powering my homemade headlamp 3wled

hi ive a lumileds 3w luxeon led headlight ive built up for cycling going thru a luxdrive 1000ma buckpack powered by a 9.6v 2500mah battery questions are is this too much of a battery and would i be better with a 4.8v 2500mah battery and what will the expected runtimes be on it with the 9.6v battery will i gain much runtime over using a 4.8v ??? many thanks

The ledsupply website states buck puck's maximum input is 32VDC, so you should be ok with your 9.6v battery.

firestarter4075 said:

hi ive a lumileds 3w luxeon led headlight ive built up for cycling going thru a luxdrive 1000ma buckpack powered by a 9.6v 2500mah battery questions are is this too much of a battery and would i be better with a 4.8v 2500mah battery and what will the expected runtimes be on it with the 9.6v battery will i gain much runtime over using a 4.8v ??? many thanks

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The Buckpuck will take care of providing the proper current and voltage to your LED.
1. Let's examine the power draw of your Lux III. Assuming a forward voltage of 3.75v @ 1000mA = 3.75 watts. The forward voltage depends upon the bin of your Lux III. i.e, it could run closer to 4v.

2. You'll be drawing a little more than that from the battery due some inefficiency in the Buckpuck (say 4 watts).

P = IV, I = P/V

For the 9.6v battery: I = 4/9.6 = 0.427 amps, so you'll probably be drawing less than 500mA from your 9.6v battery giving you 5 hours in theory.

Forthe 4.8v battery: I = 4/4.8 = 0.83 amps, giving you nearly 3 hours (in theory). In both cases the light will get dimmer toward the last 25 percent of the time period as the batteries get weaker.

- Hope this helps.

Question is runtime needed, all else equal the 4.8V battery pack is going to have half the cells in series (therefore half the weight) of the 9.6V pack. If the size/weight doesn't matter, then no point in changing anything (9.6V 2500 mAh being equal to 8xAA NIMH batteries nowadays, the weight/size difference isn't such a big deal compared to 4).

The BuckPuck datasheet says that you need a 2v drop above the Vf of the LED.
3.75+2=5.75 minimum.
Also (for red led) the circuit is listed as having a minimum of 5v input

To get the full capacity out of the cells you need to drain the cells to 1.0 volts per cell - I make that a minimum of 6 cells.
Having said that a buckpuck will work with 4 fresh NiMh cells but I dont know weather that was supplying the full current and how much run time you would get.

Out of interest I dont know what happens when the circuit drops out of regulation.

Nick

nightrider said:

In both cases the light will get dimmer toward the last 25 percent of the time period as the batteries get weaker.

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Not with the 9.6V battery it won't - the buckpuck will take care of that (it will pull more current from the battery as its voltage drops - which means you should be careful when you get near the end as it will very rapidly damage your battery due to high current draw if you're not careful). With the 4.8V battery all bets are off - it's likely to dim for rather more than the last 25% due to insufficient headroom.