I'm pretty new to the forum, but happened to stumble across your question here.
The point I bring up (we might both appreciate a solid answer) is that Fenix does not specifically list the TK-11 for use with RECHARGEABLE 123A's (see their specs page at http://www.fenixlight.com/viewproduct.asp?id=92)
Conventional lithium (lithium cobalt, I believe?) rechargeables supply 4.2 volts fully charged. This means that one 18650 is fine (and specificied as OK by fenix). Two disposable 123A's equals about 6 volts and Fenix specifically approves that battery option too, but two rechargeable 123A's would supply 2 x 4.2=8.4 volts - perhaps more than the light is intended to handle?
On the other hand, I have seen reviews that have tested runtime with the rechargeable 123As. Probably something to look into before using the TK-11 a whole bunch with RCR123A's (I personally use an 18650 - I believe the TK-11 is "unregulated" with an 18650 so output is a little lower to start with and fades with discharge, but for me the much longer runtime is well worth it and the fading brightness provides some warning with respect to a weak battery).
I'm interested to see what others think about the TK-11 + RCR123A issue.
RCR123/16340 cells only have about 650 mAh capacity (don't believe the elevated capacity numbers you see on eBay) whereas 18650 cells usually have 2200 mAh or more capacity.Yikes -- that's a pretty decent bump in runtime from using the 18650!
Yes they can be charged in both.-- And can the 18650 be charged in the Pila and Ultrafire 139 chargers?