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Old 05-28-2024, 09:09 PM   #1
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900W Microwave triggers thermal fuse.

Thanks to the wisdom and experience and willingness to share by this huge number of smart people on this forum, I am able to pinpoint and fix most issues on my RV. But, this one has me stumped and I hope someone has run into a similar issue and was able to resolve it.
My 2017 Forest River FR3 30DS has a microwave unit that is much too large for our needs. I replaced it with a much smaller 0.9cuft, 900W microwave which, according to label draws 1400W. Running it off battery, it will do fine for about 40 seconds, when some thermal fuse shuts it down. The fuse reconnects after about 15 seconds. The actual draw according to my display is 1393W.
Here is what I don't understand: 12V goes from the 300Ah LiFePo battery bank to the 2000W inverter. 119V goes from the inverter via 30A cable to a manual transfer switch and on to the 20A fused receptacle that was there for the original microwave. The triggering thermal fuse appears to be in the battery management unit, at least that is where the clicks come from. This unit should not even be involved in my circuit and this is what I don't understand. The chassis battery is disconnected during this time.

Does anybody have any ideas what may be going on here?
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Old 05-29-2024, 07:53 AM   #2
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I think my microwave pulls about 80A from my battery bank while running.

You should have a fuse of some sort between the battery and inverter to protect the wire. A battery BMS typically doesn't have a thermal breaker, but you never know. What brand battery?

Are you sure there isn't a thermal fuse tucked in around the battery?
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Old 05-29-2024, 01:12 PM   #3
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Yes Joe, there is a 300A fuse between battery and inverter.
The clicks do not come from the BMS, that was the wrong word to use by me. They come from the Battery Control Center, a unit that receives all kinds of cables from house and chassis batteries, charger, alternator etc. And yes, my battery bank is connected to this control center, but also directly to the inverter, in essence bypassing the control center. Even if the control center shuts off for whatever reason, this should not terminate the function of the inverter and the microwave should just keep running.
I am going search for other hiding places for a thermal fuse which might be, as you say, tucked in there somewhere.
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Old 05-29-2024, 01:23 PM   #4
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Make sure converter and water heater (120v) ….is OFF

Test your system by using something like a heat gun or hairdryer
They draw high amps without the massive startup current tha microwave can have

If the hairdryer works for extended times You may need to increase wiring to inverter or even get larger inverter to assist with startup load
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Old 05-29-2024, 01:50 PM   #5
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The inverter could be shutting down due to excessive heat. Be sure the inverter has very adequate air circulation.

The inverters will usually shut down if too hot and when they cool they will restart. Sounds like your issue.

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Old 05-29-2024, 03:06 PM   #6
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"Make sure converter and water heater (120v) ….is OFF"

Good point, Aussieguy.
I only have 2 circuits on the inverter: the microwave plug-in, and another one for the coffeemaker.
I don't think this is a start-up surge problem, because these spikes are momentary. The microwave runs for about 30-40 seconds, way longer than a spike would last. The click made when things shut down, is quite loud and yet I have not yet been able to pinpoint exactly where it is coming from. Frustrating.
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Old 05-29-2024, 06:40 PM   #7
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My 2k inverter sometimes has issues with the inductive start current of our 900 watt microwave. Good once running though. That said, it draws something like 130 amps off the battery. So I would go through and see if you can measure voltage drops throughout your battery/ inverter circuit, and voltage at the input to the inverter.
What is the battery bank? Wire gauge and length?
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Old 05-29-2024, 08:17 PM   #8
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Yup, that's next on the list: I will do some measurements, especially voltage drops. The battery bank consists of 3 x 100Ah LiFePO Renogy batteries, the inverter is a 2000W Renogy. Wiring is 2 x 0AWG, less than 3 feet long (should be overkill).
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Old 05-30-2024, 11:23 AM   #9
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Quote:
Originally Posted by OKHerbie View Post
Yup, that's next on the list: I will do some measurements, especially voltage drops. The battery bank consists of 3 x 100Ah LiFePO Renogy batteries, the inverter is a 2000W Renogy. Wiring is 2 x 0AWG, less than 3 feet long (should be overkill).
I wired my 2k with 1/0, and regret going so small. Wish I spent the coin on 4/0.
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