Microwave Circuit Permanently on Inverter?

[pteck]

I'm in the planning phases to install an inverter in my 27FB. Primarily to run an 800W microwave and single serving k-cup coffee maker when boondocking.

One of my thoughts in installing a 1500-2000W inverter, is to permanently connect the microwave to the inverter output, and off the house 120V circuit. Along with a inverter specific outlet in the kitchen, largely for the coffee maker.

I'm trying to keep the setup simple without major re-wiring or installing a transfer switch setup.

Trying to think of significant reasons why this wouldn't be a good idea.

Our use of the microwave largely to reheat meals, so ~4 minutes total in a couple 2 minute spurts. Any real cooking and we're using the stove. We have solar so running the batts down isn't really a concern. And if the batts need a boost, we can use our Honda eu2000 to top them off with the converter as needed, but at least wouldn't need to bust out the generator everytime we want to run the microwave/coffee maker.

If we're in a full-hookup site, power efficiency is not really a concern anyways.

Basically, this decouples the microwave from direct 120V input to the trailer. Or switching anything around when we stop for lunch or hit the campsite late in the evening. Hot food and coffee is always on tap.

Open to ideas on a better way!

[wulfraat]

800w microwave will generate ~70-80A 12v draw. 4 minutes at that current draw will be very hard on standard lead acid batteries - especially so if you have not upgraded to low internal resistance agms.

I think you are better off using your generator to run the microwave... I think you’ll cook 2 standard water-based lead acids with any amount of repeated use at that level of draw....

But others with direct experience can chime in....

[pteck]

Good point as I am currently on standard lead acid batts. It's 2x deep cycle group 27 batteries. I am very much considering AGM batts the next time around once my current batts age out. Perhaps this is me helping that progress

Let's say the load is 100amps with inverter powering the microwave. So pulling 50amps for 2 minutes at a time from each batt. While a significant load, I don't think 50 amps is unreasonable to be overtaxing the batts. No doubt they will be drawn down faster at that rate.

I'm also figuring with full hookups and the converter running (@55amps), that the microwave will then be drawing about 25amps each from the batts, which should be a walk in the park. And readily replenished thereafter.

[wulfraat]

Yeah you may be OK in the end… Just know that in general deep cycle batteries don’t like high draw loads from a longevity perspective… agms do better as they have lower internal resistance. Interstates basically suck in a number of ways. You get what you pay for......

One way or the other you’ll definitely want to replace the parallel cables between the two batteries to 1/0 gauge or better to reduce resistance between the batteries and avoid melting down that crappy little 6ga wires that come stock.

I would also keep an eye on battery temperature for the first two times you use the microwave to make sure they’re not getting too hot and boiling off water.

Finally the other thing that you may consider is putting an automatic transfer switch on the microwave circuit. Or better yet wire the entire inverter behind the transfer switch. If shore power is present the microwave will draw from shore power otherwise it will draw from the inverter / Battery. I had one of those in my last trailer and it worked out well.

[DaveP]

Pteck,

I wanted to do exactly what you want to do. Morning coffee without waking my neighbors in the campground. Here is my experience. I put in 4 6-volt AGM Lifelines batteries with a 3000 watt Magnum inverter/charger. I also put in a Magnum Battery Management System. i left the microwave permanently hooked up.

I run the coffee maker to make a pot. I run my Nespresso machine to make the espresso. I run the microwave to heat up the milk. All of that only takes my state of charge down by 2% to 3%. So, with the right battery bank, this is a win-win.

[uncle_bob]

Hi

First step: Do you do coffee and microwave at the same time?

Next up: your 800W microwave may be 800W of microwave energy or it may be 800W into the chassis. It's worth seeing which they happen to be talking about.

Assuming you are running a pure sine inverter, 800W into the microwave may be 1,000W into the inverter. At 12V you are over 80A. This *assumes* you can hold 12V at the inverter at 80A. Most typical cabling will be down around 11V and you are up past 90A. A C/2 drain turns your 80AH batteries into about 50AH.....

That's just for the microwave. The single cup coffee maker here actually pulls a bit more than 800W while it's heating up ..... It also spends a few minutes in the warmup phase.

Bob

[wulfraat]

I had a 2000w inverter in my last airstream. It worked well - but I never ended up putting the microwave on the inverted circuit.

Here’s how I’m solving the problem with my new 30’ international:

I’m visiting Lewster in OR in May next year for a 600amp hour Victron lithium platform install with integrated Victron hybrid / pass-through inverter and 800w+ of solar.

I think that will do. I’ll be able to run the AC for several hours off the batteries that way microwave will be background noise

[DaveP]

Now I am feeling a bit jealous!

Just kidding...

[pteck]

Just the discussion I was looking for everyone. With some impressive installs - capable of running the A/C! Just looking for my hot meals and drinks at the moment. My major investment has been the solar on my home 6 yrs ago (best investment ever!). So this is all for a bit convenience for the short camping trips the family takes.

I've got hundreds of feet of 2/4 gauge cable in my garage left over from previous projects. Yes, def going to upgrade the batt cables.

Yes, the C/2 draw won't be the most efficient way to ensure batt capacity. The first priority is to decouple the use of the microwave/coffee maker (not at the same time unless the batts prove up to the task) from physical hookups. I can always run the generator after the fact as necessary, longer if it must, to make up for inefficiencies

Full blown transfer switch is doable, but does add complexity in install and cost. I'd like to save my time for other projects.

This setup also has the side the benefit of being able to use the microwave when the A/C is being powered by my single Honda eu2000 generator (with EasyStart). Since it will draw from the batts instead of 120V.

The real test is whether 2x 12V lead acid batts will sufficiently support the inverter. I suspect it will without too much voltage droop. If the batts are unable to support the load, or get compromised from this type of use, I'll just go forth with upgrading the battery bank. Likely to 2x 6V AGMs or 3x 12V AGMs mounted in the underbed spaces. To increase capacity, draw capability, and reduce tongue weight, and possibly to run a blow dryer!

[wulfraat]

Cool. And another easystart user - love it!

I say go for if your backup plan is to simply drop in more horsepower if your current batts won’t support the need

....and please report back how it goes!

[TouringDan]

I think your microwave on a dedicated receptacle fed by your inverter will work. The only way you will really find out is to try it. Keep in mind that your 800 watt microwave will probably consume about 1200 watts to operate.

I have a 1,000 watt Zamp inverter and wanted to operate a small microwave. No go. I found a 600 watt microwave that specified that it needed 850 watts to operate. When I plugged it in to my watt meter it required just over 1,000 watts to operate. I have two 6v golf cart batteries. My microwave operation will need to wait until I install a 1,500 watt inverter.

Dan

[SSquared]

A couple thoughts.....

We have a countertop microwave, labeled 1050 watts input. We have 2 group 27 Lifeline AGM batteries. We tried the microwave on the inverter once, and saw that it drew over 100 amps from the batteries. And the amp draw increased as the seconds went by and the batteries sagged. We decided (without good scientific basis) that was more stress on the system than we wanted to be doing.

Our inverter is a 1000 watt Magnum inverter/charger. It has an internal transfer switch. That makes it pretty simple to put just one circuit on the inverter, and have it automatically be on shore power/inverter power:
1) Remove the romex for your selected circuit from its breaker in the electrical panel
2) Connect that romex to the inverter output
3) Run a new piece of romex from the breaker to the inverter input.

[pteck]

Very useful input!

I took a closer look at my microwave and it's rated at 700W. Helps. But single serve coffee maker is indeed rated 800W. Totally get they might pull a bit more, and the inverters are not terribly efficient being in the 75-85% range.

In addition to the batts, I do believe wiring gauge and length to be the critical factor in this. So I'll definitely focus there. Like I mentioned previously, I'm half hoping to accelerate the wear on my current batts, to upgrade to AGMs and bring them inboard. To decrease tongue weight as I'd like to mount my eu2000 generator above the batt box.

One interesting thing that occurred to me is that my vehicle has factory remote start/stop. While it won't change things dramatically, this can support the inverter with an extra 15-25amps from the tow vehicle/harness. If I'm really interested in that path, I could even augment the 12V supply from the tow vehicle with a 4-gauge wire run with relay and Anderson plug, and deliver 50+ amps.

Solar would contributing another ~5-10 amps depending on time of day.

[uncle_bob]

Originally Posted by pteck

.....

One interesting thing that occurred to me is that my vehicle has factory remote start/stop. While it won't change things dramatically, this can support the inverter with an extra 15-25amps from the tow vehicle/harness. If I'm really interested in that path, I could even augment the 12V supply from the tow vehicle with a 4-gauge wire run with relay and Anderson plug, and deliver 50+ amps.

Solar would contributing another ~5-10 amps depending on time of day.

Hi

At least on my TV, simply getting the engine going does not energize the 7 pin connector. You also need to get the truck into gear / tap the brakes. That turns on
the power to the 7 pin.

Since this is breakfast, even with my relaxed schedule, solar likely isn't going to be pumping a lot of amps at coffee time.

Now, a low noise *generator* with a remote start button .... hmmm .... that pretty much does the trick. Not any more noise than firing up the TV engine. Run for <10 minutes and shut it down.

Bob

[Dave-n-Janet]

Some real world experience:
On my 2017 26U after I upgraded batteries to 2x6v GC I was able to run installed microwave off the inverter. Never tried it before battery change out. Drew about 80 amps. Just long enough to heat 2 pizza slices at a rest area. Note, didn't have a grounded extension cord so pulled microwave from drawer to run on inverter outlet in dinette area, 400 watt solar on roof may have been contributing, don't know if this would have succeeded in full dark.

Another data point is I ran my 1000 watt coffee maker (10 cup cuisinart) three times successfully but have decided to go back to percolator as it was drawing up to 100 amps and not sure that is good for the long life of the 1000 watt inverter. Took 12-15 minutes to complete coffee prep and as I recall took batteries down about 15%. Was somewhat worse when I ran the coffee maker a second morning as batteries were at 80% to start.

Figured the inverter would shut off if drawing too much power which didn't happen. Not sure if these big current draws impacted battery life, but haven't seen any signs of that.

Dave

[mkcurtiss]

Are you not using a generator when boondocking? The inverters suck battery life like there is no tomorrow!......These new Yamaha generators are so quiet, you could literally have it running right beside you and have a conversation without even raising your voice....?

[Dave-n-Janet]

I do have a Champion dual fuel 3500 watt, but run that for A/C or if very cloudy rainy days in succession...

[BAB]

I have almost the same setup as DaveP. 30 Amp Magnum along with 4 of the Lifeline 6V batteries. Add in 600W of solar and you’re hovering in the $6,000 range. (Once the batteries go, I’ll move to Lithium.)

Obviously, I didn’t do all this upgrading just for being able to use the Keurig when we’re not hooked up or using a generator.

Long ago in a previous trailer I did the 2000W inverter trick. As others have noted, a high amp draw quickly depletes the two Lifelines very quickly.

With the Easy Starts in my A/C’s I run one unit when we stop for lunch at a rest stop. If it’s a bright, sunny day, the panels do a pretty good job replenishing the batteries from noon until sunset. We routinely run the dishwasher off the batteries if we’re boondocking. Uses less water.

I am really curious how long you can go without being plugged into a power pedestal in the newer Classics that are loaded with electric power consumers with two AGM Lifelines.

It’s all in how you use your Airstream. I just want to know I CAN be off the grid and still be comfortable.

[TouringDan]

Some more real world experience:

A couple of years ago I had two golf cart batteries and a 1,000 watt inverter (photo 1). This would run all my 120v stuff except for the microwave and it all worked quite well.

I recently decided to replace my 8 year old golf cart batteries with 2 BB lithium batteries and install a Samlex 2,000 watt inverter so that I would be able to operate a microwave (photo 2). I performed a test by running the microwave for 2 minutes to heat up a cup of water. The microwave required 977 watts and the battery power required was 1,120 watts (photo 3). The battery voltage while operating the microwave was 12.1v (photo 4) so the current out of the battery was 92.6 amps. Operating the microwave for 2 minutes reduced the Battery SOC from 72% to 71% (photo 5). I believe the test was quite successful and am very happy with the performance of the BB batteries and the Samlex inverter.

Dan.