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Caveat: If you repeat any of this, the risks are yours! Use ideas and adapt as you wish.

I thought I would share my ideas for this simple heater project to provide safe cabin heat during frosty weather when EOS seals and doors can get frozen. My cars are driveway parked and the nearest mains supply is 16m from the cars. Only after hitting the remote unlock and seeing the windows not drop, do you realize window glasses are frozen to the roof seals! Although we have a top half cover, she is too lazy to use it!:(

Design criteria:
1. Safe low voltage
2. Safe low surface temperatures, no external parts too hot to touch when running.
3. What resistor?
4. What power level?
5. How to get power wires inside car?
6. What gauge power wire for 16m run to supply point?
7. Quick release power connection, in case a forgetful driver drives off with power wires attached!
8. To fan or not to fan.
9. Stage 1 simple always on no thermostat. Stage 2 option add electronic frost thermostat.
10. Safe location inside cabin.
11. Use available parts where possible.

1. Mains power to the car is not safe! I intend to leave the power cable in the driveway during Winter. 12 volts would be an obvious choice but requires heavier gauge cable over 16m distance. That thicker cable then has to be taken inside the car. I had a 2 X 45 volt mains transformer and calculated the supply cable need only be 0.75 sq.m or 6amps for that higher voltage.

2. Any kind of heater needs to be built so it does not burn if touched. Whilst exposed hot parts can be kept inside a ventilated box, the box must not get too hot to burn or cause damage inside the car.

3. Resistors could be halogen lamps, fixed high wattage resistors or something else. Lamps can fail and would light up the cabin at night attracting attention. High wattage resistors need fixing to a heat sink of some kind with a large surface area. Several resistors may be needed. My solution was to buy a 230 volt 1800 watt cooker ring element (£4). The element resistance is 30 ohms. Running at about 45 volts, this will emit the design power of 65 watts with sufficient metal surface in the element to run at a fairly low surface temperature.

4. I decided on a power level of around 60 watts. This power will not keep the whole car defrosted! Due to their size, the rear window and sunroof glasses are unlikely to be defrosted, but I expect the front of the car, door and glass seals will be o.k.

5. Getting power into a secure car is not easy. Running wire across the bottom door seal is obvious, but the wire jacket ideally needs to be flat. Oval 6A wires aren't bad, but the best solution (I used) was a 1m long red SATA hard drive cable. The SATA connectors are chopped off and the cable trimmed to access the 4X 18gauge insulated wires which are then 'paired' to create a 2 core flat cable. This passes nicely over the bottom door seal without distorting it.

6. 0.75 sq.mm 6A oval mains wire is o.k on voltage drop over 16m length carrying 1.5A.

7. The heater has a 1m (red) tail - the SATA cable is terminated with a 2 pin inline connector. But rule number 1. is always put the heater on the driver side!

8. Most modern cars are fitted with ultrasonic anti-theft systems. Circulating air currents inside the cabin are responsible for causing nuisance alarms early morning when the sun comes up. I therefore decided NOT to fan assist the heater.

9. Without a frost thermostat, the heater will run fine 24/7 but uses electricity. I added a cheap electronic thermostat probe set to 6-8 deg. C. I'm still experimenting with locations for the temperature sensor. On the dash driver side, the front and side screens are nicely defrosted as is the area near the sensor, but the rear and sunroof glass stay frozen. I shall probably clip the sensor to the sun visor.

10. The safest location for the heater is on the driver side floor mat. The biscuit tin has several 30mm holes on two sides near the bottom, keeping the box very cool. Even so, I would not put it on a leather seat!

11. I already had a 2X45 volt a.c toroidal mains transformer. Each winding is about 1.5amp (two cars!). Cooker ring element (£4), Chinese thermostat module (£2), 25M 0.75sq.mm (£10) and some small components (£3). The biscuit tin was free! - Finish rattle can matt black.

Marked up photos are attached.
 

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Nicely done! And probably cheaper than building a garage! :- p
 
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