# 12v camper thermostat and heater

Discussion in 'General Electronics Discussion' started by ticomic, Sep 26, 2016.

1. ### ticomic

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0
Sep 26, 2016
Hi. I wish to heat a small, closed-in sleeping area in my camper ( approx 6 ' X 5 ' X 5' ) using a 200W 12 V car heater. If I run it off a deep cycle 12 v battery ( Group size 31 - 800 marine cranking amps, 105 amp hours, 185 minute reserve capacity ), A) how can I figure out how long it could run ? B) 200W 12 V heaters on the market use two 20A fuses. Could I use an old-style mercury thermostat and a 12V / 20A relay to control one leg of the heater ? . There is no danger of any water freezing, as camper is very basic, so heat can be concentrated on sleeping area only. 3-season use only. Thank you four your time.

2. ### Harald KappModeratorModerator

11,445
2,628
Nov 17, 2011
A)
A rough estimate:
200W / 12 V = 16.7 A.
105 Ah / 16.7 A = 6.3 h
This does not take into account loss of capacity of the battery due to ageing, low temperature etc.
B)
Probably yes. A 20 A relay should be good for this purpose. Note that the relay needs to be specified for at least 20 A DC break capability. Otherwise interrupting 20 A DC may cause an electric arc across the relay contacts which may burn the relay.

3. ### Alec_t

2,950
803
Jul 7, 2015
Have you done the maths to see if 200W is sufficient?
The mass of air involved is ~6kg and will need ~6kJ (6kWsec) to heat it 1 degree C. With a 200W heater that would therefore take ~30sec; so 5 mins for a 10 degree rise (assuming no heat loss)..
Do you know the thermal resistance of the walls, floor and roof of the enclosed space? Account will also need to be taken of the considerable heat loss through the inevitable roof opening (assuming you don't want to asphyxiate yourself ).

4. ### ticomic

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Sep 26, 2016
Thank you very much. The note about purchasing a 20 A DC relay is a big plus. The little heater will never run for 6 hours, given the help of two bodies also providing some heat. Looking forward to installing all this. BTW it froze here this morning. Just goes to show beautiful autumn leaves season without heat in Québec can be rather uncomfortable. Merci !

5. ### ticomic

3
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Sep 26, 2016
Thank you very much for your time. First, there is no risk of asphyxiation because one wall is only thick, opaque curtains. Also, given that 2 bodies already generate some heat, added to the fact that we just want the inside temp about 10 degrees C warmer than outside at most, this will fill the bill. It froze here this morning, proof that autumn leaves season ( and May, for that matter ) requires a little heat booster during the night. Camping is positively OUT from Nov to April incl for us, anyway. By 8 this am, it was +2 C. Coldest temps are between 6 and 7 am. Such is Northern Québec....6 mo of 'nice' temps. Note: I do have a propane/electric furnace, but that would be used only if we hit a really cold and unpredicted spell of witer air. Under normal conditions, just making toast on the propane stove will dry the air and warm the whole camper - but by then, we heve opened the ceiling traps. Merci !!