Dial a wattage, 12v heating pad

[SternWake]

Up until its recent failure, I have been using a 53 watt 115Vac old school 20+ year old timerless heating pad on my 400 Watt PSW inverter to slowly heat my 5 gallon shower bag.  Not very efficient, but the 400 watt PSW inverter drew 1.1 less amps powering this heating pad than my 800 watt MSW inverter did.

So it got wet, and failed, and that was that.  Always looking to bypass the  inefficient inverter, I search 12v heating pads and find many devices intended to be car seat heaters, but I want one only big enough to fit on the seat bottom, not also up the backrest.

So I find this one and order it:
http://www.amazon.com/gp/product/B00X75AS20/ref=s9_acsd_hps_bw_c_x_2

It arrives and I plug it in to a 12v receptacle fed with 10awg, and it is pulling 40.2 watts.  but 5 minutes later it shuts off, the light remains on but it pulls no amperage.  I pull ciggy plug and the spring loaded tip is hot enough to scorch my finger tip and the whole plug is in the stinky hot plastic temperature range.

I wait for ciggy plug connector to cool and replug it in, and the light illuminates, but it draws no measurable wattage. It has failed.

Out come the wire cutters, off comes the Ciggy plug, and it is dissected.
 It has a switch soldered onto a basic circuit board with a diode or 2 involved.  3 wires lead from plug to heating pad, but only 2 are employed, the blue and white wire, and they are no thicker than 18 awg, likely 20awg.

.

I hook these blue and white wires to 12.5v directly and a wattmeter inline reads 62 watts.  So I use an older 45 amp powerpole connector with 10awg leads and some wire nuts for temporary testing, and the heating pad is pulling upto 68 watts at 14.5v, and 55 watts at 12.8v.  After a few hours of use at 14.5v, I notice it is no longer functioning.  The wire at the wire nut had heated enough to somehow open the circuit.  Obviously the 68 watts was too much for the provided wiring.  Voltage drop was an intentional  limiting factor provided with original ciggy plug, and I had bypassed the plug and wired it much more directly to power source over thicker wire, and it was too much for it.

So I used 18 awg wire, about 6 feet of it with a 45 amp powerpole on the end, and cut off most of the provided 20awg wiring, soldering 18 to 20 awg about 8 inches from heating pad.  

It can still pull close to 70 watts initially hooked to 14.5v, but then settles in the 55 to 60 watt range once the resistance increases with warmed copper wiring.  This wattage quickly becomes too hot when used under my lower back, and I need to throttle it back.

So I pull out my 10 amp motor speed controller which has a 45 amp powerpole plug on its output and plug the heating pad into that, with my gt power wattmeter inline.  Now I can dial in any wattage from 0 to ~65 watts at the turn of a dial.  The higher the voltage available, the more wattage the heating pad can pull, thus the '~' 65 watts.

So this heating pad is dual purpose in my intended usage, to heat my shower bag when i have a surplus of solar, or am willing to use some battery capacity to heat water, or when my lower back is protesting my life.

With the PWM motor speed controller, I can effectively dial a wattage, and thus the temperature of the heating pad, and how much it consumes from my solar surplus or my battery capacity.

I basically save 20% simply by not having to use an inverter to power it.  More directly wiring it to a 12v source increases the wattage it can pull, but eliminating the ciggy plug increases reliability.  In fact I had no choice but to cut it off as it failed in short order and caused a non functioning heating pad all together within minutes of being plugged in.

While I am using a 10 amp PWM motor speed controller as a heating pad throttle, A PWM LED dimmer can also be used, such as this 8 amp version:


http://www.amazon.com/Dimming-Contr..._UL160_SR160,160_&refRID=011EMMR3ARRQEX1KEK4A

I have several of these already employed dimming my LEDS, and have ordered 4 more at ~2.70$ each on the slow boat from china and will employ one of these for the heating pad controller, as my 10 amp Motor speed controllers have no box to protect the circuit board.

PWM motor speed controllers and PWM LED dimmers are really pretty much the same thing, and much higher rated controllers are available for some more 'headroom' and possible reliability:

http://www.amazon.com/s/ref=nb_sb_n...ols&field-keywords=pwm+motor+speed+controller

I am satisfied with the performance of this pad, and being able to eliminate having to use the inverter and its 20% loss of efficiency is always a good thing. If I have grid power available my Adjustable voltage powersupply can more than cover the 12v heating pad draw and precisely recharge battery to full as well.

I do have a 115Vac 60 watt heating pad with the 45 minute timer somewhere, but the timer makes it useless for heating the showerbag.  This 12v heating pad at full output likely heats the showerbag faster, and consumes less power to do so.

So Win Win.

Perhaps others might benefit from my experiment.  I am very happy with the results, and my contempt for 12v ciggy plugs is justified once again.  Obbomed's 12v ciggy plug appeared to be a better design, as advertised, but its early failure and stinky hot plastic revealed it to be just as bad as the rest of them.  It failed at~ 42 watts after about 5 minutes of passing that much current.  Shorter thicker wiring allows the heating pad to draw nearly 50% more wattage, but I decided to limit this ability somewhat with a longer length of 18 awg, rather than the 10 awg I was originally intending to use.  The Motor speed controller effectively limits its ability to heat and to deplete batteries.

I found it was able to heat my showerbag from 72f to 82f in about 1 hour and 15 minutes, which seems faster than my previous 53 watt heating pad on the inverter could, but I do not have scientifically valid data for comparison to how well the old pad worked vs the new.

114F is about the limit I can handle when bathing with hot water  The old pad would take about 14 hours to exceed this temperature when the bag is amply insulated with jackets, towels, ect.

No  hard data yet on the new pad time to 114f, but I suspect it will be faster and use less Watt/hours Amphours to do so.

Also I can decide to allow only a certain amount of battery capacity to deal with heating water via the motor speed controller.  While I might not get that skin pinking 114f at 15 watts over 8 hours of heating, it would still be better than ambient temperature water.

My old 115vac heating pad had 3 settings, and there would only be a difference once the pad got the water to the 100 degree range. With setting of 1 to achieve 108f in 14 hours, setting 2 121f in 18 hours, and setting 3 132f in 24+ hours.

Now I have unlimited control, more efficiency, and am very pleased.

You too could have this for less than 30 dollars, but some wiring skills would be required to bypass the ciggy plug and hook it to the motor speed controller.

While I intend to use mine as a water heater and a lower back soother, I also imagine it would work great to help keep warm in winter, and dialed down with the PWM motor speed controller throttling the wattage/heat, could be less abusive to the battery capacity available.

In case some here are interested in the wattmeter I employ inline to measure current:

http://www.amazon.com/G-T-Power-Met...F8&qid=1464563069&sr=1-1&keywords=gtpower+130

I have modified this with 8awg wiring replacing the aluminum 12awg leads, but they now sell a version with 8awg leads too:

http://www.amazon.com/WindyNation-A...F8&qid=1455265595&sr=8-98&keywords=DC+ammeter

Anderson Powerpoles are a much much better electrical connector than a ciggy plug, but they do take some skills and tools to employ properly

I use 45 amp versions everywhere.  the 45s will mate to 30 and 15 amp versions which are much easier to crimp properly:
Please note that cutting off a ciggy plug also eliminates the fuse located inside a ciggy plug connector, so the wiring then becomes unprotected, and a fuse needs to be employed to protect the thinnest wiring in the circuit.

http://www.amazon.com/s/ref=nb_sb_n...&rh=i:aps,k:anderson+powerpoles+30+amp+45+amp

https://powerwerx.com/anderson-power-powerpole-sb-connectors

Ciggy plugs will at some point let you down, and the Anderson powerpoles are, in my opinion, the best replacement option, but certainly not the only option, and do not forget their lack of built in fuse.

 

[Gunny]

SternWake, how did you get all of your electrical system knowledge?  Ihave read most of your threads and am impressed by your ability to explain how things work.  I wish I was parked next to you when I install my solar.

Thanks for the tips.

 

[SternWake]

SternWake, how did you get all of your electrical system knowledge?  

An intense interest, and ~16 years experience living in a van and acquisition of the tools required to measure results of experiments.

Nothing quite like looking at a plastic battery case and having no idea what is going on inside of it to spark the curiosity.

One day it works, the next day, not so much.  Figuring out why is where the learning is achieved.  I remember being confused and making mistakes, grasping things incorrectly wishing I had someone who I could ask and receive a coherent answer from, and on this forum try to keep others from repeating my mistakes as I slowly learned.  I recall being easily confused on this subject manner, and not really finding the right questions to ask or those to ask them of, so I try to head them off at the pass when I see similar confusion and the desire to learn.

 It would be different if people's budgets were not limited, and premature battery replacement were of no financial consequence, merely a nuisance.

But a 100$ minimum, premature  and unexpected battery replacement, is a kick in the gonads to most of us, and usually preventable. 
 15 amp household outlets have spoiled nearly everybody who has grown up with the power grid, their biggest dilemma is if an outlet is within easy reach of whatever they need to plug in and power.

 

[Gunny]

An intense interest, and ~16 years experience living in a van and acquisition of the tools required to measure results of experiments.

Nothing quite like looking at a plastic battery case and having no idea what is going on inside of it to spark the curiosity.

One day it works, the next day, not so much.  Figuring out why is where the learning is achieved.  I remember being confused and making mistakes, grasping things incorrectly wishing I had someone who I could ask and receive a coherent answer from, and on this forum try to keep others from repeating my mistakes as I slowly learned.  I recall being easily confused on this subject manner, and not really finding the right questions to ask or those to ask them of, so I try to head them off at the pass when I see similar confusion and the desire to learn.

 It would be different if people's budgets were not limited, and premature battery replacement were of no financial consequence, merely a nuisance.

But a 100$ minimum, premature  and unexpected battery replacement, is a kick in the gonads to most of us, and usually preventable. 
 15 amp household outlets have spoiled nearly everybody who has grown up with the power grid, their biggest dilemma is if an outlet is within easy reach of whatever they need to plug in and power.

Well said, thanks.

 

[SternWake]

This 12v heating pad appears to be cycling on and off.  Not sure what type of safety device is employed or even if this is an intentional cycling of it for safety.


This cycling is undesirable in my mind as it does not appear the pad is actually getting anywhere near the danger zone, heat wise and will slow the heating of the showerbag.

I'll investigate/research/experiment further and report back, and urge anybody who was considering this product to hold off until I have a better idea what is going on with it

 

[SternWake]

The heating pad basically can't get my 5 gallon showerbag hotter than 101.5f. But it does appear to get the ~68 degree water to this temperature faster than the 53 watt 115Vac heating pad could. Once the water is at 101f, the pad cycles only briefly Short time on, long time off.

I think it used about 1Ah per hour to maintain 101 f overnight last night. I had a bunch of clothes over the bag insulating it, but improvements could be made in this area. The showerbag basically rests on several layers of reflectix in my passenger seat, and I bend the pad up to the shape of the bag, and stuff jackets, clothes, ect around and on top of it to insulate it.

Ideal water temp would be 110 to 114f.

There is some sort of bi metal thermal switch in the pad which opens the circuit when it reaches a certain temperature.

I likely could open up the pad, replace this with a slightly higher rated one and perhaps get upto 110f, but for now 101f feels pretty dang good after a surf.

I've not been throttling the pad back with the PWM motor speed controller.

I recently put a full bag on the pad, and rested thermocouple on top of bag which measured 69.1 degrees F.

30 minutes later, 70.5f, 2.5Ah consumed. Wattmeter says it is drawing 62.3 watts at 14.29v .
78 minutes later 73.2f, 6.794AH consumed drawing 61 watts at 14.27v.

I'm plugged in, holding absorption voltage. Wattage at 12.6v has been around 47 to 51 watts.

I'll have some more data on how many AH and watt hours it consumes over a certain timespan to get the 69.1f degree water to ~101F tomorrow as some starting point data.

Since the voltage greatly affects wattage the pad draws being plugged in and holding 14.5v, then 13.6v will throw off the results somewhat compared to if the battery was powering it at a lower voltage from 12.8 to 12.2v. Lower voltage likely equates to longer heating times to 101F but less overall AH and WH consumed.

 

[SternWake]

10 hours after placing ~ 5 gallons of 69.1 degree water on the heating pad, it was 101f.  

It could have reached that at 8 hours, I know not.


It consumed 33.129AH in those 10 hours to heat the water that much, but the battery voltage was 14.7 for nearly the first 2 hours and 13.6v for the remaining 8.  Results would be different if battery voltage was dropping from 12.8 to 12.4 during that 69f to 101f Journey

COnsidering direct sunlight can heat the showerbag water hotter faster, this is a very poor use of battery power, but it is convenient to not have to move the bag around into the sun and then insulating it late afternoon, until I use it later in the evening.

There is likely a more efficient way to heat the water electrically, conveniently, this 12v heating pad was just the convenient option and is more efficient than the household heating pad on an inverter.  But the household pad could get to and exceed the  skin pinking 114F water.

  I never hooked my wattmeter inline on the inverter to see how much it consumed for comparison, but either way, this is not an efficient method of heating water.

I'll try the PWM controller throttling back maximum wattage and collect some more data and will update this thread occassionally.  Replacing the bi metal thermal switch inside the pad with a higher rated one is not high on my to do list.  Perhaps in the fall when the 114f becomes slightly more neccessary, and the heating pad has proved itself otherwise reliable, or not.

For now the ~25$ purchase is not regretted.  101f water is more than tolerable.  33Ah consumed  to get there is a lot though.

 

[SternWake]

Forgive me for using this thread for logging data for future reference.

Tonight, I put nearly 5 gallons of H20 on the heating pad, but have limited the max output of the heating pad to 30 watts VIA PWM motor speed controller, at an initial battery voltage of 12.8v.

The water started at 69.1 degrees again, again at 11:15PM . I am not plugging into the grid overnight, so this will be battery voltage only. I'm pulling 12.9 amps as I type, 12.5V, 13Ah from full.

We'll see where the shower bag temp resides tomorrow in 8 to 10 hours or so, and how many 'AH' were consumed to get there tomorrow sometime. Wish I could think in watt hours, or even thought to record Wh last night.
Changing voltage throws Ah out of whack to a unfortunate degree, to those of us who learned to think in Amp hours. Watt hours is much more precise.

Today it was using about 1.2Ah per hour through the warmish afternoon( 75 Ambient average) to maintain 101f, well more than my 12v compressor fridge uses to maintain sub 35f.


~1 hour of ~30 watts of heating pad raised h20 temp from 69.1f top 71.2f. and consumed 1.83AH or 22.9Wh

.

 

[SternWake]

30 watts Maximum allowed for 10 hours achieved only a 82.1 degree shower bag and consumed 27.4 AH of battery capacity to do so.

Much poorer numbers compared to 101f for 33AH at ~55 watts average.

The PWM motor speed controller is also not 100% efficient, So I have to say these results make dialing in a lower wattage with it, nearly completely pointless, as I consumed only 6 less AH in nearly the same timeframe for only 82 degree water.

Perhaps used to help warm one's bed in winter at 10 to 15 watts this throttled heating pad could make some sense, but certainly not for heating my waterbag to a desirable temperature for bathing 

These consumption numbers are really too high for my system to support overnight each night, more than doubling the AH I would otherwise consume and beating up the battery much much more

With thick clouds overhead now, my solar is making only 3 amps at 11AM.  I plugged in and am feeding my battery 40 amps.
  
I am not sure I will do any more experimenting regarding temperature achieved for AH consumed.

Perhaps at 15 watts max, just to see how warm the water can get in 10 to 11 hours.