I can't tell you whether this product would work or not, but I'm a bit worried about the extent of the battery drain. Your 12 amp DC estimate is right (figure 10:1 for inverter power), so that might suggest 130/12 hours of running the mat.
Originally Posted by CoyoteThistle
However, you don't want to draw down your batteries below 50% in any event, so you really have only 65 amp-hours, so that's 65/12 hours. But your 130 amp-hour rating is at 77 degrees, where you wouldn't need the heat; at freezing (assuming the battery is under your hood or somewhere else external) you'd be down about 20%, so that's about 51/12 hours of heat. Plus your 12 amps/hour is a fast enough discharge to show some Puekert Effect, for which you can look up the equation, but it basically means that you're discharging faster than your battery was rated at and thus you'll only get, in this case, 83% of the power that's available at the rated discharge rate. So now we're down to about (51*.83)/12, or 42/12, or about 3.5 hours of heat.
A 40 liter compressor fridge, as an example, could draw anywhere from 0.5 to almost 3 amps when running, but they don't run all of the time and the colder it is, the less it'll run, and the LED lights are pretty low draws, too. So figure in cold weather maybe an amp per hour for all of your other loads. However, that one amp draw goes on across many hours of the day, so as a rough guess, figure you'd maybe want at least 15 amps available for the non-heat requirements over a day. Subtract 15 from the available 42 amps and now you have only a couple of hours worth of power to run the heat mat at 100% on. (Of course, the heat mat won't run all the time, and since I have no idea how well it works, I don't know what fraction you could divide the couple of hours by to get an accurate running time.)
All of this rough-guess math was put in here only to support the generally-accepted fact that resistance space heating (and air conditioning, FWIW) from a battery bank is real close to impractical for any reasonably sized installation. My Sprinter has 615 amp-hours of AGM batteries and I do have an electric convection heater that works quite well. When plugged into shore power, it's an effective main heat source, but when running off the batteries, it only gets run for 5-10 minutes in the morning sometimes to take the chill off.
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