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12V heating pads / "electric blankets" to lie on


  • there are various types of 12V “electric blanket” / “heating pads”
  • heating pads here are defined as those you place on your mattress and thus you lie on top of them
  • low output 45-50W full size “electric blankets” can generally be used as lower output heating pads or as throw rugs and most of these will have a 45 minute timer
  • car seat heated cushion pads are more robust and compact but are shorter and have longer cycling periods of 5 minutes which can result in getting a touch too warm and then too cold
  • for overnight hikers, consider small 5V USB powered pads
  • higher output heating pads eg 75W must not be allowed to overheat as there may be a fire risk!
    • in general, they must be fixated to the mattress to avoid scrunching or doubling up
    • they should not have insulating materials such as a sleeping bag placed directly on top - use a quilt or blanket over your body instead of a sleeping bag (or open the sleeping bag as a quilt)
    • they should not be left on all night, especially at higher settings - ideally these high wattage blankets should be turned on 30 minutes prior to bed then turned off
    • they should be replaced if damaged or wires are damaged
  • temperatures above 44degC will cause burns if you fall asleep!
  • do not fall asleep on mats/blankets which have more than 40mW/sq inch as there is a risk of overheating or burns from prolonged heat exposure of more than a hour or three
  • excessive, prolonged heat may cause you to develop extremely annoying prickly heat rash
    • if you must sleep on them, use the lowest setting for your needs - preferably below 20mW/sq inch on cold nights and even lower on warmer nights so you don't overheat while sleeping
    • some should NOT be used to sleep on as their lowest setting is too warm and there may be burns risks as well!
  • the most efficient energy-wise for all night warmth but to trunk only:
    • Comfytemp Mini Car electric throw rug modified to have a 5V USB power supply = 8.4W (runs at 48W on 12V but too hot to sleep on)
  • the most versatile and greatest ability to heat a 2P tent all night
    • ElectroWarmth T36 mattress pad but lowest setting still uses a lot of power as it is so large: 36W (max. 75W)
  • the most versatile and reasonably energy efficient but turns off after 45min use
    • Kickass blanket 23-46W

12V high output electric blankets

  • ElectroWarmth T36 mattress pad
    • “twin size” = 60“ x 36” (152x91cm)
    • 75W 7 power settings, “3.1A” on lowest setting, 6.5A on highest setting.
    • no auto-off (can get separate controller)
    • MUST NOT be allowed to scrunch up or be folded whilst on as risk of fire!
      • hence requirement to:
        • secure to mattress such as via safety pins to attach to mattress etc
        • not use with insulating material such as sleeping bag on top of blanket
        • ensure it is turned off when not in use
    • can be hand washed as long as controller does not get wet - risk of shrinkage!
    • they advise to cover with thick quilt to protect it from sweat and dirt - not sure how this differs to a sleeping bag that has been crushed by the weight of your body!
    • auto off seems to occur at around 11 hour mark
    • heat settings and power usage:
      • Setting 1: seems to run at <3A when ON but has long OFF periods so gives minimal heat
      • Settings 2-7 run at 6.5A when on but the proportion of time it is on increases as setting increases until constantly on at level 7 when it hits surface temp of 48degC but will drain your battery quickly - great to get warm quick in 15 minutes but then turn it back to setting 3 or so!
        • Setting 2 On 1 sec Off 1 sec so average is around 3.1A/37W = 17mW/sq in
        • Setting 6 On 2 secs Off 0.5sec
        • Setting 7 = 6.5A/78W = 36mW/sq in
    • not as power efficient as a car seat heater pad as it is much wider than your body and the heat is more spread out (great for heating a 2P tent air up though!) but it does provide continuous heat instead of the annoying 5 min off cycles

12V lower output "electric blankets"/ throw rugs

from largest to smallest

  • Ovela / DSFL Laura Hill and unbranded 12V car electric blanket / throw rug - 1 heat setting, no auto-off, 1500g polyester fleece charcoal/navy/brown versions, 50W, 150x110cm / 59“x43”= 19.6mW/ $AU29-66 depending upon Ebayer
  • Kickass blanket 3 settings
  • many others online - some with only 1 heat setting, some with 2-3 heat settings.
    • unbranded blue plaid 145*100cm / 57“x39.4” 800g velveteen non-washable;
      • 2 heat settings: 30deg, 40deg; (same controller as car seat heated cushions but states constant temp) high temperature automatic shutdown protection but no auto-off otherwise; $AU45
    • Aussiworld no branded red/black check coloured brushed fleece throw rug 104x84cm/40.8“x33” with temp and time control, 45W max = 33mW/ max $AU42
      • probably has 4 heat settings 35 degrees, 45 degrees, 50 degrees and 55 degrees ; time settings: 10, 20, 30, 40, 50 and 60 minutes;
    • unbranded red/black or white/black check coloured or grey brushed fleece throw rug 100x60cm/39“x23.6” 450g with temp and time control, 45W max = 49mW/ max $AU38
      • 4 heat settings 35 degrees, 45 degrees, 50 degrees and 55 degrees ; time settings: 10, 20, 30, 40, 50 and 60 minutes;

smaller 12V throw rugs that should NOT be used to sleep on unless convert to 5V USB

  • Comfytemp Mini Car 12V 48W (peak) electric throw rug
    • small 20“ x 24”
    • 4 heat settings plus 5 auto off timer settings plus the all important camping stay on all night setting
    • Memory Function remembers the last heat and timer setting
    • need a torch to see control display at night though, easy to accidentally change settings
    • machine washable (just detach the controller)
    • warning label states - NOT to sit or sleep on it and not to use whilst sleeping, don't use when folded
      • even the lowest setting generates too much warmth and leaving it on for long periods is likely to cause prickly heat rash
    • Setting 1: ~2.3A = 58mW/sq in - much too high for all night sleeping!
    • Setting 2: ~2.7A
    • Setting 3 or H seem to use same power: 3.4A (41W) - when folded over generates temperatures over 47degC and this CAN CAUSE BURNS!
    • level 1 gives
      • with power adapter at “12V” A which is probably W and blanket gives degC on bare skin
      • straight from battery 13.1V at 2.1A which is 27.5W and blanket gives degC on bare skin
    • level 2 gives:
      • with power adapter at “12V” 2.36A which is probably 25W and blanket gives 37.6degC on bare skin
      • straight from battery 13.2V at 2.26A which is probably 30W and blanket gives 38.5degC on bare skin
        • this however is not maintained and can drop down to 7W!
    • $AU55
    • NOT recommended for camping as an electric blanket unless you convert to 5V USB (see at bottom)

12V car seat heater pads

  • shorter and stiffer than electric blankets (need two for upper body and your legs)
  • most have peak output at around 48W but average 1-2A power usage as they cycle on and off in 5 minute cycles which can be really annoying as you can get cold towards the end of the OFF cycle but given the high heat output, is needed to prevent over-heating or burns
  • usually have two heat settings
    • high setting averages ~25W = 35mW/sq in (hits 66mW/sqin during the 5min ON cycle on High)
  • will raise 2P fabric enclosed tent temperature by around 2-4degC as well as providing direct heat
  • there are plenty on Ebay

USB heater pads for pets

  • generally measure ~29x24cm and are 8W power usage on high = 74mW/sq in
    • this is TOO HOT for bare human skin if you fall asleep!
    • the low and medium settings reduce heat output by varying the proportion of time the max. current is applied

Reducing heat output of 12V blanket by reducing voltage to USB 5V

some electrical theory

  • Joule's Law: power (eg. watts) = voltage x current (amps)
  • Ohm's Law: current (amps) = voltage / resistance (ohms)
  • THUS, for a heater pad of the same resistance:
    • power (watts) is proportional to the square of the voltage
    • thus changing from 12V to USB 5V reduces the power used by 12×12/(5×5) = 1/5.76
      • these adapters can only run to 10-12W maximum and thus can only be used on 12V appliances rated at up to 58-69W (although the power usage of these devices will be reduced to 10-12W when USB voltages are used)
      • ie 48W 12V system become 8W 5V and will use up 8W/12V = 0.7Ah of your 12V battery per hour instead of 48/12 = 4Ah although of course you get much less heat output
      • the ElectroWarmth T36 75W 12V system would become 13W 5V and may NOT be possible without overheating the USB circuit - even on their low 35W settings as the low settings still run intermittently at 75W

in testing with the Comfytemp Mini Car 12V 48W (peak) electric throw rug

  • thus for a 12V blanket running at 48W, current used is 4A (48/12) and the effective resistance = 3 Ohms (12V/4A)
    • if one was to change this to a USB 5V system, the current would be 1.66A (5V/3Ohms) and thus the heat output power would be 5V x 1.66A = 8.3W (and this would be well within the max power of USB of ~10W) and this, on testing, will actually convert the Comfytemp Mini Car 12V 48W (peak) electric throw rug to a 5.18V 1.63A 8.4W 18mW/sq in blanket instead of well over 50mW/sq in, and in so doing, perhaps make it usable to sleep on.
    • the controller of the throw rug still works although each of the heat settings seem to output the same heat of 7-8.4W and when folded over generates heat of about 25-26degC which is far safer than when using 12V input for this “blanket”
    • ComfyTemp blanket connected to 5V USB output, when used on a bed mattress with a light doona, the bare skin temperature lying on it reaches 34degC by 15min, 35.6deg by 45min and averages 36-37.6degC after 1hr!

how to achieve this

  • you can buy a USB to female 12V cig lighter socket cable BUT this has a chip which up-regulates the 5V USB to 12V which is NOT what we want - using this for a high output 12V blanket is likely to overheat your USB circuitry.
  • you can take the component apart and bypass the voltage up-regulator circuit and directly join the in and out red wires and then you have 5V in and 5V out.
  • alternatively, buy a 12V cig lighter DC-DC transformer as below which allows 5A at switchable voltages 5V, 6V, 9V and 12V

Reducing heat output by using 6V input

what would this achieve?

  • changing from 12V to 6V reduces the power used by 12×12/(6×6) = 1/4
  • the 48W ComfyTemp blanket (3 Ohm) would theoretically become 6V/2A/12W at max = 25mW/
    • in testing:
      • level 1 gives 10.85W and blanket gives 35degC on bare skin by 15min
      • level 2 gives 11.2W and blanket gives 35degC on bare skin by 15min
      • level 3 gives 11.5W
  • the ElectroWarmth T36 75W 12V system would theoretically become 9-18W blanket instead of 36-75W and thus use up less of your 12V battery but it would not feel that warm as it now gives only 4-9mW/

how to achieve this?

  • see below for 9V

Reducing heat output by using 9V input

what would this achieve?

  • changing from 12V to 9V reduces the power used by 12×12/(9×9) = 1/1.78
  • the 48W ComfyTemp blanket would theoretically become 9V/1.7A/15.5W at setting 1 to 9V/3A/27W at max.
    • however even at setting 1, it would give 32mW/ which is too warm for all night sleeping
    • level 1 gives 1.66A which is probably 15-17W and blanket gives 35.6degC on bare skin by 15min
    • level 2 gives W and blanket can give 37.1degC on bare skin by 15min
  • thus a 75W electric blanket would become a 42W electric blanket
    • less heat output but also less power usage
    • the ElectroWarmth T36 75W 12V system would theoretically become 21-42W blanket instead of 36-75W and thus use up less of your 12V battery and the warmth range would be a nice, gentle 9-20mW/
      • (just don't move your charger output voltage to 12V as the blanket would then want to use over 6A and the 5A charger might just burn out!)

how to achieve this?

  • you can buy a female cig lighter socket that is receiving power from a standard plus such as a 2.5x 5.5mm plug which can then potentially be attached to a 9V output via a female to female adapter and then to a 12V to 9V cig lighter socket adapter with a male 2.5x 5.5mm plug
    • some 12V cig lighter “chargers” to 9V 2.5x 5.5mm plug are only rated to 1.2A/10.8W or 2A ie 18W output
    • you can get 12V cig lighter chargers rated to 5A which would give 60W at 12V, 45W max at 9V, 30W at 6V, 25W at 5V
    • USB-C cable to 2.5x 5.5mm plug are also generally only rated at 5V 2A 10W output
    • USB-C PD cables to 2.5x 5.5mm plug are generally rated to 3A which would get you to 27W
      • at 3A max, they may be rated to 20V 65W
      • but how do you ensure it will be 9V output?
australia/heating_pads.txt · Last modified: 2023/02/20 23:21 by gary1

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