Battery problems for my custom bike light

Lumbee1

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Jun 21, 2006
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Specs:
Dual P4 CREE in series
Fatman with external log pot
3.6V 4800mAh Li-Ion

This has been a crazy learning experience for me. It seems like just a few months ago, I thought a Fenix L2T was the ultimate bike light.

The problem I am having is that I am not able to get 1A to my LED's. No matter how far I adjust the trimpot on the Fatman, I am only able to get 750mA. The battery comes with a 8.5A protection PCB but I don't think that would limit me to 750mA. Am I missing something here? Is the battery just not capable of delivering 1A?
 

Blindasabat

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Since nobody has posted yet, I'll take a shot. Your battery should have no problem pulling 1Amp, I have gotten over 1Amp from a single RCR123 and about an Amp from an RCR2. Single LED though.
But I don't know anything about the Fatman...
Maybe you could experiment and try it without the Fatman and pot to see what your draw is to see if (or eliminate) that as the problem. If you still don't get over 750mA, then you have resistance in the system or a bad battery. Some connection or material could be causing a lot of resistance.
Try another battery if you have one to eliminate that. Even 3AA or 3C NiMh to about duplicate an RCR or any other larger LiIon 17650/18500, etc.
 

cmacclel

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How are you determining 750ma at the led's?

Ideally with the fatman you would want to be just below the forward voltage of the LED's. Did you measure the current at the battery?

Mac
 

Lumbee1

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I have not measured the current at the battery. I am measuring the current at the LED's by putting the DMM in series with the LED's.

I really wanted to use the 18650's but the only configuration is to run them in parallel to keep them at 3.6V. There were not too many battery options between 3.6V and 6.6V (Vf of the LED's)
 

cmacclel

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Lumbee1 said:
I have not measured the current at the battery. I am measuring the current at the LED's by putting the DMM in series with the LED's.

I really wanted to use the 18650's but the only configuration is to run them in parallel to keep them at 3.6V. There were not too many battery options between 3.6V and 6.6V (Vf of the LED's)

4AA's would be ideal.

Mac
 

Sway

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4 in series is too much for the FatMan even with low Vf LED's, try it with 3 and you should be able to push 1A.

Later
Kelly
 

cmacclel

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Sway said:
4 in series is too much for the FatMan even with low Vf LED's, try it with 3 and you should be able to push 1A.

Later
Kelly


I thought he was only running 2 XR-E's?

Mac
 

glockboy

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houston, tx
Dual P4 CREE in series= 7volt @ 1 amp
4AA= 4.8 volt
3.6v lithiun.
What Mac say "4AA's would be ideal." for the Fatman.:thumbsup:
 

dom

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This is a quote from George at the Taskled forum -about the Fatman.

"I'd suspect your meter/measurement. Note, that including a meter in series with any current measurement will create a voltage drop due to the meter being in circuit.

To measure efficiency would need in theory 4 meters, 2 measuring current and 2 measuring voltage."

Cheers
Dom
 

Sway

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cmacclel said:
I thought he was only running 2 XR-E's?

Mac

May be I'm reading him wrong, my mistake.

Lumbee1 how many LED's are you running in series total 2 or 4?

Later
Kelly
 

Lumbee1

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2 CREE's in series.
2 18650's in parallel.

I think it is a limitation with the Fatman. I have not measured current draw from the battery but from the calculations it looks like it is very close to the Fatman maximum input. At 750mA, the light is quite impressive. I dialed it back down to 700mA to be safe and will AAA the converter to the top of the housing.
 

chris_m

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Couple of things:

To get 1A at the LED, that will be up to 2.25A at the battery with that set up (which as you recognize is pushing the boundaries of the Fatman).
To measure current with a Fatman, simply use you DMM to measure the voltage across the sense resistor (the one next to the LED connections). Unless George has changed this it's a 0.1 Ohm resistor.

FWIW, I run 3 Crees in series at 1A with a Fatman from a 7.2V LiIon battery. Even got this to work OK at 700mA with a 3.6V battery.
 

Lumbee1

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chris_m said:
Couple of things:

To get 1A at the LED, that will be up to 2.25A at the battery with that set up (which as you recognize is pushing the boundaries of the Fatman).
To measure current with a Fatman, simply use you DMM to measure the voltage across the sense resistor (the one next to the LED connections). Unless George has changed this it's a 0.1 Ohm resistor.

FWIW, I run 3 Crees in series at 1A with a Fatman from a 7.2V LiIon battery. Even got this to work OK at 700mA with a 3.6V battery.

Very interesting...

From my calculations
3.4Vf * 3 * .700mA = 7.14
Assuming 90% efficiency
7.14 / .90 = 7.933
7.933 / 3.6V = 2.203mA input

As far as the measuring the voltage across the sense resistor, where would that be located?
fattop.jpg
 
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