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Running microwave shutting down BMS

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GetOut

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Jun 1, 2021
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Hi all,

I built a 280AH battery using a 120amp Overkill BMS. I've got a 2200 watt pure sine inverter and I tried to run my rv microwave and it shut down the bms. Looking at the specs on the microwave, it takes a 1500 watt as input, 1000 watt as output. So that makes sense... 1500w / 12v = 125 amps. I guess the bms should shutdown. So I then tried to run the microwave on low power thinking that that would reduce the input amperage. Nope, still shutdown the bms. Strange, unless the input stays the full 1500 watt amount regardless of the output.

So, I am now considering swapping out to a smaller 1000 watt microwave in the hopes that it wouldn't shutdown the bms. 1000w / 12v = 84 amps. Do you think that would work?

Thanks so much!
 
GetOut said:
So, I am now considering swapping out to a smaller 1000 watt microwave in the hopes that it wouldn't shutdown the bms. 1000w / 12v = 84 amps. Do you think that would work?
Click to expand...
It is probably the surge current starting up, regardless of the operating mode. The only way to be sure is to find out what the surge is for that 84 Amp microwave. I have oftern heard that many of thes Chinese BMSs should only be run at 50 percent of their ratings because of differences in language which can get lost in translation.
 
You need to factor inverter efficiency of 85% (even if yours says higher)

1600W/12V/.85 = 157A.

Standard microwaves are on/off only. If you specify 50% power, it is on 100% half the time.

A 700W microwave pulls 1200W

That's still 118A.

Another option would be a Panasonic inverter microwave. Those are supposed to actually run @ 50% if set to 50%.

 
That makes sense, I didn't think about it running full power for less time. Thanks for the help!

The inverter microwave sounds interesting. I'll have to do more research on it!
 
GetOut said:
That makes sense, I didn't think about it running full power for less time. Thanks for the help!

The inverter microwave sounds interesting. I'll have to do more research on it!
Click to expand...

My neighbor bought one. He thought he was doing the right thing by running his old microwave at 50% power. He's now gleefully using 50% to slowly warm his coffee without spiking his load in 10 second intervals.

It's actually beneficial for re-heating. The 100% pulsing is uneven, and it requires the rest period to allow the heat added at the surface to penetrate - super hot surface, cold insides. When running at 50%, the surface heating is less severe, and the increased heating time allows for a continuous rate of heat transfer - more uniform heating/fewer hot spots.

When I last did research on it, it seemed that inverter microwaves were very common outside the U.S. Panasonic was the only reasonably available model when I last checked, and they're not particularly expensive compared to similarly powered "standard" units.
 
I have a Panasonic 1200W Inverter Microwave. When it is operating, it is pulling 55A from my 24V system which translates into 110A on a 12V system. There is no surge at start and even if I run it at 50% Cooking power it does not make any difference. Now my nuker is 14 Years Old and so the tech in it is fairly ancient in terms of these types of appliances. Would a new one use less power ? Possibly.

A difference between Inverter Nukes over the standard type... It has NOTHING to do with energy consumption, the inversion process for nukers is related to how they cook the foods even from the center out and back in, in waves. This prevents things like cooking the center of something while the outside doesn't get very cooked... We all have suffered such with the older nukers

You did not specify what kind of inverter you are using, but I am using a Samlex EVO which is Pule Sine Low Frequency that is 96% Efficient. Whether or not you have a Low Frequency or High Frequency Inverter it should make no difference.
 
That's interesting. We also had a 1200 Watt Panasonic inverter microwave and found that a Honda 2000 could run it on 50% but not on any setting higher than 50% which indicates that load drawn does depend on power setting. Our Panasonic may have been newer than yours, we went to a 600 Watt unit because it fits our kitchen better.

My understanding is that modern inverter microwaves run the magnetron on lower current for lower power settings whereas standard microwaves achieve lower power by pulsing the magnetron on and off to get a lower duty cycle delivered to the food.
 
I also have a 280AH battery using a 120amp Overkill BMS. I hedged my bet and bought eight cells and two BMS. Now I have twice the capacity for high amperage devices like the microwave. I've had no problem starting anything in my RV, including the air conditioner.
 
GetOut said:
Hi all,

I built a 280AH battery using a 120amp Overkill BMS. I've got a 2200 watt pure sine inverter and I tried to run my rv microwave and it shut down the bms. Looking at the specs on the microwave, it takes a 1500 watt as input, 1000 watt as output. So that makes sense... 1500w / 12v = 125 amps. I guess the bms should shutdown. So I then tried to run the microwave on low power thinking that that would reduce the input amperage. Nope, still shutdown the bms. Strange, unless the input stays the full 1500 watt amount regardless of the output.

So, I am now considering swapping out to a smaller 1000 watt microwave in the hopes that it wouldn't shutdown the bms. 1000w / 12v = 84 amps. Do you think that would work?

Thanks so much!
Click to expand...
I'd switch to a 24V system and be done messing with 12V rather than purchase a different microwave. My microwave running in my camper draws 47A when the magnetron is on.

You could add another battery in parallel if you really want to stick with 12V. That would cut the amps per BMS in half. I find life much easier with 24V once the system is built. No way I'd consider a 12V inverter in my camper.
 
Steve_S said:
A difference between Inverter Nukes over the standard type... It has NOTHING to do with energy consumption, the inversion process for nukers is related to how they cook the foods even from the center out and back in, in waves. This prevents things like cooking the center of something while the outside doesn't get very cooked... We all have suffered such with the older nukers
Click to expand...

Your wording implies that microwaves of any type might cook from the inside out. They do not. Microwaves of any type heat the surface of the food, and the interior is heated by conduction from the warm/hot surface.

Matter is not invisible to microwaves. Microwaves excite the first water molecules they meet. They do not jump over/past water molecules to get to others. The primary cooking benefit of inverter microwaves is the ability to heat at a true reduced power so the surface heating rate is more comparable to the rate of conduction heating of the interior.
 
I was wondering if I added a second battery in parallel, would it split the amperage draw? Or would it still try and pull the full amount from either battery?
 
GetOut said:
I was wondering if I added a second battery in parallel, would it split the amperage draw? Or would it still try and pull the full amount from either battery?
Click to expand...

As l long as the resistance is the same to each battery then the current draw will be split almost 50% to each battery. That's how it works in my setup.

Using cables that are exactly the same length will get you as close as you can to the same resistance to each battery.
 
HRTKD said:
I also have a 280AH battery using a 120amp Overkill BMS. I hedged my bet and bought eight cells and two BMS. Now I have twice the capacity for high amperage devices like the microwave. I've had no problem starting anything in my RV, including the air conditioner.
Click to expand...
That's smart!! Where did you get the cells? I'd love to see how you set that up. Any pics?
 
HRTKD said:
As l long as the resistance is the same to each battery then the current draw will be split almost 50% to each battery. That's how it works in my setup.

Using cables that are exactly the same length will get you as close as you can to the same resistance to each battery.
Click to expand...
That might be an easy solution to my issue. Would I have to add the same size battery? Or could I add a smaller 100AH Lithium battery?
 
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