Just a quick update, ive tried a few different options over the past few weeks
My finding are basically that the higher the voltage the product takes the better the heat but the bigger and heavier the battery (obvious i know)
I personally found the 7V to be the happy medium and have gone for the Gerbing gloves and the gerbing socks (but im still looking for a better sock or insole)
i created a spreadsheet with various info as i researched, it is work in progress but may be helpful if your in the market as there are very limited reviews on this stuff
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http://www.filedropper.com/heatedclothing