winter experiments: contact heating

Last winter I used a Mr. Heater Buddy to good effect.  Since I know that works I decided to play with some other options.

electric heating

Space heating with electricity is impractical off-grid.  Contact heating, however, might have a fighting chance.

I had prior experience with these forms of contact heating:

  • electric blankets (200-300w), which worked well

  • electric mattress pads (100w-300w), which worked even better than the blankets

  • heating pads (20-45w), as one might use for back pain

  • heated motorcycle vest, gloves, and insoles, (90w) which worked amazingly well

  • weatherproof pet heating pads (20w) for doghouses and to keep water bowls from freezing

The following components were used for my testing:

the mattress pad

Starting with the heated mattress pad, it is described as having 10 power levels and is marked 180w.  Amazon reviews mentioned about 20w draw on the lowest setting.  Sounds doable to me.

In practice, however, there are complications:

  1. turning the unit on automatically starts heating at the last-remembered level.  That seems reasonable, except…

  2. it runs full blast when started (preheating?), and keeps running full blast for at least a minute before dropping down to the selected heat level

  3. full blast is way over the claimed 180w rating, usually in the low-to-mid 200s.  It spiked over 300w a few times, as measured on Kill- a -a-watt.

  4. temperature is not maintained by reducing wattage, but by running the heat full blast for short periods of time.  The lowest setting, 1 out of 20, runs the pad 200w+ for 2 seconds then sleeping at 0 watts for 40 seconds (~6w is used by the controller to run itself).  I really wish it would hold 20w over time.

It would trip up the Aims inverter sometimes, the controller would reset and it took me a while to figure out why. This doesn’t happen during start-up, only during the 2-second pulses.

It wasn’t related to battery voltage;  it might trip when Vbatt was over 14v and might not when it was closer to 50% DoD.   It wasn’t hitting the isolator’s max, since it is a continuous 300w inverter that can tolerate 600w peaks.  And if it trips you can turn it on again and it will run fine (including the lenghty, mandatory power-on preheating). If you are watching bank voltage when it trips there is no worrying sag.  Connecting to the battery terminals or to the power distribution point about a foot away makes no difference.

I suspect the resets are a function of the soft start “feature” or some other unknown interaction:

H. SOFT START TECHNOLOGY The soft start technology built into this inverter protects the unit from delivering too much AC power at once by gradually increasing the AC voltage pushed out.  - [manual]( (pdf)

It might be possible to disable this feature with a bit of hardware hacking, but I’d be more likely to replace it with an inverter that doesn’t do this.

The 250w inverter in the Rockpals unit can’t even even get the pad started;  the controller screen partially comes alive then goes back out.

Conclusion:  I think a “dumb” (not electronically controlled) pad would work better here.  Give me an old model with a rheostat dial.  But since this one starts fine and runs a few minutes before one of the 2-second bursts confuses the inverter, I have been using it as a pre-warmer.  If it’s a particularly cold night and the dog has wandered off I sometimes reach over and turn on the pad, knowing it will confuse the inverter after a while.

heated car seat cover

I cut the ciggy lighter plug off the heater and slapped on a 5.5mm plug so I could jack into the Rockpals.

Heats as expected.  LO power is gently warm.  HI is a little too warm, perhaps best used for preheating.  [updated to add:  even LO was a bit too warm, I ended up putting down a thin blanket over it.  Ran 9 hours overnight and there was still one bar left on the Rockpals pack.  Calling this one a success]

Conclusion:  works fine, unknown how long it will last under constant use in a way the designers could not have forseeen.  :-)

next test:  heated vest

Motorcycle heated vests work so well I’ve been meaning to try one for portable use.

The heated vest market has matured quite a bit in the last two years, moving away from proprietary (and $$$) battery packs.  Now it’s more common to buy the vest and power it with a USB generic power brick like you would charge your devices from. Since these bricks are 5v @ 2.xA, we can see they are running about 10w of power.  Doesn’t seem like much, but my motorcycle vest was 50w and I usually ran it at the very bottom of its power range.  If it was chilly on my unfaired bike I would turn it up to about 25%.

I bought a Dr. Prepare Unisex Heated Vest but it hasn’t arrived yet I’m thinking about running a USB extension to make a quick-disconnect for heating the vest when I’m sitting at the “desk”.    :-)

I’ll update with info on the vest when it arrives.

Update:  it arrived and works well for ~10w.  The heated collar section is particularly  nice.  The battery pocket is rather small on the inside and barely fits my biggest pack.  At this moment I am running it on low at the desk, connected to a usb outlet with a 3’ extension. Observations:

  • High power (red):  100% on duty cycle

  • Medium power (blue):  75% on duty cycle

  • Low power (green)  50% on duty cycle.

At the $50 price point I would like to see slightly heavier zippers, a stretch panel on the side to make it conform better for optimal heat transfer, slightly larger battery pack pocket.

possible test:  heating pad

If I find a simple heating pad at a thrift store or something I’ll try that, too.