(This post was last modified: 08-03-2016, 05:26 PM by ShadowsDad.)
(This post has turned into a far longer one than I intended.)
Yup. Heat is another potential hazard. Storage ina car is a no no, especially in the summer. They can start to behave badly just by being in heat. They can begin a degradation reaction and then it just feeds on itself. Your car could be engulfed and you did nothing. At least to your eyes. LEDs with primary cells (non rechargeable) don't have that problem. Our cars have smaller one cell Li cells (primary) in them for emergency use. They have a 15 year lifespan. My rechargeables get used every day, except for the ones in my slung pack. They get checked at least once a month. Yes and never charge a battery without someone being present.
One of the things I like about the Opus charger is that I can program the charge rate. It also will discharge a battery for storage at a programmed rate. Rechargeables will degrade no matter how they're stored, but having them always fully charged is the fastest way to degrade them. Discharging to a half charge and putting them in the refrigerator, as close to 32°F is best. My spare batteries are in the beer/BBQ refrigerator in the shop. BTW, a discharged cell is never discharged below 2.7- 2.8 volts if you like the cell. A deeper discharge harms it and we already discussed Li-Ion bombs. Recharging before it gets to that voltage if it's in use is strongly suggested. For me, using a light every night, a few times per night, once every month or 2 is plenty soon enough for recharging, but the voltage can be tested and many lights won't allow a cell to be discharged below that magic voltage*. Unused Li-ion cells don't readily self discharge, but again, they will age and slowly degrade.
*One can be using the light and all of a sudden it will just shut off. That's either the light saving the battery, or if the battery is "protected" the protection on the cell switched it off. Protected cells are safer than unprotected. I use unprotected cells in critical duty lights where light must be had and I don't care if the battery is harmed. I also use them in my L6, but I'm very careful to match them and keep an eye on critical measurements that gauge how the cell(s) are doing. Once I get protected 26650s (KeepPower 5400mah) I'll discharge my present Basen 26650s and put them in cold storage.
When Bruce mentions damage, that includes even a small dent. One does not play games with these cells. It just isn't worth it.
Oh, and never buy cells with the word "fire" in the name unless those exact cells have been tested as being good ones. China is rewrapping used cells and they have a tendency to use fire in the various brand names. They are just a bomb waiting to happen. I just recycled 4 "fire" batteries that were absolute garbage. Another tip off is a claim for fantastic capacity. 18650s have a top capacity at this time of about 3500mah. If it was possible to make one with higher capacity, Sony, Panasonic, Samsung, and any other big players would be doing it. So if a battery claims higher than that just run away from it. You know it's garbage. The "fire" batteries I just recycled? They claimed 5400 mah capacity, but when I bought them I just didn't know. That's not the case with 26650s that I know of... yet. And good 26650s can have higher capacity than 3500mah becasuse they're a physically larger battery. I know they're available with a capacity of 5400mah.
If someone knows what they're doing, and I don't, batteries can be reclaimed from used and trashed computer battery packs taken from the recycle bin. But one must absolutely KNOW what one is doing. Short out the pack or cell or cells and one has a potential bomb. Then one must know how to determine if the individual cells are still good before using. Remember those terrible Chinese rewrapped batteries? That's how they got them. But some cells in a battery pack can be good. I was given 2 cells that were reclaimed and they're fine. But since I have no present use for them they are at 1/2 charge (it's called "discharged") and in cold storage. To my knowledge the danger potential of cells in that condition is close to zero.
Depending on capacity really good 18650 cells will cost about $7-$10, and state of the art 26650s will cost up to $15. Unless I know for sure that cells from a supplier I don't normally use are tested and found to be reputable I buy my cells from Mountain Electronics. I have no connection with them except as a full retail paying customer. Other outlets that are considered reputable are Banggood, Gearbest, and Aliexpress. Simon at Aliexpress is the manufacturer of Convoy lights.
Way too many words. I need to stop.
There's plenty of this information on The Candlepower forum, and Budgetlight forum.
Brian. Lover of SE razors.