Powering a microwave from inverter/12volt battery???

[Larry B]

  Need a little guidance on this subject. I do not think I understand microwave ratings. My wife asked if I could could figure out how to run the microwave from the inverter and 12volt battery while boondocking. (without having to start the generator).Just  for limited use- reheating coffee , warming soup ,etc.  I have a 2800 watt Magnum inverter and a 12 volt battery system. First thing I did was start a  roof top ac to test battery bank and inverter. Ran ac for 20 minutes then shut it off. Battery volts went from 12.6 to 12.3 after test and drawing 110amps (according to inverter remote contol)for full 20 min. Inverter still running cool and batery cable same. Looks good, lets try microwave. Microwave drew 147amps . My battery will not likely handly 3 to 4 days of dry camping with this energy hog. The rating on back of mic is 12.5 amp. and 1450 watts. Does this watt refer to energy used to operate mic or energy generated to heat my cup of coffee?  Why does mic draw so many amps more than roof ac ? ( I could not find the book on ac but I am sure ac unit is more than 12.5amps rating.)  Do others run mic while boondocking?  Is there some thing wrong with mic? If I need to buy a different mic what do I look for? Or is wife not going to be happy because she has to start gen.?

[TomC]

147amps is what a battery would pull at about 11volts. 1450 watts is a powerful mic. You could get more batteries. I run my mic on my Trace 2500 watt modified sine wave. It hums on the inverter and takes longer to heat. But with the Magnum being a pure sine wave-it should run the same way. Good Luck, TomC

[bevans6]

The 12.5 amp and 1450 watt rating is what the oven draws at 120 volts AC.  A 1450 watt microwave will have cooking power of around 1000 watts due to losses in the magnetron tube.  A microwave tube is either on or off, and they get the lower cooking power by cycling the magnetron on and off.  

Converting that 1450 watts to inverter amps is basically taking the wattage, adding in the efficiency loss of around 12% and looking at the voltage level of the battery under that draw.  As Tom said, the math shows that you were trying to draw around 1650 watts on the input side of the inverter, and the current of 147 amps translates this to a battery voltage of 11.2 volts.  Which suggests that your battery is too small, by a lot, and you discharged it pretty far with the air conditioner test, then you put the heavy load on it and pushed the voltage down even further.  Magnum addresses this in the manual, and recommends a 400 AH battery bank for loads over 1000 watts, and you have a load of 1650 for the microwave, so you need some serious batteries.  A fully charged battery will have a static voltage reading of around 12.7 volts under no load and no surface charge, but that same fully charged battery can drop to 11.2 volts or so under a heavy discharge load.  My truck has two pretty decent batteries but the glow plugs draw over 200 amps during a cold start - the fully charged batteries drop to just over 11 volts and I get nervous, every single time I start it.  I was far happier and far less informed before I installed the voltage gauge...

Get more batteries, charge them fully, and the 1 minute cycle to heat up your coffee will not be a problem.  Your high powered microwave will heat your food in less time than a smaller microwave, so the average power draw per cup of coffee will be about the same.  If it draws 120 amps with a fully charged large battery bank of 400 AH, you will be pulling 2 AH per minute of use so you could microwave for 100 minutes to reach a 50% SOC on your battery bank.

Brian

[bobofthenorth]

What Brian said.  Plus get yourself a state of charge (SOC) meter.  I use the Trimetric 2025 on the bus and the boat.  It's not possible to live off battery power without one.  Battery voltage is completely unreliable as a measure of discharge state.  If you don't buy a SOC meter then budget for frequent battery replacement.  And you need a bigger battery bank if you are going to do much cooking with batteries.

[Melbo]

There are also inverter microwaves. We have one in our bus that when used on lower power are more efficient in energy use.  But as previously mentioned it takes lots of battery to cook and boondock. We have to run the generator a little over two hours a day. One hour in the morning and one in the evening to stay even when dry camping.

HTH

Melbo

[TomC]

As Melbo said-when dry camping, we run the generator for about 2 hours in the morning to heat the 2-10gal water heaters (especially if you're showering together), to make coffee, charge batteries, and hair dryer. If need be also run for an hour or two at night.

We have 2-8D Lifeline AGM batteries for house for a total of 510amp/hours.  On my truck, since I have more room, I'll run 4-L16 6v Lifeline AGM batteries wired for two banks of 12v for a total of 800amp/hours. It is interesting, Lifeline also makes the same battery in 2v. So to get 12vdc, you'd run 6 together. That would give 1200amp/hours. The 2v batteries are aimed at UPS (Uninterruptable Power Source) for big inverters used in offices or even homes. Good Luck, TomC.

[bevans6]

Inverter microwaves are very interesting.  Also inverter Tig welders.  Now you know how screwed up my mind is when I think of those two things at the same time...   I mentioned that the magnetron tubes are either on or off.  Old style units fed the power to the magnetron and turned it on and off with transistors, all the power had to flow through the transistors, generating heat and using power.  Inverter microwaves use inverter technology - essentially the same deal as a pure sine wave inverter - to turn the magnetron on and off at quite high frequency, around 20 Khz I think.  That high rate of cycling makes it look like the magnetron is actually putting out less power on the low settings.  The deal with the inverter technology is that the output power to the magnetron doesn't actually flow through the output MOSFET devices so they don't generate the heat and the loss of efficiency as they control the magnetron tube.  The output of a pure sine inverter is either on or off as well, full voltage or no voltage, it just gets turned on and off so fast with a duty cycle that mimics a pure sine wave that the output is minimally filtered and turns into the next thing to a pure sine wave.  I wrote an article about pure sine wave inverters and posted it here a few months ago.

Brian

[Larry B]

Thanks everyone for the replies--- I have a read the replies a couple of times and it appears I will need  to run generator a little each day and I should increase battery bank. To increase battery or double what I have will be difficult. I have a battery like Tom was talking about 6@2volt. As picture  below shows.(the piece of wood is end of a 2x6-for size comparison) There was a time I could have had as many of these as I wanted,but I only took one. For charging purpose and for inverter use so that things "balance out" would I not have to duplicate what I got? The little sticker on battery says; Absolute11P;;  type-90A-17;;  700ampere hrs;;  8hrs to1.75VPC
     As Brian points out a smaller microwave will have to run longer to do the same heating job as the one I have. Not really gaining much with different microwave. You are correct 11.2volts is what the remote contoller was reading while the mic was running.
     It sounds as if the first thing I  need to change is how I view my system and what I expect from it, unless someone knows an easy way to increase existing battery bank  .A SOC meter sounds like a wise investment -  it will take better care of these batteries(and maybe a different set someday ) 
    Thanks again for the replies     
     
     LarryB

[bevans6]

700 AH is big enough to do what you want, so try again after a full charge and not running the AC first.  I have a feeling that somehow the battery wasn't fully charged when you started, can you equalize charge those batteries?  I can't remember if you can equalize charge AGM's, which is what they look like.

Brian

[bobofthenorth]

I equalize my Lifeline AGMs once a year.  It depends where you read whether that is a good idea or not but I think it helps. Our house bank of 3 x 8Ds is 10 years old now & I can't detect any decline in capacity.

R.J.(Bob) Evans

[bevans6]

Absolyte 11P battery:  http://www.p-s-s.com/Absolyte_IIP.htm

There is a brochure link if you want to know all about what you have.  The last page has the charging specifications so you could program your Magnum correctly.  Top of the line battery, for sure!

Edit:  I happened to take a look at the charging specification for that battery, for some reason.  It's calls out a 106 amp charge for two hours as a start, the bulk charge basically.  What I just now noticed is that is per each 100 AH of rated capacity.  I kind of knew that AGM batteries could accept up to 100% of their AH rating, but that brought it home.  It also made me realize that charging a 700 AH AGM battery with any portable charger is going to take a while - your Magnum isn't going to push 742 amps at them, it is going to have maximum 125 amps.  That is going to extend the time to reach full charge by a lot.  The Magnum should be able to tell when to go into float mode, though.

Brian

[Lin]

You have more than enough capacity to do what you want assuming the batteries are in good condition to start with.  As mentioned,the issue is getting them fully charged.  I do not know your inverter/charger, but if it can give you a reading of charging amps, you can watch to see how long it takes to drop.  The better the charge on the battery, the lower the charge amps will be.  It can therefore be impractical to try to get the batteries fully charged while dry camping since that could require you to run your generator for long periods of time just to top the batteries off.  Hence, we settle for getting the batteries almost charged.  That is not the best thing for the life of the bank, but it is a workable compromise.

If you want to get the batteries fully charged every day, you might look into getting a solar panel that can deliver in the range of 10 amps.  In that case, you can get the batteries mostly charged by running the generator for a while and then let the solar top them off.  This would not be the same system as those that want to do all their charging from solar.  For a top off solar charger as mentioned, one good panel could be enough.  It could be on the roof or even just something you set up on the ground. I was even thinking of putting one on the roof of my Jeep so it could be in the sun while the bus is in the shade.  Also, it would go with us if we went out since a portable system could be easily stolen.

In short, your system is fine as is. You just need to get a handle on managing it.

[Larry B]

That is good news. It looks like with a bit of self training and a good look at the specs on the site Brian pointed out will help a lot. According to info on the site it claims it is not necessary to equilize charge this battery. I do remember telling the remote that this was a gel battery, I need to change that to AGM. It does take a long time to recharge. After three days of dry camping the inverter may not finish recharge the battery the same day it is plugged in.(only 30 amp supply). Sounds as though I got lucky when I ran accross that battery for nothing. I spent most of the day working on pro heat and the new heating lines between it , the bus engine and house heating system. I  will change remote setting and recharge batteries tomorrow. It sounds as if a  trimetic 2025 more than ever would be a good investment.  Thanks again for all the help. I have some questions on 24volt stuff, I will be back to pick your brains shortly--- after I get this and my heating finished. Thanks.
           Larry B     

[Fred Mc]

What is the advantage of having 2V X6 bat  for 12 V vs 1X 12V battery.Also do these type of batteries have an extended life or is it similar to the 8D that a lot of use have.

Regards

Fred

[Fredward]

Just as another point of reference, we have a little 600 watt microwave and an old 2200 watt inverter powered by a single 8D lead acid battery. It runs the microwave just fine although things seem to take a little longer to cook than when connected to generator or 30amp hookup.