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Discussion Starter · #1 ·
[ I originally posted about this in another thread: Meditation Hut for just one person ]

I want to power an electric heater off a car battery, and I found a 12V 300W heater for sale on the internet:


The mains electricity supply in the UK is alternating current going from -325 to +325 volts. If you hook up an old-fashioned tungsten lightbulb to the UK mains supply, the lightbulb will give off the same heat as though it were connected to a 230V DC battery. So anyway if I have a 300W heater running off the mains, then the current flow through the heater is 300W divided by 230V = 1.3 amps.

However if I have a 300W heater running off a 12V car battery, then the current flow is 300 divided by 12 -- and that's a massive 25 amps. That's 20 times more current than a heater running off the mains!

The wire I have for my hut is supposed to be used for wiring the lights in a house -- it's rated for 6 amps. The concern with using too thin of a wire is that it might overheat and burn, because thinner wire has greater resistance. Any kind of copper wire of course has a little bit of capacitance and inductance, but for the most part it's just a very low value resistor. The heat (measured in watts) produced by a resistor can be calculated from two figures:
i) the resistance of the resistor ( R )
ii) the current flowing through the resistor ( I )

Heat produced by resistor = I²R

Note here that the current here is squared!
1.3 amps squared = 1.69
25 amps squared = 625

If I divide 625 by 1.69, then this tells me that the wire for the 12V heater will produce 370 times as much heat as the same wire being used to run a 300W off the mains.

So if I understand all of this stuff correctly, it looks like the 6 amp wire just won't be good enough for my 300W heater running off a 12V car battery. Instead I think I need to buy this other wire rated for 37 amps:

www.diy.com

Prysmian 6242Y 4mm² Twin & earth cable, 10m | DIY at B&Q
Power your mains electricity with this Prysmian Copper installation cable. It is suitable radial socket circuits and is supplied in a 10m length.
www.diy.com

This thicker wire costs £2.00 per metre. (In American, that's US$0.69 per foot, or US$2.06 per yard).

Am I making sense here? Do I need to get the thicker wire rated for 37 amps? I'm pretty sure I need the thicker wire.
 

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If the heater is drawing 25A, then you need a wire rated for 25A. The 37A wire is a bit overkill but it will work.
Also note that most car batteries have a capacity around 50 Ah, so if you run 25A it will be dead flat in 2 hours.
 

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This exemplifies a disadvantage of low voltage circuitry, you need fatter wires to carry a given number of watts. Wire size is calculated based on amperes and not volts.

For a continuous load which includes heaters, you need wiring that goes with 25% more than the expected amperes drawn continuously, namely 31-1/2 amps, requiring 8 gauge wire.You could get by with 10 gauge wire maxing out at 30 amps not continuous when you have to change the battery every 2 hours but if you are using a discarded electric vehicle traction lithium battery, with more heater run time, then the continuous duty rule applies.

The heavier wire has a lower resistance per foot (per meter, per mile or whatever) so the I squared R will be smaller compared with thinner wire of the same length and therefore the amount of heat given off per foot would be lower.

High voltages have their own disadvantages, notably insulation requirements to prevent jumping through air to another conductor, producing a short circuit, but that is another program for a future discussion.
 

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Discussion Starter · #5 ·
Also note that most car batteries have a capacity around 50 Ah, so if you run 25A it will be dead flat in 2 hours.
My hut is tiny and heavily insulated, so I will only have to run the heater for 2 minutes to get it up to temperature.

And then for the other 23 hours of the day, the battery will be trickle-charging from a solar panel or wind turbine.

I'll go buy the 37-Amp wire tomorrow, as well as junction boxes rated for 30 amps.
 

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300W isn’t a lot of heat. It will work in the confined space of a truck cab, but as you increase the cubic feet (or meters), you need more wattage.

Most small space heaters for household or office use in the USA are 1,000w, or 1500W
so I doubt that 2 minutes of running the 300W will heat your space adequately.

You don’t want a car battery which is a starting battery. What you should be thinking about is a deep cycle battery. While it might look like a “car battery” it is designed to withstand a much greater depth of discharge than a starting battery.

If the battery is listing “cranking amps” and not amp hours, you probably don’t want it.
 

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Discussion Starter · #8 ·
300W isn’t a lot of heat. It will work in the confined space of a truck cab, but as you increase the cubic feet (or meters), you need more wattage.

The room I'm heating is 2.5 ft x 3.5 ft x 5.25 ft. It's tiny -- more like a wardrobe than a room.

Most small space heaters for household or office use in the USA are 1,000w, or 1500W
so I doubt that 2 minutes of running the 300W will heat your space adequately.
Here in Ireland we typically have 2 kW electric heater at home.

You don’t want a car battery which is a starting battery. What you should be thinking about is a deep cycle battery. While it might look like a “car battery” it is designed to withstand a much greater depth of discharge than a starting battery.

If the battery is listing “cranking amps” and not amp hours, you probably don’t want it.
Could you please provide me with a link to a suitable battery?
 

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At a minimum you need to look at a Deep Cycle Marine Battery.
Those have been designed to be run down a bit and go through some charge discharge cycles that will be slower due to solar power and wind turbines in the case of sailboats.
Ordinary car batteries are just designed to give a big startup jolt, and then get instantly charged back up.

I think it will be important to understand how may square feet or cubic feet you are trying to heat to what temperature. 300W is not a lot of heating power, but it draws a lot. Any space hear I have used is 750 low / 1500 high, whether forced hot air, oil radiator or tubes. Granted the UK probably doesn't go into a deep freeze, but still 300W is not much.
 

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Discussion Starter · #10 ·
At a minimum you need to look at a Deep Cycle Marine Battery.
Those have been designed to be run down a bit and go through some charge discharge cycles that will be slower due to solar power and wind turbines in the case of sailboats.
Ordinary car batteries are just designed to give a big startup jolt, and then get instantly charged back up.

I think it will be important to understand how may square feet or cubic feet you are trying to heat to what temperature. 300W is not a lot of heating power, but it draws a lot. Any space hear I have used is 750 low / 1500 high, whether forced hot air, oil radiator or tubes. Granted the UK probably doesn't go into a deep freeze, but still 300W is not much.

It seems that those batteries are called 'leisure batteries' here in the UK. I can pick up this bad boy from my local supplier:

 

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The room I'm heating is 2.5 ft x 3.5 ft x 5.25 ft. It's tiny -- more like a wardrobe than a room.



Here in Ireland we typically have 2 kW electric heater at home.



Could you please provide me with a link to a suitable battery?
You MUST use a deep cycle battery for this. A car start battery will NOT work.

I have no idea what you are heating, maybe a children's coffin? In any case, you will want a reasonable size battery. Probably 200ah, and do not discharge beyond 50% DOD if you want it to last. That brings you to 100ah. You also are advised to discharge at approx .1C or 10% of the ah rating for best performance. That is 20A on a 200ah batt, so that would be within reason.
 

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The wire I have for my hut is supposed to be used for wiring the lights in a house -- it's rated for 6 amps.
Yeah, you need 25A for that heater.
Yeah, 6A wire is too small.
You don't need 2 pages of math to tell you that LOL. Getting lost in the theory will only impede your project.

Watt's Law is more interesting. Watts = Volts x Amps.
What happens for a given load when you divide volts by 20? It means amps multiplies by 20.
How much fatter does the copper need to be to carry 20 times the current? Say it fast, don't think about it.
Correct.


Instead I think I need to buy this other wire rated for 37 amps:
Yeah, 2.5mm2 will be a little too small, so I agree.

But I don't think you need twin AND EARTH for a 12 volt DC circuit. Of course economies of scale being what they are, it may be the cheapest option.

Also, you need to think about distance to be traveled, since if you're going more than about 6 feet, voltage drop will become a factor of concern. Because voltage drop is 20 times worse when you drop the voltage by 20 times, even if you enlarge the copper 20x to compensate.

On the other hand, if the wire is inside the space to be heated, it's six of one/half dozen of the other whether you make the heat in the wires or at the heater.

The room I'm heating is 2.5 ft x 3.5 ft x 5.25 ft. It's tiny -- more like a wardrobe than a room.
It will have to be fantastically well insulated.

So I should hook up a Raspberry Pi to the battery to make sure it never goes down below 50% charge.
I would aim closer to 70% if you want that battery to last more than 100 cycles. Lead-acid batteries really do not like deep discharge.

When you consider these serious deficiencies of lead-acid, you are often better off building lithium packs from surplus lithium batteries, where you can get 80% of the nameplate range out of them.



300W of heat is about 1000 BTU (that's British Thermal Unit, there's no bacon in it). A BTU will raise the temperature one degree per 55 cu ft of space. Add that to your calculations.
300W ~= 1000 BTUs per hour.
300 watt-hours ~= 1000 BTUs.
1 watt-hour = 3.41 BTUs.

So you can derive the BTU capacity of the battery.
If you use 30% of that 1140 Wh battery daily, that is 342 Wh or 1166 BTUs you'll be able to store.

1 BTU is the energy needed to heat 1 pound of water 1 degree F.
1 BTU is the energy needed to heat 250g=ml of water 1 degree C.


If you replaced that 56 pound battery with 56 pounds of water heated to 200F (near boiling), and let it cool to 60F, that would be 140 x 56 = 7840 BTUs stored in that water.

To store the 1166 BTUs the battery would store, you only need 8.3 pounds of water or exactly 1 gallon.

So why the heck are you fooling around with a battery?
 

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Discussion Starter · #16 ·

If you replaced that 56 pound battery with 56 pounds of water heated to 200F (near boiling), and let it cool to 60F, that would be 140 x 56 = 7840 BTUs stored in that water.

To store the 1166 BTUs the battery would store, you only need 8.3 pounds of water or exactly 1 gallon.

So why the heck are you fooling around with a battery?

It isn't clear what you're suggesting here.

Should I boil a gallon of water in a big pot on the stove in my house, and then bring the big pot outside and leave it in my hut to heat it up? Is that what you're saying?

I'd prefer to use a liquid other than water with a much higher boiling point so that there would be much less evaporation. If I use water then I'll get condensation on the inner wall.
 

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It isn't clear what you're suggesting here.
That using battery power to run resistance heating is problematic.

That pound for pound of storage medium, direct thermal storage is far more efficient than storing electricity in a battery.

Should I boil a gallon of water in a big pot on the stove in my house, and then bring the big pot outside and leave it in my hut to heat it up? Is that what you're saying?
I'm proposing using water or antifreeze as a "thermal battery" instead of using an electrical battery to make heat.

If that particular implementation plan feels most practical for you, then go for it.

Crunch the numbers and see if that makes sense for your BTU requirements.

Really you have been approaching this backwards, you started at a solution method (battery run electric heater) and then tried to figure out how to get the needed BTUs out of it. Instead of figuring at the outset the BTU requirement and then figuring how best to deliver it.


I'd prefer to use a liquid other than water with a much higher boiling point so that there would be much less evaporation. If I use water then I'll get condensation on the inner wall.
Well I leave the implementation details up to you. Just watch out that different materials have dramatically different thermal storage capacity, and water is the best material known.

If you're wondering why, and also how The One Ring can have unearthly density yet very poor heat retention... it's because mass (protons and neutrons) don't store temperature - atoms do. The thermal storage capacity of a material very coarsely corresponds to the number of atoms in it.
 

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Discussion Starter · #19 ·
Actually here's what I think I'll do:

Rectangle Font Slope Parallel Diagram


My hut will have a 12V marine battery in it, as well as a solar panel on the roof to trickle-charge it.

My hut is mobile, it has 4 wheels -- but when it's out my back garden, I'll just run an extension lead out to it and hook up the mains as illustrated in the above diagram.
 
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