Tail cap vs. led output current

[LED4LYF]

I am driving a cree xre-r2 with a 3-mode 1.4 A driver I bought from download, the power source is 2 AW protected li-ion 18650 cells. Theoreticaly what should my tail cap current be? and what does that reading tell me?

 

[alpg88]

i always thought tail cap reading closely represents battery load, and in case of led dd or dd hot wire what bulb or led draws, there is a slight difference thou, between tail cap reading and actual current, but not much.
in case of leds or hot wires with drivers, it wont tell you what leds or bulb draws, only load that battery sees, for bulb\led current you need to splice dmm's probes in series between bulb\led and driver.

as for what current you should see, well definitely less than 1.4a if you use 2x18650, my guess is around 1a. there is a way to calculate it, you need to know few variables, like driver efficiency , resistance,,,etc.

 

[Der Wichtel]

with linear drivers or resistors the tailcap current is the current through the LED

 

[AnAppleSnail]

I am driving a cree xre-r2 with a 3-mode 1.4 A driver I bought from download, the power source is 2 AW protected li-ion 18650 cells. Theoreticaly what should my tail cap current be? and what does that reading tell me?

With a driver you have to calculate the power going to the LED, factor in driver efficiency, and then find the current to get that power from the batteries.

Assume:
Vf = 3.7v
Driver efficiency = 85%
Battery voltage (at turn-on) = 8.4v
LED current = 1.40 A

LED Power:
W = Vf*I
W = 3.7v * 1.4A
W = 5.18

Driver power:
Win * Efficiency = Wout
Win * 85% = 5.18watts
Win = 5.18 / 85%
Win = 6.09 watts

Battery power
Wbattery = Win
Wbatt = Vbatt * I
6.09 = 8.4 * I
6.09/8.4 = 0.725 A

If this really is a regulated driver, it'll take more and more power as battery voltage drops. At 7.2v (nominal load voltage of 2 RCRs) it takes 0.84 Amps. Vf is variable and drops with temperature.

 

[1301]

edit: I was too late. nevermind

 

[LED4LYF]

I took a tail cap reading and got 800ma but the batteries were not freshly charged. So does this mean my led is only getting 800ma and not the 1.4A?

 

[MikeAusC]

If the 2 18650s are in series you have from 6.0 to 8.4 at the battery - that can only drive an LED by going through a switchmode converter.

The LED current will be APPROXIMATELY twice the battery current.

 

[LED4LYF]

Yes the cells are in series so my led is getting about 1.6A? That is over the spec of the driver it said only 1.4A. How can that be?

 

[DIWdiver]

Yes the cells are in series so my led is getting about 1.6A? That is over the spec of the driver it said only 1.4A. How can that be?

It's probably close to 1.4A. Losses in the driver make the current ratio a little less than the inverse of the voltage ratio. That's why Mike emphasized 'approximately'.

D

 

[LED4LYF]

Cool thanks guys that really clears things up for me.