I had this for 2 winters:
http://www.amazon.com/Mattress-Elec...d=1449107326&sr=8-1&keywords=12v+mattress+pad
One could feel the wires but it was not incredibly noticeable or uncomfortable. It took a while to heat up a cold mattress. It was able to draw 6.2 amps though the ciggy plug.
How often it cycled on and off was dependent on the desired setting and the ambient temperature at which the external temp controller resided in.
Generally I'd turn it on ~ 2 hours before bed and climb into a 120f degree bed, and then turn it off. The coldest of nights I would turn it all the way down and it would consume 25 to 35AH in 8 hours. The controller did not make any noise/clicks when it cycled on or off.
Near the end of winter#2, I cut off the ciggy plug and about 5 feet of 18awg, the amp draw increased to ~7 amps, and it heated up much faster, and would use slightly less battery power overnight with the higher voltage it received. Perhaps negated by Peukert.
Not long after this, half the mattress pad quit working. It was the far side, the side which got slept on less, and got less sharp cornered items placed upon it that stayed operational. Am not sure it was related to the higher voltage it received by cutting off the fuseless ciggy plug and extra unneeded wiring and using an Anderson powerpole instead. It could have been a contributing factor to failure, it could have failed regardless of my higher voltage modification. I was not able to discern any obvious failure point in the wires.
I went last winter without a mattress pad, and have not gotten another for this winter and likely will not.
If it is cold lately, I've been bringing my 200 watt Lasko electric heater under the covers for a bit. It does shut itself off if it gets too hot under there and it takes about 10 minutes for it to reset and function again.
A 200 watt heater on my Wagan 400 watt PSW inverter is about a 19 amp load at 12.4v. Luckily any more than about 10 minutes of use, and it is baking hot under the multiple blankets and more run time is not needed . I keep my knees up to form a tent and try to get the heat to the edges of the mattress under the covers. My inverter uses 0.24 amps turned on powering nothing, So unless I get out of a warm bed and turn it off, it will consume 2AH overnight doing nothing..
Obviously care must be used if one brings even a low wattage electric heater under their covers, so I am not recommending this, just stating that I do it and I take care when I do so, mindful of the danger.
I am not one who falls asleep easily. I found it turned itself off only because I had been turning it on and off rather quickly, indulging in a little grid powered heating, and one time it decided to not turn back on. Turning it off and then back on allowed the casing to really heat up and the heating element with no airflow, under the warm covers, obviously caused the thermal safety device to get hot enough to sever the circuit.
I really liked the mattress heating pad when it worked, but it did consume a lot of battery power if left on overnight. If turned off when I climbed into a 120f bed, it had consumed 9 to 14AH to get the bed that hot over 2 to 2.5 hours, and I can't really express how incredibly indulgent it is to climb into such a hot bed on a cold night. If you can easily replace that 9 to 14AH the next day, I highly recommend it, but using it overnight, even on low, that AH consumption could go as high as 50AH which is much harder to replace.
I really suspect that one of the heating wires broke somewhere just from being laid on, or from putting sharp items on my Bed rather than the increased voltage it received. The instructions recommend turning it on while still driving which would effectively have the voltage it received way higher than what my wiring modification allowed.
There were times when I fell asleep with it on High, and woke up completely overheated, and it seemingly took forever for the mattress to cool back down even with covers removed. It was pretty rare that I let it stay on all night long. The dial had a 1 to 7 setting, but it actually would not shut off completely until 0.5 or so on the dial, and I only knew this because I put a Watt meter on it and could see the actual voltage and amp draw and AH consumed.
http://www.amazon.com/GT-Power-Anal...F8&qid=1449109791&sr=8-1&keywords=gtpower+130
It was even more rare that I turned it any higher than 0.5 on the dial when already in bed.
Either way, heating consumes a lot of battery power, and presents some dangers too, but there are solutions to being cold as long as one is aware of the electrical consumption, and those possible dangers.
There are some 120vAC mattress heating pads too. I've no idea of their amp consumption nor performance in comparison, and of course one needs an inverter, and possibly a PSW inverter to power them.
These claim to be low voltage, so likely they transform 120vAC to 12 or perhaps 24vDC, so they would likely be less efficient than a 12v mattress heating pad, but this is conjecture:
http://www.amazon.com/Micro-Plush-L...49110052&sr=8-2&keywords=mattress+heating+pad
