Can I run my microwave off of battery power?

[3TV]

I'm almost afraid to ask this, because this might just break the space-time continuum and cause irreparable harm to the ozone layer.
But, ... using my 2000 watt inverter, can I run my 1000 watt microwave oven off of my dual 6 volt batteries long enough to cook a frozen TV dinner (4 minutes).

[MEXICOWANDERER]

It can be used with caution, in expressing CCA. 220 ampere hours alone is almost useless for inverter energy conversion. A minimum voltage threshold value must enter the fray. During the exercise a minimum voltage must be maintained. Duration of final voltage is a function of minimum volt threshold and ampere hours

[ktmrfs]

pianotuna wrote:
ktmrfs,

It is not a different number of batteries--it is the cell count.

For the same capacity in amp-hours, there are twice as many cells in 12 volt jars. Therefore the output demand per cell is 50% of that on 6 volt jars. The plates may often be thinner, too, which makes them more reactive and more easily able to output high loads for short time periods.

agreed. just saying it a different way. In the case of pair of GC-2's @ 200AH, ALL the current is supplied by one big 12V battery.

Parallel two 100AH 12V batteries and each battery only supplies 1/2 the total current. Couple that with the case that 12V typically have much lower internal resistance due to plate thickness and construction and 12V win for less voltage drop in the case of high current draw.

[pianotuna]

ktmrfs,

It is not a different number of batteries--it is the cell count.

For the same capacity in amp-hours, there are twice as many cells in 12 volt jars. Therefore the output demand per cell is 50% of that on 6 volt jars. The plates may often be thinner, too, which makes them more reactive and more easily able to output high loads for short time periods.

[full_mosey]

ktmrfs wrote:

...
The other downside is that for similar AH you will typically have twice as many paralleled 12V jars. Hence 1/2 the current from each jar. All adds up to 12V much better for high inverter load.
...

My pair(2) of 135AH AGMs exceed the 220AH of 2 x GCs. Three(3) of them at 405AHs would be a good match for 4 x GCs at 440AHs.

HTH;
John

[road-runner]

3 tons wrote:
Roadrunner, I believe you’ve identified your problem - ditch the 2 AWG cables because (at only 12v) they are causing a serious current bottleneck

3 tons

I appreciate the discussion. We looked at the same information and came to opposite conclusions. Time to move on!

[ktmrfs]

road-runner wrote:
After doing some research I've concluded than dual flooded GC2s are a poor choice for use with a large inverter. The deep cycle characteristic, a good trait for general RV use, goes hand-in-hand with higher internal resistance. High internal resistance limits how much voltage the battery can supply under a high load. Two batteries in series serves to increase the internal resistance even more. For calculations I'm assuming a 1,500 watt AC load, which would lead to a 139 amp battery load at 12 volts (90% inverter efficiency assumed). Resistance values are in milliohms.

What's the internal resistance of a GC2 flooded battery? Hard to find an answer! Trojan publishes this value for their AGM batteries, but not for the flooded deep cycle batteries. I found one online source that says the value for a brand new single GC2 is between 30 and 60 mOhms. Another source says a value of 10 mOhms or lower for a good condition 12 volt starting battery. The Trojan spec for a GC2 AGM is 1.9 mOhms, or 3.8 mOhms for 2 in series. Another source reported between 2 and 4 mOhms for a 12 volt AGM battery. Using the small-load/big-load test, I calculated 15.6 mOhms for my series GC2 setup. Pretty far from the 30 to 60 value for a single battery that I found online.

Using my measured value of 15.6 mOhms, the voltage drop from battery internal resistance will be 2.17 volts at 139 amps. My inverter's lowest cutoff setting is 10 volts even, and at 10 volts the current will be up to 167 amps with the voltage drop even more. A death spiral.

So unless I've made a math error or measurement error (always possible), running a 1,500 watt inverter load from dual flooded GC2s just doesn't work, math-wise. While researching I accidentally ran across a few posts that said (paraphrased) "I upgraded to dual GC2s and now I can't run the microwave from the inverter".

I didn't mention MSW/PSW or cable size because I think it's not the issue for me. The inverter is PSW. The cable size is awg 2, and is a minor factor considering the 2 foot cable run from the battery. awg2 is roughly twice the resistance of 2/0, and triple the resistance of 4/0. So the 2 foot awg 2 run is equivalent to a 4 foot 2/0 run, or six foot 4/0 run. Adding a foot for the series connector cable, 5 feet of awg 2 copper has a resistance of 0.78 mOhms, a minor factor compared to the internal resistance of the battery. At 139 amps this contributes 0.11 volts to the total voltage drop.

basically hit the nail on the head. Even with 4/0 cable 2ft that I have 2GC are not well suited to heavy inverter load. If the batteries are near full charge you can run a load for a reasonable period of time, but if they are below about 80%SOC often can't even get the load started. So unless you want or can always start with a fully charged battery bank, a pair of GC isn't a good choice. Been there done that.

4 do great, can start the load (100+A) even near 50% SOC. Now different inverters have different low voltage shut down points so some work better (or worse) than others.

The other downside is that for similar AH you will typically have twice as many paralleled 12V jars. Hence 1/2 the current from each jar. All adds up to 12V much better for high inverter load.

At a RV show I spent time talking to a trojan rep about batteries and loads. First question he asks users is "what do you want to run off the batteries?" If it ends of being high current draws, and that being most important his advice is 12V deep discharge. if the answer is low to moderate loads and wanting high cycle life and deep discharges then GC wins.

[3_tons]

Roadrunner, I believe you’ve identified your problem - ditch the 2 AWG cables because (at only 12v) they are causing a serious current bottleneck - I run my 11kbtu air cond off of two Costco sixers for 45 minutes to get to about 60% SOC using 0004 AWG... This inductive surge load is far greater than a 1500w MW and runs from a ProSine 2.0 with 4500w surge...Quite honestly and at my level BEST, I really can’t see how you are having a battery issue unless they are a bit older...My problems totally dissappered when I switched from 2 AWG to 0004 - with 2 AWG, I was actually STARVING the poor inverter...Been there and done that...This is also why I silver soldered the copper lugs...

3 tons

[road-runner]

After doing some research I've concluded than dual flooded GC2s are a poor choice for use with a large inverter. The deep cycle characteristic, a good trait for general RV use, goes hand-in-hand with higher internal resistance. High internal resistance limits how much voltage the battery can supply under a high load. Two batteries in series serves to increase the internal resistance even more. For calculations I'm assuming a 1,500 watt AC load, which would lead to a 139 amp battery load at 12 volts (90% inverter efficiency assumed). Resistance values are in milliohms.

What's the internal resistance of a GC2 flooded battery? Hard to find an answer! Trojan publishes this value for their AGM batteries, but not for the flooded deep cycle batteries. I found one online source that says the value for a brand new single GC2 is between 30 and 60 mOhms. Another source says a value of 10 mOhms or lower for a good condition 12 volt starting battery. The Trojan spec for a GC2 AGM is 1.9 mOhms, or 3.8 mOhms for 2 in series. Another source reported between 2 and 4 mOhms for a 12 volt AGM battery. Using the small-load/big-load test, I calculated 15.6 mOhms for my series GC2 setup. Pretty far from the 30 to 60 value for a single battery that I found online.

Using my measured value of 15.6 mOhms, the voltage drop from battery internal resistance will be 2.17 volts at 139 amps. My inverter's lowest cutoff setting is 10 volts even, and at 10 volts the current will be up to 167 amps with the voltage drop even more. A death spiral.

So unless I've made a math error or measurement error (always possible), running a 1,500 watt inverter load from dual flooded GC2s just doesn't work, math-wise. While researching I accidentally ran across a few posts that said (paraphrased) "I upgraded to dual GC2s and now I can't run the microwave from the inverter".

I didn't mention MSW/PSW or cable size because I think it's not the issue for me. The inverter is PSW. The cable size is awg 2, and is a minor factor considering the 2 foot cable run from the battery. awg2 is roughly twice the resistance of 2/0, and triple the resistance of 4/0. So the 2 foot awg 2 run is equivalent to a 4 foot 2/0 run, or six foot 4/0 run. Adding a foot for the series connector cable, 5 feet of awg 2 copper has a resistance of 0.78 mOhms, a minor factor compared to the internal resistance of the battery. At 139 amps this contributes 0.11 volts to the total voltage drop.

[MEXICOWANDERER]

2-volt cells are even more disproportionate. The two instances where I wail is reheating of liquids and baking a potato,

One owner did not want to go to AGM because of differences of alternator charging voltage so he was set up with 3 engine starting batteries in parallel for his bus. Dedicated to a psw inverter solely for his microwave. He loved it. SLOOP.

[ktmrfs]

road-runner wrote:

3 tons wrote:

As I stated, its usually NOT the GC’s that are the problem, its the equipment and/or the overall configuration - Inverter wattage is only one part of the equation, the equipment needs to be compatible in all ways to run optimally...

I can't speak for usually because I'm only a sample of one. I can say from first hand experience that the dual GC Trojans ARE the problem in my case. The battery cables are about 2 feet long, and I lose a couple of tenths of a volt in them. A few seconds after the microwave load hits the inverter, it kicks out from low battery voltage. I agree with the suggestion of a lower power microwave, but it's not worth it to me to rip out the nice factory installation. And with many newer rigs not having a gas oven, the selection of convection/microwave combo ovens is extremely limited. It's quite possible that there are none at all with a microwave cooking power under 1,000 watts.

your experience with a pair of GC is similar to mine and many others. I have a 2ft run of 4/0 to the PSW inverter, the issue with 2GC isn't as much the cables as a flooded cell GC internal resistance. Worked fine at 1000W as long as battery SOC was in the 80+% area. with 4 GC-2's it works down to 50% SOC. This is an example where the GC is NOT the ideal solution to the problem. for high draws 12V jars are much better. parallel banks reduce current/jar and the lower internal resistance limits voltage drop. But we with 4GC-2 I have what I need for occasional high draw use and maintain the life and ability to go to 30%SOC for many many cycles when needed w/o damage to the batteries.

[3_tons]

Roadrunner, My use of the word ‘usually’ is positively anecdotal, I base it only from issues I see from time to time here regarding the inabilities of GC’s (NOT!!)...Either way, you didn’t mention the cable size or if inverter is MSW or PSW - I had the same bottleneck problem due to AWG 2 cable size so switched to 0004 which since has done away with low voltage alarms...

3 tons

[pianotuna]

Hi Mex,

The Magnum attempts to NOT track input voltage by drawing greater and greater amps. However, with large loads, output voltage does sag over time, at least with my unit.

[MEXICOWANDERER]

Not only THAT

I would love to be introduced to an INVERTER that does not TRACK

The AC voltage TRACKS in parallel with incoming DC voltage. Higher battery voltage means higher AC exit voltage under load.

On MSW inverter power My commercial 1875 watt microwave set to "9" maximum cooks food slightly faster than a home microwave does on it's highest power setting. BUT GUESS WHAT?!?!?!?!?

Modified sine wave throws the electronic pad timer wacky. I had to connect MSW input to a wind up timer then max control electronic timer to be within 10% accuracy.

Lessee: 117% cooking power times 116% slow clock means = (calculus) "x" number minutes or maybe not...

Where's the *********** frying pan.

[road-runner]

3 tons wrote:

As I stated, its usually NOT the GC’s that are the problem, its the equipment and/or the overall configuration - Inverter wattage is only one part of the equation, the equipment needs to be compatible in all ways to run optimally...

I can't speak for usually because I'm only a sample of one. I can say from first hand experience that the dual GC Trojans ARE the problem in my case. The battery cables are about 2 feet long, and I lose a couple of tenths of a volt in them. A few seconds after the microwave load hits the inverter, it kicks out from low battery voltage. I agree with the suggestion of a lower power microwave, but it's not worth it to me to rip out the nice factory installation. And with many newer rigs not having a gas oven, the selection of convection/microwave combo ovens is extremely limited. It's quite possible that there are none at all with a microwave cooking power under 1,000 watts.