Cree XR-E Q5 at 2000mA with 2xAAA lithium batteries

[mailint]

ummm... camera flash do use high voltage...

the xenon flash wun be able tor un from those pity low voltage batteries gives.

it needs thousands of volts

Camera flashes use capacitors to obtain those high voltages from 2xAA batteries. There's no need to give them high voltages directly from batteries, that is what you're worried about.

 

[evan9162]

With 2xAAA lithium batteries in direct drive and Cree XR-E P4 I only reached about 50mA because of the voltage drop of the batteries when connected to the load.

The 50mA had NOTHING to do with the voltage drop of the batteries under load. Even the terminal voltage of AAA alkalines should only drop a miniscule amount under a 50mA load. I don't think you really understand the relationship between current and voltage in LEDs.

http://candlepowerforums.com/vb/showthread.php?t=77221
http://candlepowerforums.com/vb/showthread.php?t=72528

I'm stunned that you obtained such an higher voltage with 3xAA Ni-MH under the very heavy load of 2.5-3A. What was the total voltage of your batteries without load? 5V? from 3xAA Ni-MH?

NiMH cells can output a TON of current without dropping voltage very much. You should read up on the performance of rechargable cells as well. It should surprise no one that AA NiMH cells can output 2.5-3A while sustaining a high terminal voltage.

 

[mailint]

The 50mA had NOTHING to do with the voltage drop of the batteries under load. Even the terminal voltage of AAA alkalines should only drop a miniscule amount under a 50mA load. I don't think you really understand the relationship between current and voltage in LEDs.

http://candlepowerforums.com/vb/showthread.php?t=77221
http://candlepowerforums.com/vb/showthread.php?t=72528



NiMH cells can output a TON of current without dropping voltage very much. You should read up on the performance of rechargable cells as well. It should surprise no one that AA NiMH cells can output 2.5-3A while sustaining a high terminal voltage.

"Today I received my first LEDs to play with: a Cree XR-E, a Luxeon Rebel, a SSC Z-power P4, all pre-mounted on a star or PCB.

I immediately tested them directly connected to 2xAAA lithium Energizer batteries and I was disappointed by the results.

All were of similar brightness, the batteries went down from about 3.260V without load to 2.850V with the LED connected/turned on.

The currents were about 50-70mA the first time. Then, on a second measurement after a minute they were 40-60mA. Then, on a third measurement after a minute they were 30-50mA.

All the measurements last 1-3 seconds."

 

[evan9162]

"Today I received my first LEDs to play with: a Cree XR-E, a Luxeon Rebel, a SSC Z-power P4, all pre-mounted on a star or PCB.

I immediately tested them directly connected to 2xAAA lithium Energizer batteries and I was disappointed by the results.

All were of similar brightness, the batteries went down from about 3.260V without load to 2.850V with the LED connected/turned on.

The currents were about 50-70mA the first time. Then, on a second measurement after a minute they were 40-60mA. Then, on a third measurement after a minute they were 30-50mA.

All the measurements last 1-3 seconds."

Sounds like dead/bad batteries to me. AAA cells should not drop that much under that kind of load, ESPECIALLY Lithium AAAs.

Where were you measuring the voltage (cells or at the terminals of the LED)? How were you measuring current? What setting on your DMM for current measurement?

This is almost unheard of behavior for fresh cells. Something is amiss.

 

[biwema]

Camera flashs do use a high voltage which is generated from AA cells usind a driving circuit. I think that the efficiency of such a circuit is not optimal and there is quite a loss. Also it might be dangerous if the device breaks in the water. (Never touch charged flash capacitors in cameras)
For my aplication a bright LED would better be suited.

First I wanted to suggest a Luxeon Rebel for running at 2000mA instead of a CREE due to the high temperature limits of 150°C. But when having a look at those two datasheets it turned out that these LED are very similar. Even some diagrams look like an inedtical copy.

When using 2 AAA cells you wont get the voltage needed without any driving circuit. A Energizer Lihium Cell has a capacity of 1.68Ah at 0.5A (link below). Maybe they can even provide 2.5 A at a reasonable Voltage. Some AA cells can even be discharged at 10A and still have a voltage of more than 1V. When using and appropriate circuit, 2000 mA with 2 AAA cells is possible, but not for a long time.

Have a look at these threads about benchmarking Alkaline (including Energizer Lithium) and NiMH cells (AA and AAA).

Alkaline:
https://www.candlepowerforums.com/threads/64660
NiMH:
https://www.candlepowerforums.com/threads/79302

 

[mailint]

Have a look at these threads about benchmarking Alkaline (including Energizer Lithium) and NiMH cells (AA and AAA).

Alkaline:
https://www.candlepowerforums.com/threads/64660
NiMH:
https://www.candlepowerforums.com/threads/79302

Very useful. Thanks! I understand that the voltage drop of such configuration allows for such currents/voltages.
Sorry evan9162 and thanks biwema.
A doubt: if I measure 3A out from 3 batteries in series, do I have to refer to the 1A line or to the 3A line in those graphs? still 3A, right?

 

[evan9162]

The 3A line

I just did a test of my own. Using 2 Eveready Gold alkaline AAAs (pretty generic quality) in a crappy 2AAA battery holder (so plenty of extra resistance).

I measured current with the 10A scale of a DMM.
I measured the voltage of the batteries at the ends of the 4" leads attached to the battery holder.

Open circuit (no load) the batteries measured 3.22V
Directly connected to a P4 XR-E, the batteries measured 3.08V, and the XR-E was getting 180ma.

So, with crappy alkaline AAAs, in a crappy battery holder (introducing extra resistance and extra voltage drop at the point the battery voltage was measured), the battery voltage still didn't drop anywhere near what you reported, at over twice the current level that you reported.

There's definitely something wrong with your batteries, connections, or both.

 

[mailint]

There's definitely something wrong with your batteries, connections, or both.

Consider that they were used batteries...

 

[biwema]

I immediately tested them directly connected to 2xAAA lithium Energizer batteries and I was disappointed by the results.
All were of similar brightness, the batteries went down from about 3.260V without load to 2.850V with the LED connected/turned on.
The currents were about 50-70mA the first time. Then, on a second measurement after a minute they were 40-60mA. Then, on a third measurement after a minute they were 30-50mA.
All the measurements last 1-3 seconds."

In this point I agree with evan9162. Those Lithium cells are bad. With AAA Energizer Lithium you should still have more than 1V after 2 hours at 500mA; at 70mA it should run 18-20hours.

- When you measure 3A at 3 cells in series, every cell has 3A. Their Voltages sum up.

- When you measure 3A in parallel, every cell is discharged at 1A, on average. The trongest one a bit more, the weakest one a bit less. The Voltage equals the voltage of one cell.

Currents are measures in series (with a low internal resistance of the DMM), voltages in parallel (with a high internal resistance of the DMM).

Important:
If you run a LED on 2-3 AAA cells and you measurea high current, there will be a voltage drop ober the Multimeter. Maybe 0.1V to 0.5V depending on the internal resistance. Without the Multimeter in series the voltage over the LED would be higher and the current much larger. £This could destroy the LED. Try using a clamp-on amperemeter which supports DC.

 

[mailint]

In this point I agree with evan9162. Those Lithium cells are bad. With AAA Energizer Lithium you should still have more than 1V after 2 hours at 500mA; at 70mA it should run 18-20hours.

Yes but the problem is that the Vf of the LED at 500mA is much higher than 1V. The Vf of a Cree XR-E at 500mA is 3.4V so I'ld never reach 500mA from 2xAAA batteries except, perhaps, for some seconds...

- When you measure 3A at 3 cells in series, every cell has 3A. Their Voltages sum up.

- When you measure 3A in parallel, every cell is discharged at 1A, on average. The trongest one a bit more, the weakest one a bit less. The Voltage equals the voltage of one cell.

Ok, got it. Thanks for the confirmation.

Important:
If you run a LED on 2-3 AAA cells and you measurea high current, there will be a voltage drop ober the Multimeter. Maybe 0.1V to 0.5V depending on the internal resistance. Without the Multimeter in series the voltage over the LED would be higher and the current much larger. £This could destroy the LED. Try using a clamp-on amperemeter which supports DC.

0.1V to 0.5V of error due to internal resistance seems huge to me. Are you sure that the internal resistance can be so much relevant?

 

[evan9162]

Yes, it can

Many DMMs use an 0.1 ohm current shunt. Add in contact resistance for the leads, and that can be even higher. If you're not using the highest scale on your DMM, you could be introducing even more resistance. If your current meter has a 2A and 10A scale, then the 2A scale probably has an 0.5 ohm current shunt. Or worse, if you're using the 200mA scale, it uses a 1 ohm current shunt.

You need to measure the voltage drops across all parts of your circuit to see where extra resistance is creeping in. You'd be surprised.

 

[mailint]

You need to measure the voltage drops across all parts of your circuit to see where extra resistance is creeping in. You'd be surprised.

Incredible. I never thought it was so significant. I will try with different positions next times... Thanks for the info

 

[evan9162]

Cheap battery holders are the worst offenders. Then contact resistance with things like alligator clips.

I was doing some testing last night. The setup was like this:

Power supply -> DMM -> LED+resistor -> Power supply

Each step used alligator clips. Between the connections and wires, I had almost 0.3 ohms of resistance creep in. At one point, the power supply output was 4.5V, while the LED+resistor combo only had 4.2V across it.

 

[biwema]

Incredible. I never thought it was so significant. I will try with different positions next times... Thanks for the info

You can calculate the voltage drop ot the Multimeter that way:

Voltage = Resistance * Current

Example:
Shunt DMM: 0.1 Ohm
Resistance leads of Multimeter: 0.1 Ohm
Connections of Multimeter, LED and Batteries: 0.2 Ohm (maybe even more)
Current: 2A
(0.1 Ohm + 0.1 Ohm + 0.2 Ohm) * 2 A = 0.8 V

At such currents you have a reasonable voltage loss even without multimeter. Try your best to reduce all resistances of the circuit (Clean contacts of the battery, careful soldered connections (instead of alligator clips and thick wires).

If your batteries provide 3.4V, that will not be enough and will drop even more in use.