/diy/ - Do It Yourself

>>2539954
That's a boost converter, so when it's turned off there will still be a nonzero voltage across the LEDs. If instead of connecting the LED negative line to ground you connect it to the positive leg of the PSU/battery, you end up with some sort of inverting topology. Pic related.
If you do go for a 3V coin cell, you likely won't need any boosting at all if you can go for parallel LEDs, though it depends somewhat on LED wavelength.

As for making the most out of battery life, consider the posts from last thread about flashing an LED to keep some phosphor glowing. Read: >>2538168 and the replies to it.