I have been using AAA Lithiums in my EOS for about 3 months now. I haven't had any problems at all. I used the Duracell AAA's that it came with until I noticed that there was no difference between High and Med. settings. I then installed the AAA Lithiums. Lithiums are the only type of battery that I will use in all of my lights. I use my EOS at night in my house. I found that on low I still have plenty of light to walk around the house. I also go camping in the mountains of Pennsylvania every other weekend. I wouldn't use any other headlamp, and I'm thinking about getting a second EOS as a back-up to the first. I would recommend a PT EOS to anyone who is thinking about getting a Headlamp. Easy battery change, 66 hours on low using lithiums, one button operation, even using gloves. Just a great all around headlamp. Good Luck.