Calculation Help

Would anyone be interested in helping me with a calculation?
If I have a 750 watt draw at 120v (it's a heating element) and I want to run
that heating element off a 12v battery using an inverter, how many amp hours would I be drawing from the battery if I ran that element for about 4-6 hours? It would be turning off/on once the box reaches temperature.. only enough to keep the temperature somewhat constant.
It'd be nice if the formula was provided.. assuming it's not a very complicated one.
Thanks,
--
Bill




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Baldy wrote:

The starting point is that a 750w load at 12v is 62A. If your inverter is running at 90% efficiency, that goes to around 70A.
So, worst case, you need batteries that can supply 70A for 6 hours, ie 450AH plus. That assumes that the heater has to be on almost all the time to maintain temperature.
However, if the box is well insulated and the constant temperature is only slightly above ambient, the heater may only be on for a minute or so at first, and then maybe only a few minutes an hour. The limiting factor is then more likely to be the battery's capability of supplying 70A, not so much its AH rating.
So the answer is going to be something between tiny and huge - dependent on the heat being lost from your box.
There are equations that can work out what the heat loss will be, based on the materials used, physical dimentsions, ambient and maintained temperature, etc. But it may be a lot easier to run it, on its own, off a mains supply for a few hours and read the difference in the electricity meter - or use a domestic power consumption meter.
--
Sue







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Thanks Sue, I guess I was wrongly under the impression that when you converted 12DC to 120VAC the draw would not be very high. The item in question is an electric smoker that we'd like to use while camping where no AC is available. The box is very well insulated and there is very little fresh air moving thru it.

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Baldy wrote:

If you just stuck 12v onto the 120v heater, then the draw indeed wouldn't be very high (but whatever you were trying to cook would be, after a day or so..).
But if the load is taking 750W, all those watts have to come from the battery, even if there is an inverter in the middle.
I only have a vague idea what a "smoker" is - we don't have such things in the UK, AFAIK. I assume it heats the contents to about 80 degrees centigrade, as the cooking time seems quite long.
So it isn't a great deal hotter than ambient, so you won't be losing heat that quickly. Let's take a wild stab in the dark, and say it takes 20 minutes to stabilise and then loses 70W on average, thereafter.
That's (at the battery) 70A for 20 mins. Then 7A for say 6 hours. Or 21AH to get hot then 42 AH to keep it hot. So the sort of battery you would be looking at would be around 60AH. You need to make sure though that it is a "recreational" type meant for driving inverters and stuff and not meant as a car battery. Or it won't last long.
That's a lot of guesswork, though! But it should give you a starting point.
Me, I'd be tempted to stick in 700W of 12 volt heaters and not lose the power in the inverter..
--
Sue



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Thanks again. A smoker is a device that was originally used to preserve meats or fish when there was no refrigeration.. However, here in the US it is also known as BBQ (Bar B Que). The meat is seasoned and placed in a controlled environment with a controlled amount of smoke to add flavor to the meat depending what kind of meat you have, the seasoning and they type of wood you burn.
I know they have them in the UK but it may not be a high demand item such as it is in the states. I would imagine salmon and some sausages as well as hams are smoked.
You are pretty close to spot on regarding the temperature. Typically, in my smoker, I set the thermostat at 225F to 250F (107 to 120C) The long slow cooking time referred to as BBQ, tenderizes tougher cuts of meat and renders most of the fat in the fatty cuts.
This smoker has about a 1000 insulation and doesn't lose much heat so the heating element is only on long enough to keep the box at temperature, even in cold/windy weather.
Thanks again for your help, Bill

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--

Don Kelly snipped-for-privacy@shawcross.ca
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run
hours
Intersted to know how critical the temperature variation over time is on the final product quality.
Do you use wood chips etc.. to flavour
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Intersted to know how critical the temperature variation over time is on the

Actually there is quite a bit of temp variation but the average is pretty close to the t'stat set. It will go about 30F + and about the same minus.
Don't use chips.. rather use chunks. My smoker can do a great job of smoking chicken, pork ribs, and pork shoulder (butt) with 2-3 oz of wood. This is, as opposed to log burners, very efficient. Log burners will use possibly 30-40 lbs of wood for the same cook.
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Baldy wrote:

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