Battery inside unused flashlight

Why is it recommended to take out batteries from a flashlight if it’s not going to be used for a long time?

Alkalines leak corrosive substances so always best to keep them out of devices that aren’t going to be used for a while, even devices that are in use…
NiMH and Li-Ion are generally fine to keep in flashlights for extended periods as long as there is no parasitic drain.
I keep cells (no alkaleaks) in my lights at all times, even those that don’t get used often.
This is where having physical lockout comes in very useful.
Long term storage of Li-Ion cells should be 30-40% cell capacity, LSD NiMH cells I keep fully charged and I cycle all cells every couple of months.

also:
lithium primaries [CR123A] have a long shelf life, like 10+ years, and are considered low risk for leaking

(is it even possible? maybe they don;t even have acid/base inside to leak in the first place)

also 2:
carbon zinc, if anyone uses them, can definitely leak

wle

Any light and an electronic switch has parasitic battery drain. This is because in that kind of light the microprocessor is powered anytime the battery is connected. The switch is just an input telling the microprocessor what to do.

Typically, e-switch lights should be hard-locked out by unscrewing the tailcap if you are storing the battery inside. Some e-switch lights can’t be locked out in this manner and the cells probably should not be kept inside for long periods.

Li-ion cells become damaged if they are drained too far.

so the reason is the parasitic drain > battery damaged if drained too far?
But most flashlights have parasitic drain in their specs of hundreds of years…

In all the years of having flashlights and in using standard batteries, it is the heat

that makes them leak, at least for me Left in vehicles in summer, I have had new

ones leak in less than a week.

Not “most”, just the well-designed ones. You can’t take it as a rule of thumb that all e-switch lights will have a standby drain rate lower than the battery’s self-discharge. Play it safe, play the long game.

Nitecore as a whole is one example of torch brand that’ll make you regret ever leaving a battery inside and forgetting about it.