this certainly is good advice. while i feel rather stupid for not doing so earlier, i finally looked into safety re 18650 cells a bit myself. i am charging mine inside the unit, using only the included battery & charger, and i noticed that my tm (rather the bottom metal) gets really warm while charging (but only then). i dont have a thermometer, but it is close to being uncomfortable to the touch. i read that skin burning starts occurring at ca 48°C, so i didnt think this quite approached dangerous levels. i emailed the maker for clarification nonetheless, this is his (illuminating) reply:
"Hi!
It sounds all perfectly normal.
the battery should not get too hot, as the bottom metal panel is used as a heatsink during charging. If you would remove the battery you could feel this yourself, the actual battery stays pretty cool during the charge process. It was an intentional design choice to let the bottom panel get relatively hot during fast charge, and this all actually improves safety and lets the heat get out and keep the battery cool.
There are basically two charging techologies: Switching and linear charging. Switching chargers are very efficient at converting the USB voltage to battery and they produce very little heat. Linear chargers on the other hand transform the excessive voltage to heat and this is what makes the bottom panel warm up during charging. The advantage of linear charging is the simplicity of the charging circuit, less parts used to cause problems and generally much simpler technology for a robust and failsafe charging of the high power li-ion cell for years to come."
oh, and i also experience this
here is his response:
"About the light and charging, this sounds like it should be. When the battery is getting near full level, it starts to slow down charge, this is normal method how li-ion batteries are charged. The charging process stops when the battery takes only a very little current, meaning it is then full. But if you remove the cable, and put it back again, the charging circuit starts from beginning and sees that the battery voltage is already high and doesn't want to start charging until it has dropped a little bit. Otherwise chargers would practically all the time start charging again, as different batteries drop their voltage little bit even right after charging has ended, some more some less, and it would cause only unnecessary stress for the batteries to top them up all the time. So the charging controllers have a predetermined voltage level and only after the voltage of battery is lower than this, a new charging cycle is started. So if you take cable off before the previous cycle has completed and insert it again, it might very well be that the battery is like 95% full and the voltage is over the limit of starting a new charge cycle. If you want to overcome this, you just need to drop the voltage for a moment when the cable is connected. Turn the unit on and off, it will be enough, the starting of heater drops voltage of battery enough for a new charge cycle to begin even if you turn it off then immediately, and you will see the light stays on indicating a charge."
all this within a couple hours of asking, on a sunday.