Kiwifotos USB Lens Warmer, Lens Dew Heater Strip with Temperature Regulator, Condensation Prevention for Telescopes Camera DSLR Lens Telescope Eyepieces Less Than 3.15 inch (80mm) in Diameter

Works great to date. The coldest I have used the warmer is down to 7 degree C

I used this USB powered Dew Heater on a 8mm lens for night sky photography, an it kept few from forming on a humid night with temps in the 60s. It heats quickly on the High setting, but Low is recommended for my conditions.I used a 20,000mah portable power supply for over 2 hours continuously without any problems. This power supply has 10W USB-A output, so it has no problem running the heater. I didn't try to measure current, but I did plug it into an old phone charger that has a LED on it, and this heater apparently needed more power than its stated 5v 1000ma (5W) output because the light constantly switching between dim and bright. That tells me that a small, lightweight USB battery probably cannot run this thing, and that may be something you need to consider before heading out for a long photo session.

Update: The method I used to make these heat bands more effective have one warning. These bands were designed for 5v USB connections and when I changed them to work with my 12v Astro Zapper, I discovered that you cannot turn the Astro Zapper much past 1/4 turn before the bands will overheat. If you do that, the bands will work very nicely!Original: I received the smaller and larger versions of this item this afternoon, plugged them into a USB chargers and realized right away that these things were never, ever going to keep up with the dew generated here in Florida. It was hard to tell if they were actually "on" when on "high", "medium", or "low" having it wrapped around my hand. Fortunately, I had an older AstroZapper control box that I hadn't used in a good while. I cut the lens warmers' cables to just above the controllers (low, med, high) and spliced together RCA connectors that had good long wires. I connected them to the AstroZapper and slowly turned the variable controllers up gradually to where the heating bands would almost burn my hand. The USB idea may sound good on paper, but the power USB delivers just doesn't cut the mustard for controlling the dew I experience in Florida; however, if you have an adjustable 12v power supply, the heating bands work great after switching the power sources.

Really wanted to like them, they worked, but won't stay on for long exposure astrophotography sessions. Randomly turn off after ~ 1 hour. Repeated the process with 3 different USB sources, it's definitely the heater controller problem. Only buy if you plan for short sessions or babysitting the heaters to make sure they stay on. I returned mine.

I have now owned two different brands of these USB dew heater strips. Both have issues with the on/off button. After two uses the button to turn on does not work all the time. Long hold or short push, it takes many pushes to get it to turn on. then it might shut off when you push to change the temperature. Badly constructed controller. Going back to RCA plugs and related dew controller, which cost more and require a 12V battery but is more reliable. Good thing I can still use my USB Hub even with the charging ports.