I mean, I put my 4.22V 18650 into my 8*7135+XM-L light. What happens to the current, V, etc? especially as battery drops below what the driver puts out ( 3.6V?)
What happens to V, etc, when you get down around the Vf? 2.9-3.25V?
Very simple answer- it drops out of regulation, it’s no longer able to maintain the requested current and will direct drive the LED at a level limited by current battery voltage.
Another way to put it (note I do not have a graph in front of me, these are just random number’s picked out of the air)
Let’s say you’re using an emitter than has a vF of 2.85v at 750mA but rises to 3.3v at 1A (once again randomly picked numbers) and you’re running 4*3175’s for a total current of 1.4A (and at 1.4A the LED’s vF is 3.8). Remember to figure in all the total added up drop out, now let’s say your batters drops down below the needed 3.8V for the full 1.4A of regulation, as you get lower and lower remaining voltage you can follow the graph down to see what the max possible current the LED can pull is, when you get down to 3.3v the LED is only able to pull 1A, at 2.85v the LED can only pull 750mA. It doesnt matter how many 7135’s you have, even if you have the LED wired straight to the cell, if there’s only 2.85v for it to use it can only pull a max of the 750mA (which we get from the graph)
Crap, what a serious error on my part. I miss-spoke when I said that Vf must be higher than battery voltage, how confusing! I will edit my posts with strike-through to show the correct text ASAP. In the meantime understand that I simply said it backwards: LED Vf must be LOWER than battery voltage.
(it actually must be lower than battery voltage minus dropout voltage of course)
Oh that’s ok that makes sense. I thought it was that way, I see V as pressure, and if you don’t have enough, won’t go, although Vf isn’t constant in leds like most stuff, I don’t think.
Ok I’m starting to get this. CK where did you get the 3.8V from? You add up the 4x 7135’sx 0.12v, then whatever else is on the board? So thats what you were talking about Wight, with low Vf and drop out?
What would happen between 3.6 and Vf, though? Wouldn’t you still be above Vf, so you get more current than datasheet recommends?