than ambient. I'm using some store-bought limiters and running them from the 120 V AC mains power. I think the limiters cost ~$13 each and LEDs cost ~$0.40 each.

Oh, that's easy; the 18650 batteries are 4V each, so to get 12V you have them in series. 12V * 3000 mAh = energy in the batteries about 36 watt-hours. So, 7 hours at 5W (one LED), or under an hour with eight at a time.

That assumes no energy-losing other components (which is unlikely; most LED assemblies for "12V" nominal input have some required ballast resistance.

This is what I have.

"

"

The leds are strange looking and there are 5 of them.

5050 SMD LED Specs Number of Light Emitting Chips 3 Dimensions 50 x 50 mm Power Consumption 0.24 watts per SMD

You should have supplied that link from the get-go. Those aren't LEDs they are automobile lamps that incorporate LEDs. And, they probably use resistors to control current and there's no way to predict how long they might last with batteries.

.24 watts X 3 = .72 watts. Is that current or light output?

The original post:

I have (8) 5 watt LEDs that are run from a 12VDC source.

(3 18650 batteries at 3000 Ma hrs each.

I want to know the approx run time.

40 watts dissipated by LEDs with maybe 33 amp-hours of battery.

I looked at the link and don't see where he gets 5 watts per bulb though... The incandescent 158 lamp they are intended to replace only dissipates 3.4 watts at 14 volts.

Current used is 87 milliamps for one led.

Andy

.087 amps X 12 volts = 1.044 watts for one LAMP, each lamp incorporates several discrete LEDs.

Thanks.

Ahh. Say have you pulled one apart and looked inside? There may be some sort of switching IC controlling the current.

George

I bought some (on Ebay) for my truck awhile ago ('91 Dodge) they were

1157 replacements. Other than the dual contact bayonet base, they looked identical to his. They didn't satisfy me though - they looked too yellow (not amber) and the light output, compared to the incandescent ones, was too weak. So I pulled one apart and there were something like 5 yellow LEDs in series with a series resistor for each lighted surface.

IMO the crappy performance is something of a safety concern, so I didn't put them in the truck.

My lifetime motto: stay safe, have fun, don't get caught.