Using *unprotected* 18650s safely


Nov 27, 2009
Connecticut, US
Ok... So I recently cracked open and triaged several laptop batteries. I tossed any cells that appeared physically dubious and any cells that had a resting voltage of less than 3.5V. I then charged the batteries one at a time (on an Ultrafire WF-137 charger with the ~5V open voltage design) checking every 15 minutes or so for temperature. I discarded any that got hot to the touch or took more than a sensible amount of time to terminate. Every 30 minutes I checked the voltage. From the remaining batteries I tossed any whose resting voltage dropped more than .03V between 30 minutes off the charger and 1 week.

What remains is a big pile of healthy seeming 18650s (mostly LG and Panasonic but also some Sanyo) all unprotected. All free. Now... how to use them safely.

At a minimum I figure I'll reserve their use for single cell applications only. Also I figure I'll be sure to check the resting voltage prior to recharging under all circumstances. Still, it would be great not to have to worry too much or remember in too much detail how long a battery had been in use.

I noticed that my Malkoff M61 is designed (so says the web site) to fall out of regulation at around 3.4V. While I haven't tried this, it strikes me that under load, even a healthy 18650 might dip well below 3.4V during the last third or so of it's capacity sufficient enough for me to notice the dimming but not so far that the voltage wouldn't pop back up above 3.5V unloaded if I immediately discontinued usage. If true, then in my (mostly competent) hands, this is almost better than a protection circuit. I would try to avoid this level of discharge but I would have the option of immediately turning off the light before the cell depleted too much or (if necessary) continue to use it and discard the (free) cell rather than risk recharging it.

Does anyone have any experience using an M61 with unprotected 18650's?