The input watts of the microwave is a lot more than the 800 watts output.

Not sure what batteries you have but with AGMs you will often see the voltage drop below 12v but remember this is the terminal voltage.  Try setting it at 11.5v a see what happens.  When you turn it back off you will see the battery voltage climb again.

Also what size batteries do you have, what is the make of your inverter and what fusing do you have between the batteries and the inverter.  The reason why I ask regarding fusing is I had a resettable circuit breaker installed in my system that was causing the inverter to cut out.

Tim

I assume your inverter is fed with 12V?

Your 800W *output* microwave will require about 1500W of *input* power. Therefore 1500 / 12 = 125 amps.

125A is a considerable current to draw, for any length of time, from 12V batteries and you will need a properly designed and engineered system to handle it.

The other appliances you mention are all thermal an will have intermittent current draw as they cycle according to temperature - the microwave, set to 100%, will be a continuous draw and it may be this which is causing your batteries to struggle - if so then more batteries are required.

As I'm currently discussing in another thread in the "Technical" section I would never consider running such appliances from a 12V supply, 24V is the minimum and 48V is far better.

As a possible quick fix try setting the microwave to 50% power and running it for twice as long... this may work?

The data sheet for my batteries shows how long they last at different current. Drawing a very high current effectively reduces the AH of the battery.

As said about, try 50% for the microwave. It is still 100% draw but at a 50% duty cycle so the batteries may just cope.

How long is the wiring both + & - total length between batteries & inverter & what gauge. Might be worth doubling up the wire to reduce resistance. You could easily have 10% or worse going up in heat.

Add an additional battery. The more you can share the load to reduce peak load on each battery the better. I am sure you can find lots of BS payload with no value that you can chuck out in lieu of battery.p

I started with 2 batteries & now have 4. You don't have a system twice as good. But probably about 3 to 4 times better depending on how you use the system.

Gezza E wrote:

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Yes I have short heavy leads 2x100 ah Batteries

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There is a lot more to *engineering* such a system than a couple of short heavy leads,

200Ah is nowhere near enough capacity to run air conditioners, microwaves and such - at a guess something like 1000Ah of *quality* lead acid or 600Ah of *quality* lithium will be required *and* enough solar to charge them.

Read this: Peukert's Law

Your 100Ah batteries are *only* 100Ah when you are drawing 5A (C/20) from them - when you are drawing 62.5A (half of a microwave) they are probably more like a 50Ah battery

I'm currently running 210Ah of AGM and use a small generator for microwave and AC.

Mike Harding wrote:

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Read this: Peukert's Law

Your 100Ah batteries are *only* 100Ah when you are drawing 5A (C/20) from them - when you are drawing 62.5A (half of a microwave) they are probably more like a 50Ah

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 If drawing from the battery a lot more than C/20 on a regular basis you need more batteries. Firstly for the current situation so you have enough AH.

Also for the long term so the batteries last.

It is perfectly ok to draw the batteries down, let's say once a month, but doing it regularly really needs to be avoided.

If drawing less than C/20 on a regular basis as in our situation we have about 105% capacity for a new battery. But our batteries are about 4 years old so their capability is considerably reduced.

Swings & roundabouts!

Mmmmm, 800watt microwave? that's the output power not the input power which will be between 1.5 to 2.0 times the output power.

So taking worst case scenario of a sh!tty Chinese made inefficient microwave oven to be 2 times the output power we would be looking at supplying 1,600 watts at 240V AC.

Watts divided by Volts = Amps 1600 / 240 = 6.67 amps.

Now calculate the conversion from 12VDC to 240VAC is a multiplier of 20

Now factor in that most of the very high power inverters don't run at anything better than about 80% efficiency so divide by 0.80

so:

6.67 x 20 / 0.8 = 167 amps is the draw from your batteries.

Now, whilst most modern LiFePO4 can tolerate a maximum 1C current draw, old lead acid can't and they also suffer from terrible voltage drop at high drain levels which only exacerbates the current draw problem.

I would say your 3000W inverter is cutting out because the power supply is to it is not up to snuff.

For years and years I ran a 800W microwave purchased from Big W on my cheap Chinese 1500W inverter with a 120Ah battery.  Of course I had to be mindful of how much power I drew from the battery but my two 200W solar panels helped.  I must say that the battery was used and abused but still has great capacity although now it sits in my garage in retirement .