Apollo 13 inside my car ! or how to get lower AMP for a heater ?

Question

Hi, I'm having a problem with my car heater, which is completly dead. Yep houston we have a problem, winter is incredibly hard this year. it's over -20 celcuis outside most of the time. While I wait for better weather to fix the radiator and not freeze my fingers till they droped, I figure I can use for a while a small heater that could use the lighter switch. The lighter switch is 12 volt @ 15 amps, which give me 180 watts. I baught a pretty nice ceramic heater that use 120 volt @ 2.1 amps, which need 250 watts. I know with a lower AMP the heater will generate less heat but on the other hand this is way better than having -15 cold inside the car, which would probably feel like being inside apollo 13 with no heat... The heater is pretty simple, it use the AC to power the heater and use DC to power a fan. The thing I'm not use to is to modify and work with AC. So here plenty of pics that show how it's build and specs. I'm wondering 1- does I need to modify the 3 ways switch (off, lower, high) or 2-I need to change the step down transformer on the circuit. Thanks guys ! 
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jw0752 · Answer

Hi redgreenblue, What you are trying to do will be difficult as you are fighting design. Since the heater is designed for 120 VAC @ 2.1 Amps this means that the effective resistance of the heater element is 57 Ohms. If you take a 57 ohm heater and power it with 12 volts we will have a current of 210 mA or just under a quarter of an Amp. This translates to 2.5 watts which will not provide any usable heat. One option would be to use an inverter but unfortunately this would demand at least 25 Amps from your 12 volt system. While you could wire the inverter directly to the battery and use it only when the car is running you would have the expense of an inverter. If you take any of these suggestion make sure you leave some tolerance . For example for a 250 Watt heater you would want an inverter that can handle 300 or 350 Watts as a safety factor. The motor is another question. If it is an induction motor then it must have AC to operate. Some of the better inverters will run an induction motor. You mentioned that the motor was DC. You may want to check and verify this as it is unusual for the manufacturers to put additional expense into these small heaters. If they have AC voltage available for the heater element they likely are also using the same for the motor. Correction: I was too quick. Your motor is 12 volts. That leaves you with only the problem of how to convert a 120 volt heating element to a 12 volt one. It may be better to just buy a heater that is designed for 12 volts. Here is a small 600 BTU/Hr 12 volt 15 Amp model that mounts on a visor. https://www.walmart.com/ip/Roadworthy-THR6000C-12-Volt-Auto-Heater/47651732 In my opinion the cost to use an inverter to run the small AC heater would be impractical. Converting the AC heater for use in the car will be impractical also. Sorry. John

jw0752 · Answer

I will bother you one more time. You mentioned that the heater is at 460 Watts when it is on high in the last posting. At 460 watts the heater would draw about 40 amps from the 12 volts. This would also exceed the rating of the inverter. If an inverter detects an over load of this nature most will just shut down and refuse to work. You had a question about Amps and volts. In your case you have 12 volts. Depending on the resistance of the item that you attach to the 12 volts a current will flow. The wattage of a load is determined by the voltage across it multiplied by the current that it is drawing from the power source. The only way to change the current flow from a fixed voltage like 12 volts is to change the resistance of the load. Since the resistance of the ceramic heaters are fixed we don't have anyway to change the load. You may want to begin with the heater on the low setting so you don't overload the inverter or the vehicles wiring. Also keep in mind that this type of load on a 70 Ah car battery will more or less drain it below start level in an hour or so. Sorry I don't have a more upbeat fix for you. John

jw0752 · Answer

Hi Sorry I didn't look at the pictures before I wrote the first time. I have looked now and so I hopefully won't have a red face this time. The heater actually has (2) heaters that can be hooked up together to create the high and only one to create the low. From what I can see each heater is designed to handle 120 volts and put out 1/2 of the max output of the heater. Your inverter should work with this unit but it may have to have heavier wires running to the battery. If you put in a bigger fuse but don't change the wires the wires could over heat and start a car fire. Based on your original 250 watt rating of the heater I would expect the inverter to draw 25 Amps when it is running the heater. If you have the inverter inside the car you will at least capture what ever heat it is also producing. I am sorry that the visor heater didn't work. unfortunately there is a lot of poor quality stuff out there. John

the-dubster · Answer

Hi John,

The linked heaters are mounted in the engine bay nowadays.

I think the old units from the VW campers were cab mounted but - for your stated safety reasons - I guess that’s why they moved them.

Apollo 13 inside my car ! or how to get lower AMP for a heater ?

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