Join Date: Aug 2006
Location: Minneapolis, MN
Bikes: 2010 Surly Long Haul Trucker, 2009 Surly Big Dummy, 200x Surly 1x1, 1972 Schwinn Paramount P-15, Bianchi Pista, Kabuki (Bridgestone) Submariner
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I've been happily running a 3W "cool white" MR16 LED for about two years now. $16 from autolumination.com
, search for "MR16 12 Volt Super High Powered 3 Watt Luxeon LED".
I paired it with a 20W halogen 10-degree spot bulb ($9 for 3 at Lowes). While the 3W LED is not as bright as the halogen, I find myself rarely using the halogen for seeing; usually I just use it for wet asphalt or attracting drivers' attention. Yesterday yet-another oncoming cyclist (on the other side of a 4-lane road) shouted "whoa that light's bright," but YMMV.
Two years ago I measured the current draw (with the xenon strobe tail-light) around 400mA, IIRC, so I generally figure about 2 hours runtime per usable battery Amp-hour. (Apply lead-acid battery low temperature de-rating as needed. Nickle based batteries are supposedly exothermic on discharge, so they're a better choice for cold climates).
Running two of these 12V, 3W LED's gives 6W. P = V * I, so I = P / V; so I = 6W / 12V = 1/2 A, so 6 hours requires 3 (usable) Ah of battery capacity. I have been using a 7Ah SLA and figure I have at most 3.5 usable Ah in the cold Minnesota winters. Also keep in mind that SLA supposedly don't like being fully discharged, so don't do that.
Last edited by Zekat; 11-20-08 at 06:44 PM.