by Craig Carmichael
Preliminary edition, 2010/03/07
Designed as an accessory to the Electric Hubcap drive system to turn gas cars into plug-in hybrids.
www.TurquoiseEnergy.com - www.ElectricHubcap.com
Introduction
For plug-in hybrids running on electricity with the petroleum engine off, an electric heater will be required... at least to defog the car windshield if not for occupant comfort. I didn't like the few choices I found, and I decided to make my own. This 'booklet' details that project, with a few alternative heater design ideas for variations to the theme. Not much of an invention, but perhaps a nice, interesting project for some. And by making your own, you *might* even save a little money!
Special TOOLS
I'm not going to list mundane things like a soldering iron and tinsnips, and stick to what most DIYers into electrical/electronic things are likely to not have.
* 18" or larger Sheet metal break for folding sheet aluminum. Don't panic, I found a great 18" benchtop sheet metal break for just $39.99! ("Magnum Mighty Mini Bender" at KMS Tools -- also available on-line. Or you can use a 12" break by making the top and sides in two or three separate pieces instead of one piece. You'll have to figure out the sizes and tabs yourself, though.)
Hmm... that's probably about it!
PARTS
Chassis:
#18 gauge sheet aluminum 3.75" x 18.0"
#12 gauge sheet aluminum 5.8125" x 5.8125" (5 - 13/16" x 5 - 13/16")
Suitable Mesh Grille, fan and heater covers, two approx 4.875" x 4.875" (4 - 7/8" x 4 - 7/8")
Electrical Conduit Clamp
Electrical Conduit pipe ~18" length
Main Components. (Excludes screws, solder, wire, etc.) Listed are the ones I used. Some of them are discussed in detail further down:
Heater Elements Core: DBK HRKK01-16/14 36-72V L49V (approx 4-7/8" x 5.5" x 1.5")
Fan: Circuit Test, DC Brushless Fan CFA1212025MS (DC 12v, 0.25a) (approx 4.75" x 4.75" x 1")
Thermostat: Stancor, STO-140 - this is a typical dryer thermostat, 140 degrees F
(Rated 15 A @ 120 VAC or 10 A @ 240 VAC. )
Fan Resistor: 120 ohms, 5+ watts (For 12 V, .25 A fan with 36V power. Mount it in the blowing air - it's burning 6 watts)
Second Fan Resistor: 200 to 300 ohms, 5+ watts. (Fan speed option: low speed for 200-400 W, high for 600-800 W.)
Switch: 4 position rotary switch salvaged from a 120 V, 2x750 W, electric heater
(That's the one I used. Don't panic, See Switch Alternatives below)
Spade Lug Sockets - at least 5 should be high-temperature ones for the heater element lugs. (Most everything else can be soldered if desired.)
Switch Alternatives (see schematic 2):
1. On-Off-On double pole toggle switch, at least 15 amps rated. Gives 400-0-800 W or 200-0-800 W
2. Above plus a second switch, SP On-Off, 10+ amps. Gives 200-0-600 and 400-0-800 W.
3. Three separate toggle switches, SP On-Off, 15+ amps rated: gives 200-400-400-600-800 W.
(I recommend having a 200W setting - heaters use a lot of battery power.)
I found a small (~4-7/8 x 5.5 x 1.5 inches), rectangular, 36-72V electric heater core consisting of four finned resistors, at Canadian Electric Vehicles in "area 51" (take exit 51 near Parksville). They're also available for higher voltages. Prices (IIRC) were from 80 to 125 $. I think this one, the lowest voltage unit, was really made to be 1500 watts at 48 volts. At 36 volts it's 800 watts - four 200 watt elements. They're wired in series but with access to the midpoints, hence five connection terminals (seen on left side in picture). They're made by DBK and bear the numbers DBK HRKK01-16/14 36-72V L49V. They may be 'special order' as the voltages on the DBK web site are all 110/240. It seemed pretty much ideal for the car heater I wanted to make, so, having not yet made the heater, I bought it. It was much pricier than the $7.99 'off the shelf' heater from XS Cargo and less than its 1200 watts, but still half the cost of the finished 430 watt golf cart heaters and with substantially more heating power. I suspect if one ordered a bunch, they'd make them however one desired, eg, 1500W at 24 volts.
Some components for the heater.
I used the cube heater element with finned resistors.
The 120 volt, 2 coils, element at the front could have been tapped at 1/3 and 2/3 with high temperature crimp connectors to obtain two sets of three - 36 V, 200 W elements (total 1200 watts). This heater would have needed a larger chassis.
I measured all the parts and put the chassis dimensions on paper. I cut the aluminum sheets with a crosscut blade on the radial arm saw (safety goggles, earmuffs, no piles of sawdust to catch fire), and trimmed the bits with tinsnips. Then I marked the dimensions on the actual pieces and foto'ed them - too simple!
However, having assembled and built one heater, I found that the internal space was a little small which resulted in needless headaches. And the bottom piece would have fit better if it was made to fit just inside the top/sides piece instead of to the same dimensions. So I recommend cutting and bending the sizes typed into the fotos rather than the original handwritten sizes, assuming the same size core and fan.
These chassis dimensions are of course to fit the parts I used. Many fans are available in the same frame size, but from 3/4 to 2 inches thick. Possibly the same may apply to the heater elements unit. If the fan or the heater core is thicker or thinner, the thickness of the unit should adjusted to leave about 1/2" space between the core and the fan. 1/4" space, I found, made for difficult fitting of the dropping resistor and the thermostat in the air space between them.
The top and two sides are made from one piece, and the bottom is a separate thicker piece. The bottom is the mounting piece and for the intended mounting with a conduit pipe connection, it should be fairly stiff.
Heater chassis top (and sides) piece, #18 gauge aluminum, showing dimensions and cutouts.
Heater chassis bottom piece, #12 gauge, showing dimensions and cut-outs.
So! The break was just strong enough - pulling both handles, it was flexing a bit in the middle on the 17" bends, but it did the job.
I sanded the cut edges on the stationary belt sander (otherwise use a file), straightened them up a bit with pliers, and in a fit of decorativeness, polished the top on the polishing wheel. (But I suppose I should paint it all black now so it doesn't make glare at the dashboard reflecting sunlight - and all those awful glaring headlights that seem to have unfortunately become ubiquitous on the streets.)
Bending the bottom.
The top piece of metal C-clamped down tended to slide out with the pressure.
The answer: more C-clamps, tighter!
I also used my space saving technique of C-clamping the entire tool to a workbench when I need it rather than bolting it down.
The bender proved much more flexible for doing boxes than expected. Where 'regular' benders have movable teeth that are a pain to set up to allow compound folds, with this one you can just C-clamp down any old short top clamp piece instead of the full length one. Cool!
Do the long folds first, and the short ones after, as per these pictures.
The chassis. The bottom could be made a little narrower so it would fit inside the top piece edges on both sides. might look better. (Typed dimensions are for this.) As it was, I had to do a small bit of hacking and have one side inside and one outside.
Heater element installed.
The plastic mounting lips of the heater elements assembly had slots for bolts, but that would have meant trying to do them up from the inside with the ends sticking out. So I drilled smaller holes in the lips elsewhere, to thread sheet metal screws into from the outside.
Fan installed. The mounting holes again expected screws from the inside poking outwards. Here I just used fat screws that threaded into the plastic. The holes were also a little far inwards from the corners, but washers on the screws, and then the grille over it, solved everything.
The assembled heater with grille. (heater core side.) The switch side (left) is the bottom.
Wiring
I'm only going to raise a few points about the wiring. I trust those who embark on this project are able to follow the wiring diagrams, cut, strip, crimp and solder wire, and connect up the parts as shown. In the foto under the schematics, you'll note how crowded it looks, and indeed, the typed chassis dimensions reflect the need for a bit more breathing space.
With the noisy 12 volt fan I used, it was an advantage to lower the voltage to even less than 12 volts so it would run slower. However, if the simple dropping resistor was too large, the initial voltage was too low to start the fan. With the largest practical resistor, 120 ohms, the voltage came up from about 4 to about 10 volts as the fan picked up speed. (Above about 135 ohms, fan starting became unreliable.) This resistor dissipates about 6 watts, so in addition to being a 5 watt resistor, it should be mounted between the fan and the heater core in the airstream to keep it cooled down. The 200 to 300 ohm resistor to the 400 watt element pair is an option: the fan will get about 9 volts when the elements are off, and 11 volts when they are on, for high speed operation when the heater is on high(er).
In the actual unit, I put a 12 volt zenor diode across the fan to ensure it didn't get overvoltage. In practice, it seems with the right dropping resistor(s) from 36 volts to less than 12, this is a needless precaution. However, the right value of dropping resistor will need to be determined for each model of fan (and naturally depends on the proper voltage of the fan as well as its current draw). Many 12, 24 and 120 volt fans are available in a few standard frame sizes like the one used, and I even have a 'plus and minus 15 volts' fan the same size, but I have yet to see one 36 volts. (Is that a good argument for a 24 volt car motor system?)
Schematic - as made (4 posn. rotary switch)
Alternate Schematics - three on-off switches, or DP on-off-on plus SP on-off switch
Wiring up the heater.
The mounting plan for in the car is to have the heater atop a pivoting piece of conduit pipe (with the power line inside it), able to pivot to face the windshield or into the car (including at the driver). However, it is to hit the dash and not be able to do a 360º turn so that the wire can't get twisted up.
Safety
It is imperative that a fuse, about 30 amps rating, be placed in-line with the heater, at the power source end; not in the heater. If a short circuit occurs anywhere including in the cable, the fuse needs to blow and shut off the power to prevent fire. (The cable may become worn underfoot somewhere or sliced by, eg, a folding or sliding seat mechanism part.) Even if the power source is after the main breaker for the motor, the fuse should be used. The rating of the main breaker is so high it might not blow, and regardless, you don't want the motor to quit while you're driving because of a problem with the heater. (What was that big air crash into the ocean, where a main breaker blew and the plane lost all its electrical power because of a problem that started with the stereo headphone jack at one seat?)
A good cable to use is probably two-wire #12 gauge "cab tire" rubber sheathed wire. To avoid possible confusion (and a possible burned out fan - the polarity doesn't actually matter for anything else), the white wire should be taped red and used as the positive lead, with black as negative per DC wiring standard colors.
Ideally, the power to the heater should go through the motor controller's power "solenoid" relay so that it shuts off with the car key and electric motor system, or another "solenoid" relay also operated by the car key. If the heater can be left on when the car is off, sooner or later it probably will be and it will discharge the batteries to zero.
The heater will of course work without the thermostat, but it ought to shut off if it overheats. Reasons it might overheat include the fan quits working or the blades get stuck on something, a jacket or other cloth is draped over the heater, or it has become full of dust that impedes air flow or even catches fire.
An alternative safety device is a heat fuse that permanently burns out at a certain temperature. If you make the heater without any overheat safety device, you're putting your car and yourself at some risk. The clothes dryer type thermostat has a fairly high amps rating - 15A at 120 VAC - as it is expected to turn dryer elements on and off regularly. Here we have 22 amps at 36 VDC when the heater is on high. (If you can find an economical thermostat with a higher rating, buy it. That's the one I found.) But we're only counting on it to shut off in an emergency, ie if the fan quits working or the air is blocked. The condition that 'tripped' it is expected to be fixed quite soon and the heater should be turned off in the meantime.
When I decided to make this 36 volt electric heater for the car last year, I found only one 36 volt heater choice to buy, a 550 (...or is it 430?) watt golf cart heater, and it seemed overpriced. A new web search (Feb 2010) reveals several 36 volt heater choices, and the prices have come down somewhat. In essence, they all seem much like this one - a heater core, a fan, and a switch.
For an advantage, this heater has 'low-med-high' (200-400-800 watts or even 200-400-600-800 watts), where the commercial models I saw all just had 'low-high', half or full power. I recommend having the 200 watt "lowest" power setting to avoid draining the batteries needlessly fast while driving, as well as, ideally, lots of watts to warm the car on cold mornings. For this, 800 watts is minimal, even if it's a lot to ask of batteries already driving the car.
If I find 800 watts isn't enough when it's (for Victoria BC) very cold, perhaps I'll make a second, 1200 watt, heater with that helical coil element from the $7.99 household round coil heater, the "+ & - 15V" fan, and the other parts I have collected up - there are enough parts in the box to make a 'free' second heater!
One alternative to running down the batteries with the heater right at the start of a trip is to use a 120 volt heater and pre-warm the car. (I did this just once and it was very pleasant to get into a warm car with no ice on the windshield.)
An even better idea would be to tie the heater into the car's battery charging system so it would effectively run off the mains from the charger while the car is plugged in.
Unfortunately turning on any heater in advance requires the driver to don his parka, go out in the cold to the car and turn the heater on, then go back in the house.
So a further improvement would be to be able to operate the heater remotely from inside the house. This would probably be done by having a "solenoid" relay just for the heater and some button or switch feature of the charging unit. The heater would be powered: (a) if the car key is on, or (b) if the car is plugged in and the remote switch on the charger is set to "heater on". Thus the driver could leave the heater set to "on" after arriving home, turn off the car, and have it activate when desired with the charger plugged in, but without fear that it might be forgotten and drain the batteries.