# Help: Is there a Battery (runtime) calculator anywhere

#### blakelidge

##### Newly Enlightened
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
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
Thanks,

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

Dave