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

see also:

• heaters for camping
• the problem with cold air whilst camping
• keeping warm when camping or hiking
• heat injury, burns, heat rashes and heat illness

• 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

• ElectroWarmth T36 mattress pad
  • https://electrowarmth.com/products/copy-of-bunk-warming-pad-non-fitted-12-volt-bunk-warmers
  • $AU122 “Made in USA” but actually made in China¹⁾ - may be available on Amazon.com.au
  • I have bought one of these and am very happy with it however they are hard to get in Australia without paying ridiculously high delivery charges for international postage
  • “twin size” = 60“ x 36” (152x91cm) 2160 sq in
  • 37-75W 7 power settings, “3.1A / 37W” on lowest setting, 6.5A on highest setting.
  • auto-off after about 12hrs
  • 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
• Boussac / IWP 12V electric blanket 9 settings with 2/8/12hr timer
  • 175x65cm / 69“ x 25.5” / (1762 sqin); nice plush finish; no obvious smell; nice controller easy to use in the dark;
  • power output:
    • Setting 1 = 35W = 20mW/sqin; this is the setting for longer duration use overnight;
    • Setting 5 = 48W = 27mW/sqin;
    • Setting 9 = 58W = 33mW/sq.in; not as warm as some but at least it is unlikely to burn your skin;
    • this could be the perfect 12V electric blanket for camping in southern Australian conditions for the price
      • can be “washed” as it does unplug so a hand wash might be OK but perhaps avoid getting the plug wet
      • I have bought this one so will have to see how well it lasts - I would advise tying it down so it doesn't crumple up during the night - just add some tarp clamps and bungee cord around mattress
        • so far so good, I am loving it
  • https://www.aliexpress.com/item/1005008224104103.html ~$AU50
• Aliexpress generic 12/24V electric blanket 2 settings
  • 40W low, 85W high; cheap material and strong off-gassing smell in reviews
  • 150x50cm or 150x70cm and need to select 12V or 24V model.
  • https://www.aliexpress.com/item/1005003427526442.html $AU27-34 for the 12V models

• 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/sq.in $AU29-66 depending upon Ebayer
• Kickass blanket 3 settings
  • $AU69 1.9A/23W 10.5mW/sq.in, 2.85A/34W 16mW/sq.in, 3.8A/46W 21mW/sq.in with 45min auto off
  • 142x98cm / 56“x 38.6” / 2162 sq in
  • a great option if you only want a low 23W power usage and don't mind the enforced 45min auto off function although not as warm as most others on high power
  • https://www.kickassproducts.com.au/buy/kickass-12v-heated-blanket/KA12VBLANKET
• 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/sq.in 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/sq.in max $AU38
    • 4 heat settings 35 degrees, 45 degrees, 50 degrees and 55 degrees ; time settings: 10, 20, 30, 40, 50 and 60 minutes;

• 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 as I have done (see at bottom)

• 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

• 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

• 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

• 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!

• 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

• 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/sq.in
  • 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/sq.in

• see below for 9V

• 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/sq.in 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/sq.in
    • (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!)

• 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
• HOWEVER:
  • 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?