I understand that the batteries differ and so do the environmental conditions, LED power requirements, and use and recharge patterns. But is there a general trend assuming ‘normal’ flashlight use as to how long it takes for the battery to become less than useful and in need of a replacement in a ‘modern, bright’ flashlight?
I guess what I’m asking is a ballpark: would it be months or (many) years?
Same lifespan as regular Li-ion cells: 2-3 years hard use (cellphones etc), 3-5 for lower use devices, 5+ for rarely used devices, especially if you don’t charge to 100% and then put into storage.
I’ve some old panny B’s that must be 7+ years old now, they do fine in lower power draw lights.
Nice summary and answer! 
To expand, the higher the current draw on the cell(s) the sooner you’re likely to see the consequences of “aging”. Lots of hot-rod lights on the market are limited only by the amount of power the cell can supply.
As the cell ages, peak output will likely reduce as the higher internal resistance increases and the voltage then “sags”.
Thanks a million! May I ask about yet another aspect of it: I live in the tropics - how much does the temperature matter?
LiIon cells usually like it pretty warm. 30-40°C are great operating temps for them. Prolonged near-limit use in higher temps will be an issue, but just for general use, they will like tropic temps better than temps below 10-15°C.
I found this thread which refers to a study from which this set of graphs is taken:
Ah, in storage they definitely discharge faster. Most/all chemical reactions are sped up by higher temps. But LiIon batteries can also be damaged by discharging them quickly (e.g. turbo flashlight) below ideal operating temps. IMO for just taking the light out of the pocket and cranking it to turbo, 40°C is better than 10°C. But I have no real data to show, just anecdotal evidence from my RC days. I used to put my batteries in the direct sunlight for 10-20mins when flying in colder months, or keep them in inside pockets of my jacket in winter. Around 30-40°C they performed noticeably better than below, more power and less voltage drop during high power spikes.
But my drone sucked 60-80A from a 850 mAh pack… xD
Another graph from the same source showing, as mentioned above, that storing batteries charged to 50% or lower benefits the storage time and possibly the longevity as well.
Each manufacturer has a different voltage to store them, it is stated in the data sheet.
As much as lowering it to 50% is too general.
I personally have all my cells charged to the maximum because I use them when I need them.
If I discharge them to give them a longer life, then when I need them the most because I am going on a night hike with my nephews and nieces I don’t have them charged, I will have long battery life but little or no battery usage.
Summary enjoy your flashlights and batteries and when they don’t work buy another one.
I can put them all in storage mode with the charger but I would not be able to use them, because I would have to charge them first.
It’s up to each one.
Hypothetically, what’s more helpful to extend the battery life (all the rest being equal):
running the flashlight between 50% and 75% at the expense of more frequent charges, or
running the flashlight between 25% (or less) and 100%, and not having to recharge that often?
As a practical matter, you’ll never be able to tell the difference.
Number of charge cycles plays a critical role in longevity.
I have 30Q batteries that are 8 years old and still perfect.
My 3 year old samsung phone has 1500 charge cycles on. The battery is 75% what it used to be new.
I have a bunch of LG and 30Q batteries that are at least that old from my vaping days. Some of them need to be re-wrapped but other than that, they seem to be in fine shape.
Where do you go to see charge cycles? I knew how to find it at one point and can’t remember how I did it. I wonder where I’m at on my phone.
What’s your charge/discharge routine?
Those graphs answer to your question, more warm + higher storage voltage = faster aging.
This is generally the most agreed on procedure.
You need to work out what would work best for you, eg. 3 cells stored fully charged, and accept they’ll have a lifespan of eg. 3+ years.
Or 6 cells stored at c.50%, which last 7+ years.
Both options give similar runtimes if the lights were to go out right now.
Lots of these are generalisations, chemistry plays a part, if I remember correctly, current generation of li-ion cells (NMC) have better longevity than the previous Lithium cobalt cells.
LFP have low capacity and a lower nominal voltage, so we don’t use them much here, but theoretically they’ll outlast "regular’ chemistry 18650s.
It’s splitting hair, but the graphs seem to show that storage at too high temperature may be more detrimental than storage at too high voltage (if one has to choose).
Different brands may behave slightly differently, but generally yes. Note also that 50% storage number that gets thrown around is just the “optimum storage voltage”.
Storage anywhere under 4.20 is better than completely fully charged, obviously as you approach “optimum” which is about 3.7v, you have a trade off with stored amount of energy.
Most of my lights are single cell, so I don’t have to worry about balancing them. I tend to keep my cells at (very roughly) 4-4.1v but I’m not pedantic about it.
This all said, cell life might not bother some if new cells are readily available and fairly inexpensive.
