Running a microwave on a car battery with the engine running - will the alternator blow up?

Thread Starter

LMF5000

Joined Oct 25, 2017
101
I'm pondering backup power for my apartment and am considering buying a pure-sine inverter large enough to run a microwave (it being the most useful heating appliance). My microwave uses 1,700W (measured at the plug) so I'm guessing an inverter with 2000W continuous rating is required.

Battery-wise I don't intend to haul around the kind of batteries needed to reliably run this much power. I was thinking of using my car. The problem is 1700W @ 12V = 141A. I'm fairly certain the battery can take it (a microwave only runs 30 seconds at a time and starting current in normal car use is probably 200-400A).

The question is, if I keep the car running to take some load off the battery, and assuming an alternator rated say 80-120A, will the alternator's output voltage dip safely to the level where it only delivers up to its current limit, or will it stupidly try to maintain normal charge voltage (14-14.5V) and burn itself out trying to power the inverter by itself?
 

Sensacell

Joined Jun 19, 2012
2,908
Its gonna take more than 140 Amps.
The inverter is not 100% efficient.

Ever try running your strarter for 30 seconds? This will not end well. I think it would barely work with 4 batteries parallel.
 

DickCappels

Joined Aug 21, 2008
7,906
How about marine deep-discharge batteries that are recharged with an appropriate AC line charger? (I like BobTPH's suggestion as being more practical.)
 

crutschow

Joined Mar 14, 2008
27,960
How do you intend to connect between the car battery and the microwave in your apartment?

A small backup generator would be more practical and likely wouldn't cost much more than that large inverter.
 

Janis59

Joined Aug 21, 2017
1,355
Assume that inverter have 85% effectivity. Thus 1700 W demand 2000W in the input. Thus for 12V it makes 2000/12=167 Amperes. From that (in the best case, if You have largest of large alternators) 85 Amps are given by alternator and 82 Amps are taken from battery. Thus, assumingTou have an 85 A*h larger than average battery, You have left 1 hour of work. In the case of average small 45 Amp alternator and 55 A*h battery, Your time budget is multiple times shorter.
 

MrChips

Joined Oct 2, 2009
24,458
During a power outage when one has to rely on emergency backup power one needs to go into energy efficiency mode.
If one has to cook food in an emergency then using a microwave oven to do so would be off the list of options. A stove fueled with wood, charcoal briquettes, camping fuel, kerosene or propane gas would be at the top of the list.
 

dl324

Joined Mar 30, 2015
13,328
I'm pondering backup power for my apartment and am considering buying a pure-sine inverter large enough to run a microwave
We lose power several times a year, usually during the winter, and we're okay with eating sandwiches and things that don't require heating. If we're without power for more than a day or so, we'll use our fireplace, wood stove, charcoal BBQ, firepit, or propane burners.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
101
How do you intend to connect between the car battery and the microwave in your apartment?

A small backup generator would be more practical and likely wouldn't cost much more than that large inverter.
Hey Crutschow! I'm glad you asked. From our other interactions on this forum and your helpful replies to my past questions, I recall you're somewhat fond of safety and best-practices, so you're going to love the idea I pondered as part of this system. [Before reading further make sure you're lying down and have an alcoholic beverage handy].

Our garage under the apartment complex is powered by a cable coming down from the main CB panel in the apartment. There's a sub CB panel in the garage with am RCD and some other protections, and the output feeds the plugs and lights in the garage (reminder: I'm from Malta so building codes are different to what you're used to).

So, my idea was to have the inverter connected across the car battery, and a suicide cord (wire with male mains plug at both ends) connecting the inverter output to a plug in the garage. Current then flows backwards through the garage sub-CB, up the cable and into the main apartment CB, from where it gets distributed to all the plugs and bulbs in the apartment. Obviously all input switches will be off (there's a switch built into the electricity meter, and a second sub-CB in the apartment between the meter and the main CB, so that's at least two layers of switches to prevent islanding and harming a utility worker, or feeding mains AC into the inverter input).

I know, I know, bad idea. I wonder if am RCD will get harmed by passing current in the "opposite" direction to what it expects? (since in this case load and source get switched)

Anyway, I've abandoned the idea after reading this thread, but I will make a second post for all the answerers in general to explain the background and my reasoning.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
101
Thanks all for the lively discussion. Let me provide a bit of background. The reason I'm asking is because the 500W pure sine inverter I'm using as backup power for my home office (powered by a 12V 36Ah deep cycle battery) died yesterday. First the digital display said "internal data error" but it continued giving normal 230V output normally for a few hours, then after I turned it off and on again it stopped giving output altogether and now just displays the error and switches on the fault light. Nothing obviously burned on the inside. The brand is Carutu if anyone has any insights.

Anyway since I now need a new backup inverter, I was wondering whether it would make sense to get a much bigger one to be able to run more things around the house, and the only realistically useful thing I thought of was the microwave since it only needs 30-second bursts of cooking. That lead to the question of what to use to power it. Doing it with batteries would need more size, weight and cost than a cheap generator so it's a no-go. So I thought of the car, using a combination of alternator and starter battery - hence the question.

In hindsight after reading your posts I have decided against the idea. The hob in the kitchen runs on a gas tank, and the BBQ (aka grill) in the garden also runs on gas, so these will be the primary cooking methods during power cuts.

I've considered a generator but aside from the cost and ongoing maintenance it will take up a lot of space and the noise will bother the neighbors. I primarily use the inverter to power the internet modem, laptop, lights and fans so I can keep working from home during power cuts and for this my current setup works well, giving me about 8 hours of runtime (based on real-world testing).

Now I'm just wondering what size of inverter to get. 500W is decent and leaves a lot of extra headroom but a 300W would save some space and cost. Then there's the 2000W that would add capability if I could find a battery to handle it...
 
Last edited:

nsaspook

Joined Aug 27, 2009
8,788
Lots of electrical loads have higher power ON loads than steady state so 500W or more would be better. My 2KW model has low idle current so it's great for instant standby power.
 
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strantor

Joined Oct 3, 2010
5,677
You can get a high output alternator for your car. Probably depends what car you have but just Google "high output alternator for ___your car___." I put a 320A alternator in my last truck.

Know that an alternator will not put out its rated amps at idle. I think they redline the engine to get the advertised number. My "320A" alternator think couldn't even do 100A at idle, so a typical factory 60A alternator might not deliver more than 20-30A at idle, not sure, never tested it. Google "output curve" for any particular alternator you might be interested in.
 
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