Induction Cooking (the kind that uses magnets)

[akrvbob]

Rather than do all the math for you, let me just say that my 1200 watt microwave draws 2 amps per minute. So a 5 minute burn only draws 10 amps from the batteries.

That's why an induction stove and a microwave can work, they are very high draw but very fast and when you break it down by the minute, it's very reasonable.
Bob

 

[ccbreder]

two amps sounds kind-a low. My 700 watt microwave draws 10-11 amps on 120 volt. Using an inverter from my battery would have to drain >100 amps at 12 volts. But, right, that is only for a few minutes. If I heat my sock for 3 minutes, it would be a bit more than 5 amp/hour from the battery.

 

[akrvbob]

two amps sounds kind-a low. My 700 watt microwave draws 10-11 amps on 120 volt. Using an inverter from my battery would have to drain >100 amps at 12 volts. But, right, that is only for a few minutes. If I heat my sock for 3 minutes, it would be a bit more than 5 amp/hour from the battery.

 My microwave uses about 100 ah at 12 volt and I'm giving it an extra 20% for inverter inefficiency and Peukert affect so 120 ah. 

If it draws 120 amps per hour, divided by 60 minutes, that's 2 amps per minute.
Bob

 

[ascii_man]

 My microwave uses about 100 ah at 12 volt and I'm giving it an extra 20% for inverter inefficiency and Peukert affect so 120 ah. 

If it draws 120 amps per hour, divided by 60 minutes, that's 2 amps per minute.
Bob

Current is measured in Amperes (not "amps per hour")
Energy can be measured in watts (W) or ampere-hours (A*h)

A 1200 W microwave: 1200 W / 12 V = 100 A

It draws 100 amps. Which will deplete a 100 Ah battery in one hour.

This can also be expressed as: 100 A * 1 min * (1 h / 60 min) = 1.66 Ah/minute, but that's unnecessarily complex.

Operating a 100 A device for 5 minutes uses 100 * 5 min * (1 h / 60 min) = 8.33 Ah.

Since 100 A (Amps! Not Amp*hours!) is a lot of current, Peukert's law will be noticeable, similar to how your "100 MPG*h gas tank" (20 mpg * 5 hours) will give you 5 hours at 50 mph but substantially definitely not 2.5 hours at 100 mph because your MPGs at 100 mph are less than if you were going the 50 mph rated speed.

The ampere should not be confused with the coulomb (also called "ampere-second") or the ampere-hour (A⋅h). The ampere is a unit of current, the amount of charge transiting per unit time, and the coulomb is a unit of charge. When SI units are used, constant, instantaneous and average current are expressed in amperes (as in "the charging current is 1.2 A") and the charge accumulated, or passed through a circuit over a period of time is expressed in coulombs (as in "the battery charge is 30000 C"). The relation of the ampere to the coulomb is the same as that of the watt to the joule, and that of metre per second to metre.

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[ascii_man]

I think I realize why people get confused about current.

We speak of speed as "miles per hour" or "kph" or "meters per second". Current is the rate at which charge is transferred, and an ampere is one coulomb per second. So just remember that an Amp has the "per second" hidden inside it, kind of like when someone describes his speed as "fifty clicks". When you hear Amp hours, that's current times time.

 

[GotSmart]

I think I realize why people get confused about current.

We speak of speed as "miles per hour" or "kph" or "meters per second".  Current is the rate at which charge is transferred, and an ampere is one coulomb per second.  So just remember that an Amp has the "per second" hidden inside it, kind of like when someone describes his speed as "fifty clicks".  When you hear Amp hours, that's current times time.

I understood everything up to this. 

I thought it was amp minute?   AM / PART (%) HOUR

But then you are not supposed to drain 100% of your battery

 

[akrvbob]

A 1200 W microwave: 1200 W / 12 V = 100 A

It draws 100 amps.  Which will deplete a 100 Ah battery in one hour.

This can also be expressed as: 100 A * 1 min * (1 h / 60 min) = 1.66 Ah/minute, but that's unnecessarily complex.

Operating a 100 A device for 5 minutes uses 100 * 5 min * (1 h / 60 min) = 8.33 Ah.

Since 100 A (Amps!  Not Amp*hours!) is a lot of current, Peukert's law will be noticeable, similar to how your

I have no idea what your point is.

You reach the same numbers I reached except I added in a guess for inverter inefficiency and Peukert affect.  I think failing to include them in real world usage is a mistake.  But that's up to you to decide for yourself.

Forgive me for not using the exact right terminology you like. What I said is still correct.
Bob

 

[ccbreder]

We are just picking on your terminology, Bob. Over 100 amps are flowing from the battery, and the wires better be up to the job. The overall drain from the battery is the amps flowing by the time being used. amp hours. Maybe 2 or 5 amp hours from the battery storage, but still 100 amps per minute while operating.

 

[GotSmart]

Bob, not everyone processes information the same way,  Having things put two different ways, (Yours and ASCII_MAN ) gave me the understanding on how to work the formula's in my head. 

Thank you both!

 

[ascii_man]

I have no idea what your point is.

You reach the same numbers I reached except I added in a guess for inverter inefficiency and Peukert affect.  I think failing to include them in real world usage is a mistake.  But that's up to you to decide for yourself.

Forgive me for not using the exact right terminology you like. What I said is still correct.
Bob

Yes, you should include inverter inefficiency and Peukert effect.

Units are essential when working a science or engineering problem. When the units work out, it's a good indication that you probably set up the problem correctly and did the algebra correctly. It also demonstrates an understanding the concepts (e.g., the relationships between force and mass, power and energy, time, speed, and distance, current and voltage, etc.).

I couldn't tell whether your numbers were close due to rounding or just coincidence.

A layperson would never use "amps per minute" or "amps per hour": those would be for a rate of change of current, i.e., how fast current was rising or falling.

I responded not to be rude, but because I have observed other capable members of this forum get very confused and struggle with basic calculations like this, and a big source of the confusion is the use of incorrect units like "amps per minute".

It's not a pedantic personal preference. Famous unit conversion disasters include:
2004: Tokyo Disneyland: Derailment due to broken axle. Miraculously, it happened at the very end of the ride, so all passengers were safe.
http://www.system-safety.org/ejss/past/current/pdf/main1.pdf

1999: Mars: NASA's Mars Climate orbiter ($125 million) disintegrates after its 296 day journey:: Lockheed sent thruster data in pounds, while NASA expected newtons, causing the spacecraft to enter the Martian atmoshphere
http://spacemath.gsfc.nasa.gov/weekly/6Page53.pdf

1984: Spinal Tap: Stonehenge

1983: Air Canada 143
(Air Crash Investigations dramatization):

 

[ascii_man]

I understood everything up to this. 

I thought it was amp minute?   AM / PART (%) HOUR

But then you are not supposed to drain 100% of your battery

Just remember that an ampere (A) is one coulomb (C) of charge per second. Rates are always "stuff per time", and the per means "divided by". So we say "miles per hour", "feet per minute", or "meters per second". Current is a rate, but the "per time" is the "per second" baked right in to the definition:
1 A = 1 C/s
So don't add another "per time"

If you do hear a time after ampere, then it is ampere-hour, meaning the current multiplied by time: "It draws 1 amp and I ran it for 10 hours, so my battery has 10 A*h less in it".

 

[ascii_man]

We are just picking on your terminology, Bob. Over 100 amps are flowing from the battery, and the wires better be up to the job. The overall drain from the battery is the amps flowing by the time being used. amp hours. Maybe 2 or 5 amp hours from the battery storage,

so far so good but...

but still 100 amps per minute while operating.

Fumbled at the goal line. Just 100 amps is good.

The point of all this is that we need to be both concerned with energy and power (how fast the energy is used). High power for a minute or two doesn't use much energy, except that when running off batteries we care about Peukert. And batteries or not, whether the wires can safely handle that much current does matter: a lot of houses only have 100 amp service. Which is a lot more useful at 120 V than at 12 V, but the wires don't care about voltage, just how many electrons are flowing through them per second.

A lot of electrons make a coulomb a charge, and a coulomb per second is...one ampere.

 

[akrvbob]

I guess i have a different goal which is help people and communicate. Letting them see that the extreme short time of power use makes it practical to use a microwave off solar.

It really does use 2 amps per minute when it's on and that is so simple and clear that I will continue to say it that way and I will not give anybody a physics lesson on top of it. If the question is "How much power does your microwave use?" the answer is--2 amps per minute.

I've said many time that to use a microwave I recommend 400 watts of solar, 400 ah of battery, a 2000 watt pure sine inverter and I use 2/0 (two-ought) cables with it.

If the question comes up here how to run an induction cooker, I will say it again. But so far we are only talking about is it possible--and it is.
Bob

 

[ascii_man]

How much power? 1200 W
How much current? 100 A
How much charge / how much battery capacity? about 2 Ah/minute (that's amp-hours per minute)

 

[akrvbob]

How much power? 1200 W
How much current? 100 A
How much charge / how much battery capacity? about 2 Ah/minute (that's amp-hours per minute)

The one thing we hear repeated constantly on any thread about solar or electricity is people saying they just can't get it. That it's all gibberish to them. And this is a perfect example of why. 

They just want to know if they can use a microwave and they get a physics lesson instead. 

I look at it as I will give them basic information they can use and apply and if they want to know more and build on it then I'm glad to go over how I arrived at that by adding a little bit at a time. I've found that if you can keep it simple enough, they can take it in a little bit at a time and start to build a solid foundation of 12 volt knowledge

I attribute the majority of this site's success to my willingness to make things very simple and not overwhelming. But at the same time I tremendously appreciate members like Sternwake who take those of us who are ready to an even higher level of knowledge, I've learned a lot from him and will be forever grateful.

I felt like this thread needed a very simple explanation and if there were any further questions we could advance. I still think that's right.
Bob

 

[ascii_man]

...
They just want to know if they can use a microwave and they get a physics lesson instead. 
...
I've found that if you can keep it simple enough, they can take it in a little bit at a time and start to build a solid foundation of 12 volt knowledge
...

Without the incorrect units, there would have been no "physics lesson".

I agree with building a foundation out of small pieces. But I think it's better if each of those bite-sized pieces are mostly correct, rather than each sowing a little confusion.  Given that the third paragraph of the Wikipedia entry on the ampere begins "The ampere should not be confused with the coulomb (also called "ampere-second") or the ampere-hour (A⋅h).", how do learners benefit from perpetuating this common mistake by saying "amps" when you mean "amp-hours"?  Why not just say "amp-hours"?

Perhaps I'm wrong.  And perhaps it's better for an elementary-school teacher to consistently use incorrect grammar all day, because a lot of people find English difficult and confusing, so immersing the children in bad grammar is somehow better.  And learning to play sports is hard, so we should teach kids a cross-handed grip in baseball and to carry a football 6 inches away from their body with one hand.

 

[ascii_man]

Getting back on topic, we really want to know how much of our battery (assume 400 Ah) gets used up by running our induction stoves for 10 minutes.

Bob: 2 A/min * 10 min = 20 A
ccbreder: 100 A/min * 10 min = 1000 A
ascii_man (method 1): 100 A * (10/60) h = 17 Ah
ascii_man (method 2): 2 Ah/min * 10 min = 20 Ah

The first two answers are really confusing to people; they'll spend the next hour trying to figure out how "20 amps" relates to their 400 Ah battery. The last two, by using correct units, give a result that allows clear, apples-to-apples comparison.

 

[GotSmart]

Getting back on topic, we really want to know how much of our battery (assume 400 Ah) gets used up by running our induction stoves for 10 minutes.

Bob: 2 A/min * 10 min = 20 A
ccbreder: 100 A/min * 10 min = 1000 A
ascii_man (method 1): 100 A * (10/60) h = 17 Ah
ascii_man (method 2): 2 Ah/min * 10 min = 20 Ah

The first two answers are really confusing to people; they'll spend the next hour trying to figure out how "20 amps" relates to their 400 Ah battery.  The last two, by using correct units, give a result that allows clear, apples-to-apples comparison.

It is my humble opinion that there are people out here that do not have minds capable of grasping higher electrical theory. I know I have been trying for many years.  

A "close enough" formula such as Bob put out is understandable.  The 20 A obviously gets subtracted from the 400 AH battery.  (Which in reality can not be drained of 400 A in an hour, as that would kill it) 

When you start inserting the amp second in there, things get excessively confusing.  (Whats with them kumbia's Bubba?)  :huh:

We're not rocket scientist building a Mars Explorer, just people wanting to cook dinner. 

So a 1,200 Amp device draws 2 Amps a minute from the battery bank. (Or something close to that)  Ideally I would need a 2,000 (Amp?) inverter wired directly (With a fuse) to the power source.    Is that right?

 

[ccbreder]

asscii man; you are making things far too complicated for someone that just wants to heat the beans. You should not quote me out of context to make your point. My explanation was only to insure that you understand, each minute your microwave is running, 100 amps are flowing through your wire going to your inverter. It is still called amps, and it is how we size the cables and fuses. Like Bob pointed out, you need 2/0 cable from your battery. I don't care about all the theoretic details, I want the right size wires, the right size battery, and the right size charger, if I use my microwave. I need about 2000 watt inverter, that will draw about 100 amps when operating, that is not 2 amps per minute, that is 100 amps every second and every minute it is operating. Then we can find a sufficient battery to supply that for the 2 or 5 minutes the microwave is heating 4 or 5 times during the course of my day. That and recharging is a different thread.

 

[highdesertranger]

I am sorry I ever asked. lol. highdesertranger