Small microwave in Dodge caravan. Can I use an inverter?

[rlphoto]

Hi Guys, Almost spring and camping season! Anyway I want to power a small micro wave in the dodge caravan tow vehicle. (because that is where we keep the food) And I want a microwave to heat meals with. We can usually heat stuff to eat with out messing with cook pans and stuff. Now what size inverter do I need to buy, I realize I must hook it direct to the battery. I have a small microwave already it is about 950 watts. I am not sure of my alternator wattage, Any pointers on doing this?

Thanks!

Randy

[Arne]

if the inverter and wiring is big enough, I don't see why not... small microwaves are something like 600 watts?

And the time in use should not be a big deal... it will be a BIG draw on the battery, around 50 amps, so wiring is a big deal... and don't try to do a roast... or you might roast your battery.... for 5-10 minutes max, I'd go for it.

[brian_bp]

The advertised power of microwave ovens is the output power - the rate it's actually delivering energy into the food. The input power requirement is typically about 50% higher, so that 950 W microwave probably uses at least 1400 watts of electricity to make 950 watts of microwaves. It's an efficient way to cook, but it's also an extreme electrical requirement.

That's one honking pile of current (e.g. 120 amps at 12 volts is 1440 watts to run the 950 watt oven), so the alternator is not going to keep up... the battery will make up the difference, then the alternator has to charge it back up, just like running the van's starter motor (but not quite as extreme as the starter).

[rlphoto]

Thanks for all the help, the full load wattage is 950, the cooking wattage is about 600. Thanks for clarifying that anyway. So it will not be as bad.

If I have a 160 amp alt, it will give me 1920 watts. If I have the 136, it will give me 1632. That might be at a certain RPM? not idle? anybody know this?

I can get a good quality 1500 watt inverter for less than $200. Got to get Some heavy battery cable also. Haven't figured out where to mount it yet. Maybe just mount it under the hood and run an extension cord.

Thoughts??

*****UPDATE*****

Alternator is 140 amp at 1680 watts...... at what RPM though?

[asianflava]

I have a 1200W (2400W peak) inverter and it barely runs my 900W microwave.

[rlphoto]

I have a 1200W (2400W peak) inverter and it barely runs my 900W microwave.

Do you run at idle? How long do you run your microwave? What is your alternator size?

Just trying to get an idea about my situation.

[brian_bp]

If I have a 160 amp alt...
That might be at a certain RPM? not idle?

Alternator output is usually directly dependent on speed up to some point, then you don't get much more current by running faster. Since modern automotive cruising engine speed is routinely 2000 rpm or lower, the alternator needs to be well up in its output by that point (if the alternator runs at the same speed as the engine - it can go faster or slower by changing pulley size); I have noticed diagnostic instructions which call for testing the output at 2000 rpm. The examples I've seen are at basically full rated output by 3000 rpm, and may not produce any more at higher speeds. At idle, I would expect only a small fraction of peak output.

It's an almost random example, but there is an output curve for some alternators sold for welding applications on the Zena web site.

[rlphoto]

If I have a 160 amp alt...
That might be at a certain RPM? not idle?

At idle, I would expect only a small fraction of peak output.

Good info, I appreciate it. Anyway since the van is not here right now I am guessing from memory that the alt pulley is considerably smaller than the crank pulley. Maybe 2 to 1 or 3 to 1? The alt could be possibly be running at 2000 rpm at idle right? Maybe even enough for %50 output? you figure the alt has to stay above the headlights at idle plus what ever else is normally running.

That graph gives a good idea anyway.

[xrover]

I recommend mounting under the hood. I had one in the passenger area and I always worried about someone or something arcing the terminals. And you have to run battery cables to it as well to carry the load.

[Arne]

If you draw 50 amps with wires 15' long, you need to use #6 wire..... to get high amps with low volts, you need big wire....

http://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html

[wlooper89]

Though not an electrical expert, I know a simple formula that may help with planning. It is Amps = Watts/Volts (watts divided by volts). Essentially a microwave using 800 watts on 120V AC will draw 6 1/3 amps. From an inverter on a 12V battery it will draw ten as many amps or about 63A, plus any normal inefficiency in the inverter circuits.

As others have said, a battery can take that for a short time and using heavy wire to the inverter. We have a microwave oven in the trailer galley. I looked at the numbers and decided to just use a Coleman propane stove unless there is shore power, which we have at most of the campgrounds we visit.

But if you really want to run a microwave oven from the tow vehicle battery and alternator, I would say go for it! Maybe just make sure there is enough juice left to start the TV engine.

Bill

[wannabefree]

Not to poop on the parade, but...

Typical auto alternator produces 65A.
You need about 5A just to keep the car running. Down to 60, or 720W.
Your micro is 600W (could be much more, 900 is typical these days).
It wastes 300W due to inefficiency. Others already covered this turf.

Your inverter also wastes power. Figure 50% of what you are using gets converted to heat (but not where you want it) .

So 900W + 450W, you need 1350W.

Where does all that power come from? If the alternator can't deliver, you get it from the battery.

Alternator delivers 720W, battery delivers 630W, everybody's happy.

Now a good battery in peak condition will crank a car about 5 minutes. It is delivering maybe 40A to do that. OK, 630W is 52.5A, or a bit more than cranking a car. So maybe figure 4 minutes of microwaving before you won't be able to start the car again.

All this is just guesstimating, and YMMV. And you could run the car for half an hour after nuking that popcorn to put a bit of charge back in the battery. And you could get a heavy duty alternator (delivers about 95A) for several hundred bucks and sneak up on that power deficit a bit. Or you could spend $200 on ebay and get one of those cheapo 2 cycle gennys, and not spend it on an inverter.

I'd really urge you to consider a generator. Cost wise it will be a wash, and the genny will never leave you stranded, but it does add weight.

[asianflava]

I have a 1200W (2400W peak) inverter and it barely runs my 900W microwave.

Do you run at idle? How long do you run your microwave? What is your alternator size?

Just trying to get an idea about my situation.

No engine, this is straight off the trailer battery with 4ga wires connecting them. After a few seconds, the inverter will go into protection and you have to reset it. It's no big deal, I'll just have to be plugged in to use the nuker.

[brian_bp]

Your inverter also wastes power. Figure 50% of what you are using gets converted to heat (but not where you want it) .

There certainly is loss in the inverter, but I don't think they're anywhere near this bad.

[Alphacarina]

Most modern inverters have efficiencies in the mid 90's - 95% is not uncommon. You'll need about a 1200 watt inverter

Few inverters are designed to be mounted 'in the weather' so I would be very careful about mounting one under the hood - If there's any chance it could get wet doing 60 miles an hour in a driving rainstorm, then it's not going to last you very long

Key to good inverter performance, especially under the heavy loads a microwave will put on it, is the battery in the system - You can't run an inverter just off an automotive alternator because the regulator in the alternator is designed to track the battery voltage . . . . and the 65 to 70 amp load the inverter is going to place on the battery is probably too much for the little starting battery in your car. For what you desire to do, I would mount a larger than normal, heavy duty deep cycle battery (maybe a Group 27) in place of the starting battery in your tow vehicle and then run 6 gauge wire from the battery to the inverter, which should be mounted someplace where you can ensure it stays 100% dry

Other than that, I don't see a problem with what you want to do

Don