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Summary

I'm thinking about putting an electric heater in my car that I could turn on remotely 15 minutes before I leave.

Goal

My goal is for the car to warm enough and, hopefully, the windshield will defrost at least partially.

Parts

  • 75Ah Deep Cycle Battery
  • 1000W Power Inverter
  • 500W Heater with Remote

With the numbers above, I believe I should be able to operate the heater for nearly 2 hours.

Notes

  • I don't want to remote start the car and let it idle.
  • My car is close enough for the remote, but not in a garage.
  • I'd take it home over the weekend for charging.

Questions

  1. Is this setup feasible?
  2. Any other comments?
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    If there is an outlet accessible to the car I think you'd do better with a block heater. They warm the coolant, so the engine is warm and you have heat and defrost almost instantly. Easier on the engine too. Don't think you're going to get enough time out of that battery. – dlu Oct 30 '16 at 7:08
  • What's wrong with a remote start? Also applying rain-x or something similar to the windshield would be a lot lot easier and safer. – Ben Oct 30 '16 at 15:36
  • Remote start will require the engine to run which I don't really like, plus I feel like trying to hook up those components as a fun project. I guess most people will opt for the remote start... – Nick Oct 30 '16 at 17:47
  • It would help to know more about your operating environment - how cold will it be, is there AC anywhere nearby? Is there a reason why you want/need this to be battery powered? – dlu Oct 30 '16 at 20:46
  • i'll put this forth, I've tried this before when i was younger using an extension cord and an ac ceramic 500w heater and it doesn't work very well. you need a way to point the heater so that the air blows against the windshield without losing too much energy. – Ben Oct 31 '16 at 11:24
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You could do something like this, but I don't think a 70 amp-hour (Ah) battery will be up to the task – assuming you only want to charge it once a week. As a general rule you don't want to discharge the battery beyond 50% (35 Ah in this case) and capacity goes down as the discharge rate goes up. The capacity rating you will need will also depend on the type of battery that you're using (a traditional flooded cell lead acid battery would need to be larger than an AGM or gel battery). So the first thing you'd want to do is to up your battery capacity, around 140 Ah for an AGM or gel battery or around 220 for a flooded cell. You can use this calculator to experiment with your options.

The advice to go with a DC heater is good, heating loads are generally DC anyway, the only practical difference between a DC heater and an AC heater is likely to be the fan motor and the resistance of the heating element (to account for the higher voltage). Staying DC will both save you losses in the inverter and money that can go into a bigger battery.

To do this project safely you'll need to find a secure way to mount your battery, and you'll also need to plan for both over-current and over-temperature protection (the heater(s) may already include the latter).

I'd be inclined to design the remote heater control around a small capacity relay that operates a larger relay for the heaters. I think that would help hold the cost down.

I'd also suggest looking into using the heater to heat the water in the heater loop of the engine cooling system and using your actual heater/defroster to handle the job. I think you'd be less likely to have fire problems and you'd save space and mounting issues. It would also help to warm the engine so that you'd have heat once you start driving.

All of that said, if you have AC power available, I'd suggest looking into using a block heater to warm the water in the cooling system. That would give you heat right at the start and would have the engine already up to (or close to) operating temperature. If you wanted to defrost you could add a relay to bypass the heater fan switch and run the defroster as you are planning to do.

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Yes, this is doable. Just 2 things:

  • save yourself the inefficiency (and expense) of a power inverter and converting DC (from the battery) to AC (for your AC heater), and get yourself a DC heater
  • your heater will be unsupervised. These things get hot. Make sure it's not touching anything that will melt or catch fire. I'd set it on a piece of plywood, and if you're setting it on a seat, a piece of plywood behind it too.
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  • I originally wanted to go with a DC heater but couldn't find any that are more than 250w and none that can be operated with a remote – Nick Oct 30 '16 at 16:43
  • You could use more than one heater. The reason you can't find ones that can be operated with a remote is, probably, that it is too unique a use case. You could use a separate relay, something like this would get you close, or you could use a remote with a smaller capacity to operate a larger relay for the heaters – I'd want a relay with at least a 30 A capacity for a 250 W heater. – dlu Oct 30 '16 at 20:53
  • @Nick I have several that are in storage, they are 500 and 1000 watt units I will see if I can find a model and make. – spicetraders Oct 30 '16 at 21:35
  • Look at DC Thermal heaters. – spicetraders Oct 30 '16 at 22:48

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