discharge curve graph of alkaline battery in the flashlight?

[rgbbv]

Hi

when i am browsing on the net,i always see graphs which show discharge curves of flashlight battery that show the voltage change,the capacity change or the lumen change when the current is constant.
does someone has a graph,that will show me the voltage curve in direct drive flashlight,when current is changing during the battery voltage decrease over time?(especially,on the first second and minutes of operation till it drop to about 2.4v).
i mean if we take fresh 3*aaa alkaline battery with open circuit voltage of 4.8v(3*1.6v=4.8v)and internal resistance of 0.15-0.30ohm(each one),put them in direct drive flashlight of xr-e cree led.how does the battery voltage curve and the current curve will look like over time?(the graph may consider only the internal resistance of the battery,or it could consider also the resistance of the flashlight parts:wires,springs contacts and etc.whatever you have)

PS.
when i said direct drive,i mean the battery is connected direct to the led without regulation or any current limiting component.

thanks in advance.

 

[ronv]

Hard to say. A lot of variables.
But if you look at the Cree it's typical Vf @ 1 amp is 3.3 volts so the initial current will be very high - maybe to high.

 

[ronsimpson]

Most LEDs have a very high variation in Vf. You should think about Vf min and Vf typical and Vf max.

 

[rgbbv]

Thanks for your replies.
so lets take one case that the vf of the xr-e led(as specified on xr-e data sheet)is 3.7v on 1000mA.

 

[ronv]

Here are the curves for the LED current and 2 curves for some AAA batteries, so you can get an idea by using the Vf of the LED and the 1 amp (where it starts) and the .5 amp (where it gets in a few minutes). You can probably get an esimate from those.

https://www.electro-tech-online.com/custompdfs/2013/03/XLamp7090XRE.pdf

**broken link removed**

 

[audioguru]

The absolute maximum allowed current for a warm white or neutral white Cree XR-E LED is 800mA. The max for cool white is 1A.
With new alkaline cells and no resistor then it will not last long and will probably overheat in the first few seconds.

Maybe the flashlight has resistance wiring?
Maybe the LEDs were ordered with a certain forward voltage?

 

[ronsimpson]

I don't know of a flashlight with out something to regulate current.
By the time you get to 1A LEDs there is a PWM.
I have some 20mA LED flashlights with a PWM.

 

[audioguru]

I have a few pens with an LED flashlight in one end. They use three little button battery cells that limit their own current.

 

[ronsimpson]

I just looked up a 6.8mm button battery. For three batteries the internal resistance is 15 to 45 ohms.

I did make a line of LED products with no resistor. The 1/2 cent for the resistor was a problem. I very reluctantly left the resistor out. The resistor would have extended the battery life! By using Chinese batteries with high resistance the product worked good enough. And the cost of the resistor more than payed my wages in the first year alone.

 

[rgbbv]

Thank you very much for your replies.
i had been seen before the curves for the alkaline aaa batteries that ronv gave me,but i didn't think about using its method to check how does the battery voltage drop.so thank,ronv,for turn my attention to this possibility.
thank you very much ronsimpson and audioguru for your information.every sentence and rhetorical questions that you wrote was related to the issue that i try to understand.
so thanks again.
and best regards

 

[rgbbv]

Hi again
i thought i can use ronv graph,but unfortunately i could not.the graphs are constant current curves.and i was aware to it,but i thought at first that i can combine between the 0.5A and 1A graphs.unfortunately,i couldn't.because at direct-drive flashlight the current is constantly decrease as the voltage decrease all the way down and the capacity of the battery play a roll here.
so if someone can share the graph that i am looking for i will very happy

 

[MrAl]

Hello there,

What are you going to use this for?

I ask because the LED forward voltages vary a bit from piece to piece so one might not perform the same as the other because even a small voltage difference could make a large difference when doing direct drive. A series resistor calms this difference down a bit but if not using one then you'll see a wide variation in discharge times.

It may be that you'll have to do this graph yourself with your particular LED. That may be the only way to get the right graph because it is very hard to predict this sort of thing on the fly. You might also see a difference with different ambient operating temperatures.

Generally if you know the current during the discharge time you can estimate the run time based on the capacity of the battery, but it seems you dont know that yet.

 

[ronsimpson]

You don't know the current because it is very dependent on battery voltage.

You can break up the project into small pieces.
1) Take the resting voltage of the battery and the internal resistance and determine the LED current. Then pretend that current is constant until the voltage drops 0.1V. Now take the battery voltage, at 0.1V lower, find the current and run for another 0.1V until the current gets too low.
2) You can do the same thing but every 10 minutes (or 1 minute) do the math again. At first you may need to only run for 1 minute because the voltage is dropping fast. Later step out the time out to every 10 minutes because the LED current is down and the draw is much less.

Any way it will take some math and graph drawing. I did this 5 years ago with a graph and with real parts and found the current drops greatly at first. From 20mA to 10mA was a short time. From 2mA to 1mA was much much longer. From 200uA to 100uA was days.

 

[rgbbv]

Thanks guys.
so i see that it is not so easy to find general graph for this issue,and i understood from your replies what are the reasons for that.
MrAl,about your question.i am going to had a resistor to direct drive flashlight,and i tried to see on the graph the difference between both.i know that with resistor the runtime will be longer,and it is safer but i tried to see,exactly,how fast are the changes in the battery voltage and current overtime.
Thanks ronisimpson for your explanation how to make that graph.though,in that graph i am going to decide(to guess)how much time it will take for the battery to drop 0.1v,while i tried to find a graph that will show me that situation on real.

thanks for your help.
best regards