My new Coast HP21 came in the mail a couple weeks ago and I just wanted to get on here and share my experience. I stalked these forums and did a lot of reading on the use of rechargeable batteries in the HP21. However I didn't find much since the light is fairly new. So when I got my HP21 I read the packaging very carefully to make sure that I wouldn't void my lifetime warranty. The packaging states "Do not mix old and new batteries. Do not mix Alkaline, lithium standard (carbon-zinc), or rechargeable (nickel cadmium or lithium ion) batteries." After trying the 4 D-cell batteries that came with my light with good results, I inserted my 4 fully charged 10000mah Tenergy D-cell batteries. When I pushed the button the led's came on very bright for about 10 seconds, and then got very dim. Even when I cycled the switch, the leds would not come on bright. So I called Coast and spoke with a tech. After telling him the whole story, He advised that it sounded like I had a defective light. While I was waiting on my replacement, I read about rechargeable batteries in the x21 which is similar, and found a good thread on here about the x21 being unregulated and that rechargeable batteries were not a good idea. At the end of the thread someone stated that they had found a solution, and that was to use a d-cell spacer, and 3 8000mah rechargeable batteries. When I got my replacement I tried this with my 10,000 mah batteries and had success! I am now very pleased with my flashlight. The hp21 is actually a little brighter with 3 10,000 mah tenergys that it is with 4 alkalines! (both checked with a volt meter) So far, I have used my light no longer that 30 minutes, and cant tell that my light is dimming. My only complaint is that the beam doesn't focus down like I expected. I have compared it to my Maglight with upgraded terralux 600-700 lumen bulb, and it really blows it out of the water. Also my Brinkman 3000 cp spotlight which has a much more focused beam wouldn't seem that much brighter if the coast focused down like the spotlight. I even shined the hp21 out the window on the road in front of my 02 Ford Taurus, and the Hp21 washed out the car headlights. I think that this light is very impressive!! I will post some pictures on here when I can figure out how to make them do this light justice. Pictures I have taken seem to really underestimate the power.