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Microwave on Aux Battery?

24K views 80 replies 18 participants last post by  tgblake 
#1 ·
OK, here's the scenario:

-6 month old 100AH AGM battery fully charged, voltage reading about 12.6
-700W Microwave (uses about 1000W)
-HF 1500W (3000 surge) inverter connected with 2 feet of #6AWG (doesn't get warm at all)
-1 cup of water in mug
-No solar, just a 150A isolator that charges battery whenever I drive - works great for TV and most loads....

I turn on the microwave... it sound different than on household AC, kind buzzy
Voltage meter drops to about 11.8 volts.
About 3 minutes in, inverter drops out
Water hot but not quite boiling yet....

Your thoughts are welcome! I'd like to be able to do a 5-10 microwave cooking thing occasionally....

I'm wondering if I ran the van and let the battery isolater link the batteries, would that do any harm to the van electrical? I have the stock alternator, not the HD one. I tried it but I have a 25A fuse going to the isolater. It popped. I know, I need a bigger fuse and will try that, but....

I was hoping to not have to start the van to use the microwave. Insights are requested, please :D

Ed
 
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#2 ·
#3 ·
I'd wager there's a few things going on here...

- That 700W microwave probably does use 1000W, or maybe closer to 11-1200W. I've read of some 700W micros tripping a 1000W inverter. You have a 1500W inverter, not a problem there, but just to note the actual power draw.

- Harbor Freight inverter? Its probably a modified sine wave, not a pure sine wave. Many electronics will do OK on a modified sine output, but microwaves prefer a pure sine. They will be noisier and take longer on modified sine wave.

- Its only 2' of #6 wire, but I would say that is undersized for a 1500W inverter. Looking at the wiring specs for a Magnum 1500W it specs a #2 wire for a short length, for a 5% voltage drop. The specs for my Magnum 1000W specs a #1 /0 wire to handle the maximum possible draw at low voltage. Even though its not feeling warm, your #6 is likely undersized, leading to voltage drops, which is never good.

- The DC current draw for a 1000W output is large. When a lead-acid battery has a large current draw, it drops voltage, and capacity as well. (advantage: lithium) Your 100AH battery is probably not large enough to run the 1000W output needed for the microwave. Upgrading to a 200AH battery would help.

A few of those combined leads to a low voltage at the inverter, after a bit of use the voltage drops further, reaching its low-voltage cut-out.


As for running the van to do this, you're right, it would likely need a MUCH larger fuse, which may not be good idea.


In sequence I would look at


1: updating to larger cabling for the inverter, at least #2 , if not #1 /0
2: upgrading to a 200AH battery
3: changing to a pure sine wave inverter
 
#4 ·
I have pretty much the same micro setup (700 watts) only I have 2 - 100AH AGM's and a cheap 2k inverter from HF. It works fine. Sometimes I do turn the engine on to give it an extra boost, (like early in the AM when the batteries are at their lowest after running the fridge all night and before the sun comes up) but I never have a problem during the day or at suppertime.

I think you need a second battery.
 
#7 ·
KOV,

How big is the fuse from your van battery thru isolater to aux battery(s)? stock or upgraded alternator?

Did you ever notice what the voltage is while microwaving?

I remembered that someone had a similar setup, but couldn't remember it was you.

Thanks,

Ed
 
#10 ·
I have a similar setup except that my battery is size 8D about 230 amp hours. My inverter is pure sine wave from Xantrex. Inverter is only 1000 watts but capable of much more for a very few minutes. It indicates that battery current is about 120 amps; that is, the 700 watt microwave is using 1000 watts or more. If I run the microwave for more than 3 minutes I let the battery rest and inverter cool down for a couple of minutes before continuing. I have an 80 amp fuse from the engine battery to the combining relay. I never run the engine with the microwave or my 750 watt hot water heater as I think it might blow the fuse as I assume that all the current would come from the alternator as it would see well below 12 volts. You need both a bigger battery and a pure sine wave inverter.
 
#14 ·
OK, here's the scenario:

-6 month old 100AH AGM battery fully charged, voltage reading about 12.6
-700W Microwave (uses about 1000W)
-HF 1500W (3000 surge) inverter connected with 2 feet of [URL=http://www.promasterforum.com/forum/usertag.php?do=list&action=hash&hash=6]#6 AWG[/URL] (doesn't get warm at all)
-1 cup of water in mug
-No solar, just a 150A isolator that charges battery whenever I drive - works great for TV and most loads....

I turn on the microwave... it sound different than on household AC, kind buzzy
Voltage meter drops to about 11.8 volts.
About 3 minutes in, inverter drops out
Water hot but not quite boiling yet....

Your thoughts are welcome! I'd like to be able to do a 5-10 microwave cooking thing occasionally....

I'm wondering if I ran the van and let the battery isolater link the batteries, would that do any harm to the van electrical? I have the stock alternator, not the HD one. I tried it but I have a 25A fuse going to the isolater. It popped. I know, I need a bigger fuse and will try that, but....

I was hoping to not have to start the van to use the microwave. Insights are requested, please :D

Ed
Hi,
On the #6 wire.
If the input to the microwave is 1000 watts, and the inverter is 90
% efficient, then the current from battery to inverter is about (1000/0.9)watts / 12volts = 93 amps.

This voltage drop table: http://www.solarseller.com/dc_wire_loss_chart___.htm
says that a #6 wire can be up to 2.9 ft long for 2% voltage drop at 100 amps current flow. 2% voltage drop is only 0.2 volts, so while it seems like something heavier than #6 would be a good idea, its probably not the basic cause of the problem.

I agree with the others that you probably need more battery.

Gary
 
#15 ·
OK, I do run #6 gauges wires too:D
but I do run it in parrallllleL:eek:
So, my total summary of a cable 6 / 3 = 2gauges.
It was much cheaper to by jump-cabels #6 gauges cut it and run 3 connected in PARALEL :D
 
#16 · (Edited)
Lots of good info and experiences here. Thanks!

A few thoughts...

autogateman,

The math doesn't work. To get 4000 watts from a 12V battery (with no wire losses), you need a 333Amp battery. I'm guessing that the welder you use pulls less than 1000 watts, and that's why it works. I've looked at uNik's posts and he uses 3 batteries to weld.

KOV, Gary,

It appears that a second battery is in my future! What I don't understand is how KOV's battery stays at 12.x volts when mine drops into the 11v range almost immediately. I think we have the same microwave, based on the description (HD, 700W Magic Chef). At a 5 amp discharge rate, my battery pretty much offers about 24 hours of use, which confirms 100Ah rating.

KOV, where's you get that battery? Brand, model number? Do you run the microwave at full power or lower power for a longer time?

My thinking is to beef up the connection between the van battery, isolater, and house battery. On my Battery Doctor Wizard, there is a button that puts the van and house batteries in parallel. Seems to me that would be the same as a second house battery. I'm going to try an 80A fuse from van battery to isolater and see what happens with the "parallel button" on.

I'm hoping the van+house batteries will work for occasional use. Not sure if running the van is a good idea because it's hard to determine how much of the alternator's amps are available to contribute to the required 80+ amps. Don't want to create any alternator circuitry repair issues! OTOH, it might make it all work, and I have no problem running the van to use the microwave.

Thanks guys,
Ed
 
#17 ·
Lots of good info and experiences here. Thanks!

A few thoughts...

autogateman,

The math doesn't work. To get 4000 watts from a 12V battery (with no wire losses), you need a 333Amp battery. I'm guessing that the welder you use pulls less than 1000 watts, and that's why it works. I've looked at uNik's posts and he uses 3 batteries to weld.

KOV, Gary,


Thanks guys,
Ed

The short term amperage capacity of the battery is much higher than 100amps. My welding machine draws 20 amps AC at full current. That's almost 3000 watts.


Sent from my iPhone using Tapatalk
 
#19 ·
Ed, I also have #6 cable from the batteries to the inverter. The batteries are from Sam's, 2/12v AGMS ($180 each as I recall). If I use the micro it's only for 2-4 mins usually on high power. If I remember I'll do a voltage check for you with it running on just the house batteries and then combined with the alternator. Memories aren't always what the used to be:laugh:
 
#22 ·
Thanks in advance. I see you already stated having two 100 Amp-hour AGMs, which sounds very doable to me -- plus I could go higher if needed. I plan to build a frame to support batteries under van from the body instead of the frame so I can make it larger if needed.

We just got back from trip up north and many campgrounds closed after October 15th. Even if we stay in hotels occasionally, I want to be able to run microwave and coffee maker without having to rely on shore power.
 
#25 · (Edited)
Running the microwave off the house batteries.

I did some testing this morning and this is what I came up with. Keep in mind I’m no expert on this by any means and I’m just reporting my results for everyone to consider and make suggestion.

First the set up. 300 watts solar panels running thru a 30 A Tracer 3215RM MPPT controller feeding two 12V Duracell 31AGM batteries (800 CCA each). The house AGM’s are connected to the OEM starting battery with 4/0 cable (grounded to the body) then thru a Blue Sea 6006 single circuit on/off switch to a Blue Sea SI-ACR charging relay. The inverter is a Harbor Freight 2k model. The microwave is a Magic Chef 700w model.

The house batteries are always connected to the PV panels and I keep the on/off switch in the off position so as to not charge the house batteries from the alternator or the starting battery from the PV panels (typically).

The house batteries have had no load on them from over two weeks as of today and were fully charged from the PV panels thru the charge controller which was reading 13.8v and flashing that the AGM’s were fully charged. I turned off the PV panels and turned on the inverter and the voltage dropped to 13.1v (the dorm fridge started up as it is wired on the same circuit. I put a cup of cold water in the micro and ran it on high for two minutes at which time the voltage was down to 11.4v (the water was nice and hot, no problems from the inverter. I shut the inverter off, turned the PV panels back on and after waiting about 15 minutes the voltage jumped up to 14.2v from the PV panels and when I turned the inverter on again it dropped to 13.9v and after two minutes of heating the cup of water again the voltage was at 11.2. After shutting the inverter off the voltage went back up to 12.6v and stayed steady for several hours. Next, I turned the switch on connecting the house and starting batteries but surprisingly got the same results as when they were not combined. I’m not sure why because before putting in the PV panels I could just turn the switch on or off and in the off position I would get 12.6v and when combined whatever the alternator was putting out (usually 14.1V). I’m not sure if the PV panels are back feeding the isolator as the green joined light stays on with the switch from the starting battery in the off position now and I’m not getting 14.1v to the house batteries when the switch on as I expected I would. I do have 14.1v going into the switch so I know the cable from the starting battery is ok and as I just discovered this I’m not sure what happens when the house batteries are fully charged by the PV panels. Any thoughts or suggestions are more than welcome.

The final results, to recap, the microwave ran fine off the PV panels for 4 minutes over the time span of about 20 minutes. The inverter never made any strange noises, the dorm fridge was running the whole time as the microwave was. Of course when camping the house batteries are not going to be at 13.1v in the early AM, more like 12.4v but while camping two weeks ago the microwave ran fine off the inverter with the house batteries at 12.4v in the AM although it took ll day to bring them back up to full charge. I guess the biggest problem (as far as I’m concerned) is why the alternator isn't feeding 14.1v to the house batteries immediately when I try to feed the charging relay from the alternator.

I hope this doesn't cause people even more questions or concerns about posing their microwave from the inverter.
 
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#26 · (Edited)
Running the microwave off the house batteries.

... I put a cup of cold water in the micro and ran it on high for two minutes at which time the voltage was down to 11.4v (the water was nice and hot, no problems from the inverter.
Ah, the typical test... a cup of water! :D

I feel better already. I had the impression that the batteries under heavy load stayed in the 12's. My single battery sunk into the 11's too.

I changed a few things, since original post...

1) added a 80A circuit breaker to the main batttery-to-isolator wire (instead of the 25A that blew)

2) tried with van off and on.

Results....

- Very hot water after 3 2 minute sessions
- Inverter fan went on a few times, but heat sink wasn't hot. KOV, did yours?
- Van running didn't seem to make a huge difference.. seemed like the engine was trying to decide whether to help or not. RPMs went up and down a bit.
- Microwave sounded a bit "buzzier" than on pure AC, but everything seemed to work
- seems like my single 100A AGM battery will do it, especially with parallel hookup from the van battery. (like having 2 batteries, but probably OK for very occasional use). My isolater appears to be able to do that. Maybe running van is a good idea in my scenario.
- No fridge..... yet

Ed

ps. same Magic Chef Microwave. I hooked up a Kill-A-Watt meter and, on high, the microwave drew between 650 and 850 watts.
 
#28 ·
After sleeping on this overnight, trying to figure out why the house batteries aren't getting 14.1v from the alternator when I connect them I'm wondering if I should just remove the isolator relay from the circuit? With the PV panels it's not really needed (as long as I remember to open or close the switch when necessary. This is how I did it I'm my Sprinter (without solar).

Any suggestions???
 
#29 ·
After sleeping on this overnight, trying to figure out why the house batteries aren't getting 14.1v from the alternator when I connect them I'm wondering if I should just remove the isolator relay from the circuit? With the PV panels it's not really needed (as long as I remember to open or close the switch when necessary. This is how I did it I'm my Sprinter (without solar).

Any suggestions???
Get clamp-tester (~12dollars in Har-Freight) and measure amps on each circuit.
(on +12vDC and ~120vAC)
 
#34 ·
Update:

Microwave on AC, 1085 watts on the Kill-A-Watt meter,
On inverter, 850 watts on the same meter.... meter may not like measuring un-pure sine wave, or the inverter isn't making enough watts, even with aux+van batteries in parallel.

Water gets hotter a little faster on AC

For short term use, the Aux+Van batteries is OK, especially the few times I will probably use it. It does bother me that the microwave has a distinct buzzzzzzz on the inverter, that's not there on AC.:(

I may return 1500W and upgrade to 2000W Inverter at HF... perhaps a second battery at some point... (I would like a small fridge for trips! :D)

Ed
 
#35 ·
I'd "guess" the microwave doesn't like the modified sine wave power, which may also affect the power factor. You'd have to measure current and voltage to compare to kilowatts.

Do you know if your 1500 watt inverter is same brand as KOV's 2000 watt inverter? Apparently they are not all equal according to what I've read.
 
#36 ·
I did pull my GE microwave out of my van and ran some tests on it connected to house current. While it doesn't address battery or inverter function directly it did give me some insight, plus it confirmed some data I needed.

In order to use Amp clamp I had to buy a cheap extension cord I could modify. While at store I checked specs on new microwaves and they all ran right around 1.5 watts of power usage per watt of rated capacity. The 700-watt microwaves were all rated to use 1,050 watts, and the 900-watt microwaves were rated at 1,350.

Mine doesn't have power rating on it, but specs show 700 watts (I thought it was 600 watt microwave). The nameplate states that it uses .95 kW (which may include power factor correction).

I first ran it with 8-oz of water for 2 minutes, and it started at 9.5 Amps. After one minute it had dropped to 9.2 Amps.

I then heated 16-oz of water for 4 minutes, and it started at 9 Amps and dropped to 8.5 Amps after 2 minutes. As the microwave itself warmed up the power consumption decreased. In a way that's expected.

I then heated 64-oz of water for 3 minutes and the current remained at 8.5 Amps. Apparently that's the steady-state current draw after the microwave gets to maximum temperature.

While checking electrical power consumption I also checked water temperature in and out, which can let me compare how much of the power is converted to heat.

While I was at it I checked my parent's Cuisinart coffee maker (for Keurig cups) and it pulled 11 Amps for +/- 40 seconds and then 6 Amps for +/- 10 more seconds. This compares to 1425 watts on nameplate.

My camping Keurig coffee maker from the van is also rated at 1425 watts, and it pulled 11.3 Amps for +/- 40 seconds and then +/- 7 Amps for 26 more seconds. Not surprising the coffee is hotter after brewing which makes it taste better to me (my wife had commented that coffee tasted better also).

I can get into water temperatures tomorrow, but the important thing I learned is that the microwave is more energy efficient with greater load. So if you have limited power from batteries, it's best to utilize it more effectively.
 
#38 ·
Thanks, that's great to know. I would have gotten one of those instead if I had seen one -- less power and seems a little smaller too. The fact that it takes about 3 minutes to make a cup of coffee isn't an issue for me. My wife on the other hand has a hard time waiting for the first cup.
 
#39 ·
Regarding water temperature gain in my 700-watt GE microwave (dates back to 2006 so newer may be better):

8 oz X 2 minutes = 69 degrees F / 72 degrees F (multiple readings)

16 oz X 4 minutes = 92 F / 96 F

16 oz X 2 minutes = 47 F

64 oz X 3 minutes = 20 F


Conclusion #1 : Electrical power draw was not affected noticeably by load in microwave. We can assume current is independent of quantity of water in microwave.

Conclusion #2 : Less total energy is used when more liquid is heated all at once. That's to say that if making two 8-oz cups of hot chocolate, it's significantly more efficient to heat both at same time for longer period than to heat one first and then the other.

Although I didn't test it specifically, I'm fairly sure that heating two cups of soup or two TV dinners don't take twice as long as one at a time. That's a change we'll have to make in future because my wife tends to heat one thing at a time even if there is room in microwave to place more. A rack to stack items vertically for heating may be a good item to own.
 
#40 · (Edited)
Disappointed in CT!

I tried a few things...

Upgraded to 2000 watt HF inverter.... microwave still buzzes, but works for a while.

Removed my isolater and hooked main battery in parallel with aux battery. Figured two batteries would be better than one, but 80 A circuit breaker on aux battery popped after about 2 minutes. My thinking is the the AGM aux battery has a bit higher voltage than the van battery so it takes most of the load (which is more than 80A). Van battery doesnt seem to contribute much, even when paralleled with aux battery.

Running the engine made no difference whatsoever... not sure why....

The other problem is that the wires from the van battery to the aux battery are longer (8') than the wire from the aux battery to the inverter (1.5'), No way to get 'em closer together.

The apparent solution is a second aux AGM battery (a-la-KOV) that would be right next to the first one under my couch.

Disappointed because the cost of a second battery is about 3 times the cost of the microwave! ... and I'm not sure how much I'm going to use it (in a location with no AC outlet nearby).

I did find a really nice butane single burner stove with a dozen cans of butane for about $30. This may be a better way to approach cooking in my "overnighter-designed") van! These little cookers get great reviews and are used by restaurants, campers, boaters, etc.

Ed
 
#42 · (Edited)
I tried a few things...

Upgraded to 2000 watt HF inverter.... microwave still buzzes, but works for a while.

Removed my isolater and hooked main battery in parallel with aux battery. Figured two batteries would be better than one, but 80 A circuit breaker on aux battery popped after about 2 minutes. My thinking is the the AGM aux battery has a bit higher voltage than the van battery so it takes most of the load (which is more than 80A). Van battery doesnt seem to contribute much, even when paralleled with aux battery.

Running the engine made no difference whatsoever... not sure why....

The other problem is that the wires from the van battery to the aux battery are longer (8') than the wire from the aux battery to the inverter (1.5'), No way to get 'em closer together.

The apparent solution is a second aux AGM battery (a-la-KOV) that would be right next to the first one under my couch.

Disappointed because the cost of a second battery is about 3 times the cost of the microwave! ... and I'm not sure how much I'm going to use it (in a location with no AC outlet nearby).

I did find a really nice butane single burner stove with a dozen cans of butane for about $30. This may be a better way to approach cooking in my "overnighter-designed") van! These little cookers get great reviews and are used by restaurants, campers, boaters, etc.

Ed
Ed, not sure I'm following your thinking. Does the microwave stop working or is it the buzzing that you have issues with?

Sorry I just read the first page.....
So I have a couple of thoughts, first your 100w AGM should most likely be closer to 13.0- 13.1 to be fully charged. You may want to check the manufacturers site for info. It should be measured with no load at all and standing for a half hour or so to get a good measure. AGM's need a little better than straight charging coming from an alternator, they really should be on a programmed charge, bulk/absorption/float, to keep them healthy.

the microwave may not like the modified sine wave, in the manual for that model inverter it Says if a device buzzes or cycles on off it probably doesn't like the wave don't use it with this inverter, I'm adlibbing a bit here but I believe that was the gist.
If it has worked without buzzing in the past either the voltage may not be enough or maybe it's going bad from the dirty power??? Does the inverter get warm quickly? Fan come on?

I guess I'm not following why you need another battery. That's just storage of current and it doesn't appear that's the problem unless that battery itself is bad but you said six months. Do you have a 3 cycle charger you could charge the house battery on, I would try to get the battery healthy and then rerun the tests.

I would bet if you ran the same microwave thru a $3k pure sine inverter it would sound just like in the house and not pull near as much energy.
I have the same model and was happy it made me a pot of coffee each morning!

Last question I was confused about, when your isolator kicks on the PV panels are completely isolated correct?
 
#41 ·
Ed, for the bad-weather occasions we want hot water, we plan to use the Caldera Cone in our stainless steel sink.

https://www.traildesigns.com/stoves/caldera-sidewinder-system (Yes, the stoves can be DIY, but these guys have added good value.)

We have used this alcohol system with great success while backpacking. It is incredibly efficient and reliable and the alcohol fuel is readily available at convenience stores. On the trail--rehydrating everything--a $1 bottle lasts two days. For use in the van, we like that the fuel is not pressurized and as clean as it gets.
 
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