I see direct drive thrown about every which way, but I can’t seem to get a grasp of what it actually does long term.
From what I’ve read, using direct drive with a single emitter and a single battery will run at 100% efficiency. That seems to make sense, but what happens as the battery is drained? Do the amps supplied slowly drain along with the voltage? Do the amps supplied stay constant as the battery is drained?
I’m trying to figure out roughly how long a KP protected 14500/Efest IMR will run a triple XP-G2 in direct drive as I’m more for functionality over wow factor in a more expensive mod. Also wondering if an LSD AA NiMH can direct drive the triple?
I believe that people here are a bit reluctant to direct drive (as in: directly connecting the led to the battery, no driver used) their leds from a (single) Li-ion because until not so long ago it would just destroy the led (just a while more back everyone was happily direct driving their incan's). Since we have the copper boards, the xml(2), the xpg(2) and the Nichia's can handle being driven directly, with the benefit of having no driver losses and maximising output. Drawbacks are loss of modes (even direct drive with a pwm-controller causes some voltage loss, I found), and no battery protection (but you can use protected batteries for that, or just watch your output drop, as I do with direct driven lights with an IMR battery).
What happens when the battery drains is that it supplies less and less voltage, so the current through the led (and thus output) drops with it, until the minimal voltage is reached at which the led lights up at all, but the current will be so low near that voltage that the battery will still be able to supply that for a very long time (the light will give a moonlight output).
As for your runtime question, I'm a bit lazy now to do the math exactly, but doing some rough guesses (on Efest IMR 14500), if the electrical path in the light is well done with not much resistance anywhere, each of the three xpg2's will see about 1A, so 3A with a 0.7Ah battery will give about 14 minutes runtime.