Quote:
Originally Posted by coachster
it could work if you insulate the stock over the warmers, but consider this. it you tape the warmers to the stock, heat will more likely escape the exposed side faster then penetrating the stock and warming the air inside to then warm the battery. the best way to transfer heat would be direct contact with the battery. air in an insulator.
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Assuming that you were to install the warmers and insulation indoors. Then the battery would be initially at room temperature (nice and warm). When you are outside, you won't care about the heat that is being transferred from the warmer to the battery (because the battery is already warm). What you are concerned about is the heat loss through the insulation. Coachster is correct, installing the warmers on the outside of the stock will not keep the batteries above freezing as long as if the warmers were installed inside the stock (assuming the same insulation thickness). The reason is that with the warmers on the outside, there is more surface area for heat loss. Also, if the warmers are inside the stock, there is the extra insulation of the plastic stock.
P.S. Heat transfer is always going to occur across a temperature difference. If you just wrap the battery with insulation, the heat loss will come from your battery - and over time, it will cool down to the outside temperature. By placing a warmer inside the insulation, the heat loss will come mainly from the warmer, not your battery (note: your battery will also get warmer, storing heat that will be released when the temperature of the warmer drops below that of the battery), thereby extending the time in which the batter remains above freezing.
OK - I'll stop rambling now.