1. Any tips to reduce risk of fire?
Some folks suggest charging cells in a fireproof metal box. While that would increase the safety factor, I just charge cells on my stove. The kitchen sink is 3 feet to the right of the stove and has an easily opened window just above it, if things were to get really out of hand. Additionally, I never leave the room while cells are charging.
2. Any warning signs of impending doom?
In most cases, a damaged cell will give some warning signs that can alert the user to potential problems. Damaged cells will become abnormally warm when nearing full charge on the charger. They also will not hold their charge well. If a cell charges to 4.20 Volts, for example, and within an hour or so, the voltage has dropped to less than 4.00 Volts, the cell is either damaged, or at EOL. As Black brought up, a means to measure cell voltage is really a requirement when using Li-Ion cells.
Another possible sign of a damaged, or worn out cell is if the cell just doesn't seem to perform as well as it used to. The output of your light may be noticeably less, or the runtime may seem much shorter. In either case this is a sign that the cell should be replaced.
3. With this setup, am I at very little risk of my TK11 exploding? What can I do to be safe(er)?
Keep in mind that, particularly with LiCo Li-Ion cells, you are always at a higher safety risk than when using nickel based, or alkaline cells. None of the various types of Li-Ion cells were ever intended by the manufacturers of the cells to be used in flashlights. Our hobby has just "borrowed" them from their primary use, which is the building of battery packs for electronic devices, tools, and such. These devices all have their own unique, built in, finely tuned to the particular application, protection systems, not only in the device itself, but in the charging system, as well.
If you familiarize yourself with the proper handling and care of Li-Ion cells, you should be fine. One thing I will mention here, is that when you are using multiple Li-Ion cells in series, it's just as important as when using primary lithium cells, to monitor the cell's voltages so that they remain "matched" in capacity. Never use cells of different voltage, capacity, or SOC (state of charge), in series applications. Also, the use of protected cells is almost mandatory, particularly if you are new to using Li-Ion's.
4. Do I have to do anything special to prolong the life of the 18650's? Is it bad to charge cell when almost full? How about almost empty? Is it good or bad to run the cell down? If its bad to run the cell down, will the cell or the light "shut off" to prevent damage?
All chemistry of Li-Ion cells prefer a shallow discharge, as opposed to discharging them fully, each use. You can do as jasonck08 suggested, and it is safe, but your cells will not last nearly as many cycles.
As has been mentioned, storing unused Li-Ion cells at about a 40% SOC will prolong their calendar and cycle life considerably. Also, storing them in an airtight container (eg. a Ziploc bag) in the fridge, can significantly extend lifetime, as opposed to storing them at room temperature.
Personally, I try to avoid "topping off" Li-Ion cells if they have only been run down a tiny bit. For example, if I have a light that runs for one hour, I don't consider recharging the cell until it has been run 10, or 15 minutes. This doesn't mean you can't top them off whenever you want, but the time spent at the end of the charge cycle, is the hardest part of the charge regimen on the cell, and causes the most wear.
As far as running cells down until either the protection circuit, or the lights circuitry shuts the light down, this is a bad habit to develop. Both of these functions are designed as a safety, to protect the user, not the cell. It's best to avoid this situation, if you want your cells to last.
You see this is what's scaring me. I can read stickys and posts till I'm blue in the face but that isn't going to make me any less concerned. After reading your post now I'm reconserding, maybe I should just stick with primaries:thinking:
Hope this helps both of you guys. It's not as much of a PITA as it sounds, to care and feed for your Li-Ion cells. Once you develop the proper routine, you get used to it. Keep reading, and with that, and some experience, you'll do fine.
Dave
