Am I asking too much from my batteries?

Lumbee1

Newly Enlightened
Joined
Jun 21, 2006
Messages
105
I have a dual CREE light that I want to run at 1A. The LED's are in series and I am using a Fatman for regulation. Total LED Vf is ~6.8V. The Fatman is a boost converter so I needed a battery pack less than 6.8V. I purchased a Li-Ion 18650 3.6V 7200mAh battery pack and a Li-Ion 18650 3.6V 4800mAh battery pack. The 7200mAh pack was defective. With the second battery pack I am only able to get ~750mA to the LED's. Further adjusting of the trimpot does nothing to increase the current.

Am I expecting too much from the 4800mAh battery pack? Would a working 3.6V 7200mAh battery pack supply 1A to the LED's? Do I just need to look for other alternatives?
 

chimo

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Sep 16, 2004
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Ottawa, Canada
Math time.

Let's say the LEDs are 6.8V when in series at 1A.

Total power to the LEDs is 6.8 Watts.

Let's assume an 80% converter efficiency.

Power the batteries must supply is 6.8/0.8 = 8.5 Watts.

If your battery pack is in parallel, let's assume the loaded voltage is 3.6V

The pack must supply 8.5/3.6 = 2.36 Amps.

That is around 1C for the newer 18650s so it should be OK. Perhaps you have a wiring issue.
 

PhotonAddict

Enlightened
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Dec 10, 2004
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357
Location
Mtl, Canada
I believe the Fatman is limited to a max. input current of ~2A, so you may be running into that restriction. A measurement of the current being drawn from the batteries might help.
 

chimo

Flashlight Enthusiast
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Sep 16, 2004
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Location
Ottawa, Canada
Just checked George's site (TaskLED):

"Input_current should be around 2A or less for optimal performance of Fatman. Fatman can run up to around 2.2A max. When running at high input power it is recommended to thermally epoxy the back of the Fatman PCB to a heatsink or the body of the flashlight."


My efficiency swag may be a little low. If it is 85% then the batteries would only have to supply around 2.2A (90% => 2.1A / 95% => 2A)
 
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