# Calculate 12v Battery Runtime When Using Only 3.6v

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?

• How do you step down the voltage? So you plan on putting the LEDs in parallel?
– Rev
Nov 16, 2016 at 7:15
• Parallel and I plan on using a voltage regulator to step down from 12v to 3.6v. Nov 16, 2016 at 7:17
• Switching or linear regulator? If switching, what's the expected efficiency?
– jonk
Nov 16, 2016 at 7:34
• Actually I just realized I have a buck converter laying around so I would be using that. Nov 16, 2016 at 7:47
• You should really put details like that (and more) in the question. Without it, this is impossible to answer... Nov 16, 2016 at 13:34

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).

• With a linear regulator (and from a prior post, the OP has to have this all working in a few hours' time), the efficiency will be 30%. So the time will be a LOT less, if that's all he has to work with.
– jonk
Nov 16, 2016 at 7:42
• Battery would be a 80Ah starting battery. Nov 16, 2016 at 7:45
• @jonk: Supplemented info for linear regulator.
– Rev
Nov 16, 2016 at 7:46
• Yeah a regulator would be out. I have a buck converter that I can also use. Nov 16, 2016 at 7:52
• @Mr.SCW: Well, then everything should be clear? ;) (PS: a buck converter is also a regulator, the LINEAR is the thing you do not want)
– Rev
Nov 16, 2016 at 8:00