Help needed calclating run time?

I got my cree's wired up today, thanks to all the tons of advice found here on CPF.

But i would like to know how long will they stay on for?

I have:

3 Cell 11.1v 5000Mah Li-Polymer Battery that auto shuts off at 9 Volts.

4 Cree DX Q5's wired in series with a total forward volatge of 14.4V (measured) at 980mah constant, provided by a Blue shark from sanwiche shoppe.

Cant remember how to work it all out. Cheers.

Anyone?

14.4V and 980mA is a 14.1W draw. The pack has 55.1Wh, so the light will run for nearly 4 hours.

gav6280 said:

I got my cree's wired up today, thanks to all the tons of advice found here on CPF.

But i would like to know how long will they stay on for?

I have:

3 Cell 11.1v 5000Mah Li-Polymer Battery that auto shuts off at 9 Volts.

4 Cree DX Q5's wired in series with a total forward volatge of 14.4V (measured) at 980mah constant, provided by a Blue shark from sanwiche shoppe.

Cant remember how to work it all out. Cheers.

Click to expand...

Well, let's see. . .

The power being consumed by your LEDs is 14.4 v * 980 mA = 14.1 watts. The voltage of your battery will drop as it's discharged, and it might average, say, 3.5 volts/cell over the discharge period. So the total energy you can get out of your battery would be about 3.5 v * 5000 mA-h * 3 cells = 52.5 watt-hours. What you don't know is the efficiency of the regulator. If it were 100% efficient, you'd get 52.5 w-h/14.1 w = 3.7 hours. Something like 70 - 80% is a likely efficiency range, so you should get something like 3.7 * 0.7 to 3.7 * 0.8 = 2.6 - 3.0 hours.

But why not just measure it?

c_c

Measure the current draw from the battery pack. It should be higher than that to the LEDs. Divide 5.0 Ampere hours by the current (in amps) and that is your theoretical ideal run time. So say you draw 1.25 amps from the batteries. Your run time would be 5.0/1.25 or 4 hours. Max in theory. If you run your light in short bursts you may get that. If you simply run it straight through and the batteries are getting warm, may be reduced. Possibly a worst case 20% reduction of run time if your current really isn't much past 1/4 of total battery capacity.

Let us know what the battery current draw is, that will also help to calculate the efficiency of the driver.