Although you're better off having a heated lower layer for trapping the heat. I use a heated waistcoat that goes above my base layer. The battery is about the size of 3 Tim Tams and lasts all night, but mine is only 7.4v
Surely a 12v one could be tethered to the main battery, for those of us into goto and imaging
Haha, Oh come on, it's not like your living in Finland or Canada. If your cold buy one of Geoff's freezer suits, or wear your ski gear, or just put on a hat (that's where most heat loss occurs). I have some of those chemical hand warmer packs that I have carried around for years and never used. How many time will you snag the cable on the tripod or scope before you rip it off?
I have had a "Jett" Heated Sleeveless Jacket for about 10 years.. It works really well when I have worn it in isolation. This isn't all that often as it doesn't work all that well in combination with my one piece freezer suit, which is what I normally wear. It runs about 4 to 5 hours on the battery pack and I also have a second battery pack for it which I keep charged. I am on my second set of battery packs with the first set lasting about 7 years, before they failed to hold a reasonable charge level.
Whilst these things might sound like a bit of a gimmick / novelty, I think one of these sleeved and hooded heated jackets from AEG / Bunnings would work very well in combination with a set of freezer pants. Particularly on the basis that it has temperature control, whereas a freezer suit doesn't. Depending on the type of observing you are doing you can actually get pretty hot in a one piece freezer suit; particularly if you are using a large aperture scope and continually moving a ladder around and climbing up and down it. On countless occasions I have had to peel back the top half of my freezer suit after a few hours observing and then your top half gets really cold fast. It would be really nice just to be able to turn the top half temperature down a bit, when you start to get hot.
Depending on the type of observing you are doing you can actually get pretty hot in a one piece freezer suit; particularly if you are using a large aperture scope and continually moving a ladder around and climbing up and down it.Cheers,
John B
Can confirm but I don't mind as I'd rather be laughing at my companions moaning about how cold it is whereas I'm getting a little too warm
does not compute 
. Thoroughly recommended along with some thermo boots and gloves.
