Torch Battery Runtime Calculations

[computenik]

Morning All,

Finalising Battery Design but need some assistance please to work out run times. As am basing off runtimes from other torches i have and figures i have found on the Web.

Battery is a Custom Built 14.4V Li Ion Pack (3400Mh 18650 Batterys 4S4P)
Head is a Cutter Electronics 3x XML 2 U21C a PCB (LED's in Series)( Cree XML2 have a Vf3.3V @ 3000mA)

So i have Worked out
3400 mAH Batteries @ 3.6V = 14.4V 13600mAH = 195.8 WH

Capacity mAH / Device Consumption mA * 0.7(External Factors) /1000 = Runtime
(13600 mAH / 3000mA) * 0.7 /1000 = 2.98 Hrs

Also wish to work out Watts if anyone can have a formula for me? Each Cree at 3A producing ~1000lm

 

[DIWdiver]

Well I don't know where you got your math, but my calculator shows that 13600 mAH/3000 mA * 0.7 = 3.17 hours (notice I dropped the /1000).

Watts is pretty easy, just volts times amps. If you want to calculate at the LEDs, 3.3V * 3A = 9.9W. Power drawn off the batteries will be somewhat higher, but can only be calculated if you know or assume a driver efficiency. If you know the driver efficiency, power at the LED is Vled * Iled , while power at the battery is Vled * Iled / efficiency.

You assume a driver efficiency of 70%, evidenced by the 0.7 term in your formula. This is pretty low for a quality switching regulator, so it's pretty conservative in that condition. With linear regulators, this could be either high or low, depending on the battery and LED configuration. With 14.4V battery and 9.9V LED, it's probably fairly accurate, though the efficiency will change over battery voltage during discharge.

 

[lucca brassi]

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