Re: Which batteries for my new Fenix TK-41....1.5 volt Alkaline or 1.2 volt NiMH?
+2 on NiMH
To expand on what the others have said, The TK41 IS a digitally regulated light. This means that the light output doesn't change as long as the batteries have the power to supply. Turbo, for instance, is 800 lumens. The light will continue to produce 800 lumens regardless of what type of batteries are in there until the batteries don't have enough power to run 800 lumens anymore. Once the batteries aren't powerful enough for turbo mode (800 lumens), the light will automatically step down to the next mode (high, 335 lumens). If you were to use alkalines, not only would your output be the same, but you'd be throwing away cells that weren't completely dead because they will no longer have the power for your higher modes. NiMH can just be recharged or topped off and be good to go another round.
I'm not sure where you purchased your TK41, mine did not come with Eneloops. That's a pretty sweet deal. I use Eneloops on my TK41. I started out using alkalines, but I use this light nightly and my battery costs were out of control. I use it on Turbo mode 90% of the time and I don't specifically remember noticing any difference in runtime between Eneloops and alkalines on Turbo, but for some reason I feel like Turbo actually lasts a lot longer on Eneloops :thinking:. The general consensus on CPF is that Eneloops are the best purchase for most AA rechargeable applications. This is explained in great detail here on this forum and is answered many times, in much greater detail than I can possibly explain. If you search eneloop (test, review, etc) you will be overwhelmed with info. My suggestion is a total of 16 Eneloops. While you use one set you can charge one set. This means you can almost run the light continuously on Turbo mode forever depending on your charger set up.
As Yamabishi mentions, you'll need a charger. Maha/Powerex are also highly regarded on the forum. The Maha Powerex MH-C9000 is often considered the best charger for AA NiMH. It has features that you won't find on other chargers. While you probably won't use those features the day you get it, you will want to as time goes on. For now, you could plug the charger in, stick the Eneloops in there, and they'll automatically start charging for you. Once it says "done" you're good to go. Later you can read up and learn all the awesome features of this charger and really optimize and understand your batteries. I started with other chargers, then within a month they were sold and I had the C9000.
I hope this helps. Do some research here on CPF. The amount of information available is absolutely insane. Enjoy!