How long have people gotten out of the Streamlight rechargeable batteries just sitting in weaponlights? I got 2 of the rail mount models for my 45-70 and to replace one on my AR. I don't use my lights that much but I don't want to need it and it not work.

There is very little self discharge if that is what you are asking. It will keep over 80% if unused for a year. If kept unused in air-conditioned conditions it will keep over 90% charge sitting around for a year.

If asking about the number of recharges, it should be good for 200-300 recharges minimum before degrading to 80% of its original max capacity. Charging while not hot, charging when not completely empty, and not charging at excessive speeds will increase the number of recharges before the max charge capable is only 80% of original capacity is reached. The battery industry considers 80% max of original capacity the "end of life" for a rechargeable battery.

I generally top off all of my rechargeable batteries once a year even if I did not use them. Some people will tell you you should keep Li-Ion batteries at 40-50% charge in the fridge to maximize life. While this is true, I am perfectly happy to lose 10% of my recharges by topping off once year for 30 years by keeping them in my gun safe in my air-conditioned home.

Also, if a cell ever gets to below 0% charge it will be permanently damaged and possibly unrecoverable. Starting at 40% might give me only a few years before my batteries discharge to zero and I would rather risk 10% of my charge cycles than risk my batteries being completely dead because I did not check on them after multiple years or had them in a mini fridge at a remote location where the power went out or fridge died and they were left without A/C for an extended period at under 40%.

As for how long your weaponlight will last if left continuously on..if you are asking how much run time a new 18650 cell will give you fresh off the charger... it will be at least an hour even if you did not store in climate controlled conditions and let it sit for a year but the output will slowly decrease as you use it and the voltage drops. Intermittent use and allowing the cell to rebound will help keep maximum output up. You should only be using a weapon light intermittently and it should spend most of its time OFF when being used propperly.  A 20 minute run time can cover clearing structures for 40 minutes.

Second, you should recharge the light after any significant use beyond just checking to see if it is working. Would you shoot half a magazine of ammo at the range and just put up your rifle for home defense with half a mag or top it off first? Same should be for the weaponlight.

Finally, the cells that came with my Streamlight were generally not the highest capacity cells. I believe I have some 2800 or 2900 mah Streamlight 18650 cells and I generally only buy 3500 mah Keeppower 18650 (not 3400 or 3600) which have the best balance of capacity and high charge rate/high discharge rate performance. The Keeppower 3500 mah protected cells are the best protected cells and the unprotected 3500mah are the best unprotected cells. 90% of my 18650 are Keeppower protected 3500mah at this point and I gave away most of my Streamlight batteries. Why? Some devices dont have their own protection circuits so I want protected cells for them and a 3500 mah cell at the END of its life (80% original capacity) has the same capacity as a brand new 2800 mah cell. Plus you can buy Keeppower 3500mah protected 18650 for just $10 each.

You should also keep in mind 18650 do NOT like to work below freezing, can't be charged near freezing, and don't even like to be stored below freezing. Keep them indoors and keep spare batteries near your body warmth in cold weather.

All batteries especially NiCad and MiMh batteries have a set number of charges that can be expected. I’ve been repairing Streamlight’s for my department for years and I’ve always been a proponent of letting the battery discharge as much as possible before charging. That being said, if you rely on your flashlight for work, that’s not always an option. The other option is to drain the battery all the way until it turns off. Place it in the charger for at least 8-12 hours to fully charge. I’ve got batteries still in service from 2016. Treat them right and they will treat you right.

I usually rotate weaponlight and held held batteries a couple times a year, just so I know the weaponlights have fresh batteries.

Lithium ion cells do not have have the memory effect of NiMH and certainly not like NiCad. So no reason to  cycle them. NiCad should ALWAYS be 100% cycled. NiMH can be "spruced up" to full capacity every once in a while by a discharge, full charge, full discharge, full charge cycle.

There is ZERO advantage to doing this to a modern LiIon cell. It will only wear it out faster. Maximum life will be achieved by only charging to 80% and only discharging to 20%. But for a flashlight, several hundred charges is good enough and your light will be noticably dimmer at 20% capacity. I would still fully charge.

Noone should be using NiCad for ANYTHING at this point IMO. It is outdated tech that is horrible in EVERY way. NiMH is for cells that are OK to have low capacity vs volume, where weight does not matter at all, and high voltage, charge, and discharge rates are not needed. The most recent 2000+ cycle life, low self discharge NiMH tech is perfect for your small alkaline battery replacements, but horrible for almost everything else, IMO.

My only NiMH cells at this point are AA and AAA Eneloops which have a 2,100 charge cycle life. I did a refresh cycle on the batteries that were mostly sitting in storage at the 10 year mark. Some of those were so old they were the 1,400 cycle life early gen versions. They all still performed fine.

Anything negative you might read online about the limitations of LiIon cells sitting in storage or ever needing to be cycled is generally from cell chemistries that have not been used in well over a decade. My handheld and weaponlights for LE use have been LiIon for the past decade (except for handguns where I still use 123 cells in my 7 various X300 variants). To maximize LiIon cell life, charge at 1C or less, charge when the battery and air are both at room temperature, and don't leave them on the charger after they are full.

From what I've seen with the Protac HL-X the batteries don't self discharge. I'll toss the 18650 in the charger once a quarter and it might take 200mah to top it off and that's with a daily function check before shift.