Microwave on Invertor

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pcatterall

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We have a 5kw pure sine wave inverter which happily runs our fridge when away from shore power. We do have a small ( 600w) microwave which we thought could be used to do some quick warm ups.( say 5minutes )
The microwave starts and runs fine but stops after a minute then starts and stops.
We have 2 X 80Ah leisure LA batteries Is the problem that the batteries have the capacity but not the 'umph' to run the microwave? The leads are all short and beefy.
 
2X80Ah and 5kW don't quite match tbh! How old are the batteries btw?
what does the inverter say when shutting down, any led indicators flashing low voltage?
would be easy to get a DC voltometer across the inverter beefy as you say cables and run the microwave again and see if the voltage dips - bet it does and that's your problem.

V.
 
Your microwave produces 600w of microwave energy, it consumes at least twice this from the batteries. The 600w is the output not the input.
I suspect the batteries have not enough oomph left to keep up.
Some microwaves are very fussy on the waveform of the supply and some pure sine wave inverters are not as pure as they should be.
 
agree with Boater Sam, but I think if the inverter is not quite right, device will not work fullstop.
Working for a min and stopping goes more to running out of juice
 
Like Boater Sam said, microwaves draw about twice the output rating. Your 600W microwave therefore will draw at least 1200W, more at startup, which at 12V means at least 100A, so 50A per battery. This is something most batteries can handle for a short while at least (like when cranking an engine). However, not very long, and you will also have a big voltage drop on the cables from batteries to inverter, especially if your cables are undersized. So first thing to do is have a look at the battery voltage as this happens. I expect the inverter is shutting down due to input voltage from the battery bank dropping below its threshold. Then the load is removed, the battery bank recovers a bit, the inverter resumes, only for the cycle to repeat.

Your battery bank is definitely on the small side for this job, and getting stressed a lot by using them that way. Upgrading cables (if too small) can help with reducing the voltage drop on the cables, but ultimately you would really want to upgrade the bank significantly, consider switching or adding Lithium or using gas for cooking ;-)

The 5kW inverter is definitely massively oversized for that battery bank, and having a too large one increases waste through losses (they're not 100% efficient).
 
vas said:
agree with Boater Sam, but I think if the inverter is not quite right, device will not work fullstop.
Working for a min and stopping goes more to running out of juice
Click to expand...
Inverter is pretty inefficient at generating high wattage output.

Batteries aren't very big (even if new) and "leisure batteries" aren't very good at chucking out intense power anyway.

Those factors combine to say you are probably asking too much of the batteries and inverter. It will help if you run the engine, so the batteries have assistance from the alternator to support the greedy inverter....but it may still be a struggle.

There's a massive difference between using inverter to charge a laptop, vs running microwaves, coffee machines, kettles and power tools. My experience of trashing a 3000w inverter by trying to run an induction hob at nominally 300w (supported by 3x150Ah AGM batteries) leads me to believe the expense just isn't worth it. Either you just have a very small inverter for small jobs, and use conveniences on shorepower only, or you have to fundamentally specify the entire system to meet those bigger needs.
 
pcatterall said:
We have a 5kw pure sine wave inverter which happily runs our fridge when away from shore power. We do have a small ( 600w) microwave which we thought could be used to do some quick warm ups.( say 5minutes )
The microwave starts and runs fine but stops after a minute then starts and stops.
We have 2 X 80Ah leisure LA batteries Is the problem that the batteries have the capacity but not the 'umph' to run the microwave? The leads are all short and beefy.
Click to expand...
Run your engine before operating the microwave and keep it running whilst it's on. What happens?
 
Watch the voltage when you put the microwave on, and I bet you'll see it plummet.

Use either a cheap handheld multimeter (£10 from eBay or Lidl) or fit a Smart Shunt.
 
Thanks all. Very useful. We can cook on gas but it does heat the cabin very quickly, we thought that quick reheats in the microwave would help in that respect. I guess I could bring the starter battery into use as well to try for more oomph? Will test the volts as suggested and try with engine on.
 
Using your starter battery to add power is going to end in tears, we have a 700w microwave recently fitted for the same reason as yourself, pulls 1500w when running even on half power as like induction jobs it cycles the power to reduce output so goes between full and off which certainly works the battery load, we have 1600w multipus and 450ah battery. Also to note we have flat bed microwave not turntable which actually uses a bit less power input.
 
pcatterall said:
Thanks all. Very useful. We can cook on gas but it does heat the cabin very quickly, we thought that quick reheats in the microwave would help in that respect. I guess I could bring the starter battery into use as well to try for more oomph? Will test the volts as suggested and try with engine on.
Click to expand...
Sounds like a recipe (pun intended!) for having no power to start the engine, but if you are lucky you'll have a warm meal whilst you wait for help! Running the engine might help, but most standard alternators aren't going to put out enough amps at fast idle to run without also drawing from your struggling batteries. Combined they might get you through (and should at least help put charge back in the battery if you leave the engine running afterwards). I'm guessing you'll end up running the engine for a lot longer than it takes to microwave a meal though!
 
The trouble is that you want to take that 10Ah or 12Ah out of the 160Ah batteries all at once, in only 10 minutes.

The batteries are rated at 80Ah each, but that rating will be based on a discharge time of 10 hours, probably.

1200w at 12v is 100A, and the batteries are just going to hate it.
 
Your 2 small batteries will not like providing 100+ Amps even for a few minutes. And they are probably not in the first flush of youth which makes them even less capable.
Check your cable sizes too, you need an absolute minimum volts drop.
 
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