What is the best way to calculate the runtime of a 12v battery if I am using 12 - 1 watt LEDs using 320mA @ 3.6v? Can someone post the formula?
Ok, we have:
- 12 LEDS with Vf=3.6V and Iftot=320mA when in parallel (If=24mA per LED).
- 12V battery with let's say 1000mAh (you haven't specified the capacity)
- The battery voltage will be regulated down to 3.6V with voltage regulator
Assuming you use a switching regulator with 100% efficiency we can calculate:
Ibat = Vf / Vbat * Iftot = 96mA
Thus the battery would last:
t = 1000mAh / 96mA = 10.41h
Since a switching regulator has more like a 80-90% efficiency, expect 10-20% less.
If you use a linear regulator like the LM-series, the difference between the input and regulated voltage is dissipated as waste heat. Then it would just be:
t = 1000mAh / 320mA = 3.12h
And your regulator would get pretty hot (P= 12V - 3,6V / 320mA = 2.7W).