What is the best and safest way to tell when I need to recharge my 18650 cell used in a flashlight? For example would I need to recharge it as soon as I notice it getting dimmer or when the flashlight wont turn on again?
I know I have to be very careful with unprotected 18650’s
If the light doesn’t have low-voltage-protection (and even if it does…) you should recharge the battery as soon as the light will no longer go into high modes, or if you notice it dimming. Or sooner. It doesn’t hurt a lithium-ion battery to top it up, even if it still has plenty of charge left. I usually recharge mine well before they’re even half-drained. I want a light with a full charge, so it’s good to go when I want to use it.
Charge it when it is convenient to charge it, or when it becomes inconvenient not to charge (noticeable dimming, lack of high modes).
Get a cheap multimeter from AliExpress.
There’s no easy/simple answer. Depends on the light/driver.
If it’s regulated, you won’t see any dimming until it falls out of regulation. When that happens depends on the mode and the LED voltage, so it might start getting dimmer when you still have 80% juice left, or when it’s just about ready to cut off.
If it’s unregulated, either just parallelled resistors, FET-driven, etc., then it’ll start dimming gradually right from the first time it’s turned on. You might be able to eyeball it, as it dims then you’ll need to crank it up to medium when low used to suffice, or high when medium used to suffice, etc.
If linearly regulated, as the cell fades, maximum brightness just gets less and less, so that low is low, but switching to medium is barely brighter than low, and turbo also almost imperceptibly brighter. Same happens with Li cells.
So… depends on the light.
Best thing to do, ultimately, is just periodically check the cell with a voltmeter.
Most lights I have, I don’t recharge the cells unless I can see them visibly fading, or if it’s just been, well, what seems like forever. I don’t use one light exclusively, so carry #1, use #2 indoors at night, keep #3 for going outside, etc., so each one gets rotation pretty much daily, some more, some less.
Helps that I always have at least one spare cell ready to go, but in a pinch can pull the cell from one to feed the other. My only limitation is using flattops in lights that need buttontops.
And I use unprotected cells almost exclusively, and some lights do not have LVP, so you just need to be able to eyeball if they’re getting “too dim”.