@lightseeker2009: This mod would be for enthusiasts only and would require a re-design of the head to be a commercial proposition...
I did a test as reported in
this post where 4*Eneloops on turbo were OK for up to 1/2 hr, but got excessively hot beyond that. The amount of user awareness required in this configuration would be similar to that for working with unprotected Li-ion batteries.
At the moment, the head will drop to a lower output level when the current delivered by 8AA batteries increases to a given point.
The problem when using 4AA is that this current is being delivered by one group of cells rather than the two parallel groups of the 8AA situation (where each group supplies half the current). To protect in the 4AA situation, the currrent limit in the head would need to be half its present value.
It's hard to see that manufacturing a third head design for a light that was only good for half an hour, would be a viable commercial proposition for Fenix. I think we're on our own with this one.
@orbspeed1: Because the TK30 and TK40 have different input voltage limits, you would only be able to run a single 18650 into the TK40 head. As to whether turbo mode would trip the battery protection, I don't know, but since the TK30 will not give 630 lumens on 1*18650, I suspect it would.
