Help: Is there a Battery (runtime) calculator anywhere

blakelidge

Newly Enlightened
Joined
Oct 13, 2008
Messages
14
Hi, first post in the forums so apologies if I am putting this in the wrong slot.

I hope to be building some lights for the mountain bike soon. I am provisionally intending to use three of these http://uk.farnell.com/1574561/optoelectronics/product.us0?sku=cree-xrewht-l1-0000-00d01.

Spec is:
  • LED, COOL WHT, 107LM
  • Colour, LED:Cool White
  • Angle, viewing:90°
  • Current, forward If:1000mA
  • Voltage, Vf max:3.7V
  • Beam Pattern:Lambertian
  • Case style:Custom SMD
  • Colour temperature, max:10000K
  • Colour temperature, min:5000K
  • Current, If max:1A
  • Depth, external:9mm
  • Length / Height, external:4.4mm
  • Luminous flux, min:107lm
  • Termination Type:SMD
  • Voltage, Vf typ:3.7V
  • Voltage, reverse protection:5V
  • Width, external:7mm
These are rated to handle 3.7v max (each) and they run at 1000mA These will be driven with a buck driver (which I understand will accept an input voltage range from 6v to 30v and "step-down" the output voltage to drive the leds.

My question:
How do I calculate the run-time I am likely to get from any particular battery for these three leds, is there a formula I could use?

Ideally I was hoping to be able to use my Makita 18v Li-Ion pack, these are rated at 3.0ah.

Appreciate any help/ideas as it keeps getting darker and I should be out riding :)

Dave
 

MorePower

Enlightened
Joined
Nov 4, 2006
Messages
643
Location
Wisconsin
Runtime will depend on the efficiency of the converter, voltage sag from the battery, true capacity, and LED Vf at a given If.

If the numbers you gave were accurate (in reality, you'll likely see significant deviations with regards to Vf at 1000mA If), you'd be looking at the following:

P led = 3 x 3.7V x 1.0A = 11.1W
Battery = 18.0V x 3.0Ah = 54Wh

Let's say efficiency is 95%, which for a good buck converter may be possible.

P in = P led / Eff = 11.1W / 0.95 = 11.68W

Therefore Runtime = Battery / P in = 54Wh / 11.68W = ~4.6 hours

I'm thinking you could run a 1000mA BuckPuck with the LEDs wired in series. That would give you ~7V over Vf, which is more than enough for the BuckPuck. Efficiency in this setup would be around 87% or so based on the datasheet from LEDDynamics.

I'm sure someone will be along to check my numbers shortly... :)
 

blakelidge

Newly Enlightened
Joined
Oct 13, 2008
Messages
14
Thanks,

Your reply was really helpful (and makes me happy) because now I know it's well worth getting on and building some :)

Dave
 

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