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There is one thing we often forget about eyepieces and heating straps we loop around them when in the focuser - metal conducts heat, glass does not.
So, you wrap the heater around the metal casing of your eyepiece, that you've slotted into a metal focuser that is fixed onto a metal tube. Net result, you've got a fantastic heat sink to suck the life straight out of your battery, and your eyepiece still fogs up.
A heat strap should not be used to heat up a cold eyepiece. It isn't designed for this. And if your eyepiece is a 1kg lump of glass, you are dreaming if the heating strap will have any effect.
Heat straps for eyepieces should be used to help maintain the heat that is already in a warm eyepiece. Some of the posts below talk about heating your eyepiece in your pocket, which is part of the solution. In a cold environment, you should look at keeping all your eyepieces warm during the whole night, not just while in your pocket. A heat pouch just won't do much to heat the core of a 500g eyepiece, and nothing to a 1kg lump. But if the heat is already in the eyepiece from the start, the heat pouch and heating strap will perform best for you.
There are many ways people have devised to heat their eyepiece case. Some wire a series of resistors into their case. Others put a few of these hand heat pouches into their case. Me, I've gone real high tech - a hot water bottle in a cosy, and a couple of towels over the lot. The heating strap I use then does a fabulous job for me over the three hours of a sketch I'm doing.
Remember, as soon as you put the eyepiece in the focuser, it will be losing a bucket load of heat through it. So to make the most of heating straps and whatever you have in your pocket, the best solution is to have the eyepieces warm from the start. Glass does not conduct heat well, so don't solely rely on momentary heat solutions.
Mental.
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