I have been carrying some Rotary lights, modified to High CRI, that make over 350 lumens on a fresh 16340 battery.
Today I was using one of my Rotaries, and it seemed dimmer than usual, so I put the light on my light meter and it read 200 lumens. I then removed the battery and measured the voltage at 3.28v, so I put it on a charger.
A light meter lets me check if I need to recharge, without removing the battery.
When lumens drop by 50%, I usually notice, and my light meter is very useful for confirming whether it is time to recharge.
I like being able to check lumens, without having to open the light to test the battery.
here you see 391 lumens,
from a battery over 4v
my light meter is nothing fancy, I just put a cap from a vitamin jar over the sensor. I used an HDS to calibrate the meter, by adding a couple of pieces of paper under the white plastic cap.
The meter is surprisingly accurate, after calibrating it to a 200 lumen HDS, the meter correctly read the minimum lumen level of the light at 0.08… iow, the meter is accurate within the entire range of brightness of an HDS.
fwiw, sometimes I use unprotected 1200mAh IMR 18350 Keeppower cells, and the lights do not have any LVP built in.
I was concerned about overdischarge, so I did a test to see how low the lumens drop, when the battery is approaching overdischarge levels.
I found that with a battery depleted to 2.6v, the maximum output of my Rotary was just 15 lumens.
I find this very reassuring, since I will certainly notice if my light wont go over 15 lumens, and even at that level, the battery is not yet in danger of overdischarge. Plus an IMR is safer than an ICR cell, not as likely to vent with fire after a deep discharge cycle…
With a Protected ICR cell, I run the risk of tripping the protection and being left in sudden darkness, but with an IMR, I get lots of warning time to replace the battery, as the light keeps getting dimmer, without sudden darkness.