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Discussion Starter #1
I am installing AGM house battery with a very minimal electrical demand, only coupla LED lights, microwave, tea kettle, Maxxair Fan, and am just keeping it simple with using an on/off switch to parallel with my van battery only when I need to for recharging. Not going to be camping overnight in the van, just travelin'. Was wonderin' does it matter when I turn the switch on and join the batteries, like does the engine need to be off and alternator not spinning before I switch it on or can I switch it on or off when going down the highway if I had a mind to? Would a sudden load on the alternator mess up anything? Just not sure how all this works. Thanks.
 

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2016 136WB low roof diesel, converted to an RV by Sportsmobile, TX
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It won't matter when the connection is made - most relay or MOSFET based battery isolators switch in when the voltage on one side exceeds 13.<mumble> volts, allowing the batteries to charge only when the alternator is making power. A brief change of load isn't going to do much - after all, you still have the chassis battery in place to absorb any shift.

I would switch on (joining the batteries) only when the engine is running, to prevent a deeply discharged house battery from preventing the system from starting. The only time I would switch on when the engine wasn't running would be if the chassis battery was too discharged to start the vehicle and you were using the house battery to boost it - and I'd only do that if you had sufficiently meaty wires between the batteries (like 2-0 or better).
 

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What size and type of switch are you planning on using? It would have to be pretty heavy duty! Many here use an isolator because the contacts are rated for high current.

For example, my Battery Doctor is rated at 125A. For about $55, it does what you want to without you having to remember when to turn it on or off. For reverse linking (to jump start the van battery from the aux battery) you just press a button. There is a trigger wire on the BD that you could switch with a light duty switch if you wanted to keep the connection from being made.

A simple solenoid isolator does the same thing.... it lets you control a high current connection with a low current switch.
 

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Discussion Starter #4 (Edited)
The one I was looking at was rated 12v 500 amps I believe. I have this one on my boat that has 4 deep cycle batteries and handles it okay. Still not sure which way I want to go on the isolators. Just trying to keep it very simple.

I have another question that is puzzling the heck out of me. I read on a long thread about Microwaves not working with single battery 105amp and 1500 watt inverters and just don't understand why there's a problem.

I used this calculator https://www.batterystuff.com/kb/tools/ac-to-dc-amperage-conversion-run-through-an-inverter.html

According to the calculator a 700 watt microwave rated at 1200 watt (10 amp AC) would draw 110.4 amps DC per hour through an inverter supposedly taking into consideration the inverter draw and would draw 110.4 amps DC per hour.

I may be misunderstanding the math. Seems like that would be just 1.84 amps DC per minute (110.4 divided by 60). To heat up a cup of water for 4 minutes would only take 7.36 amps DC out of your battery. Am I calculating this wrong? There's somewhere around 80 to 100 amps DC usable in a charged up 105 amp deep cycle battery which seems would be sufficient. Why would a 1500 watt MSW (modified sine wave) inverter not handle that. Would the MSW make make that much difference?
 

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Although the math seems like it should work, 100+ amps is a lot of current. Even though you are only using 7+ amp hours, it is still running the 100 amp current while the MW is running. This leads to a big voltage drop which reduces the effectiveness of the Microwave. I found that starting the engine and connecting to the alternator increased capacity to the point where it would work. Without the extra current from the alternator, I found the voltage drop was too much and the microwave would not function effectively.
 

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2014, 138WB, High Roof, Gas, SW MT
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Hi,
I think you are confusing amps with amp-hrs.

Amperes is the rate at which current is flowing -- in the fluid world, its like gallons per hour. Its the RATE at which electrons are flowing by in your wire.

Amp-hours is an amount of energy -- in the fluid world its like gallons. For example, 10 amps flowing for 2 hours is 20 amp-hrs, and 10 amps flowing for half an hour is 5 amp-hrs. The fact that your battery is rated at 100 amp-hrs means that it can supply that amount of energy -- it has nothing to do with the maximum rate at which you can remove that energy (amperes) from the battery.

"amps per hour" makes no sense. In the fluid world, it would be like saying gallons per hour per hour.

Your 700 watt microwave probably uses about 1000 watts from the outlet, and with an inverter that is 90% efficient, the battery would have to supply about (1000/0.9) = 1300 watts.
The current draw from the battery would be about (1300 watts / 12 volts) = 111 amps. Your microwave will result in this much current being pulled from your battery as long as the microwave is on. It may be that your battery is not capable of supplying that much current -- the manufacturer could probably tell you.

If you run the microwave 1/4 hour, the the number of amp-hrs you will use is: (111 amps)(0.25 hrs) = 28 amp-hrs.

If you try to keep the 100 amp-hr battery from draining below 50% state of charge (as is often recommended for battery life), you can run the microwave for about half an hour before it will be depleted to about 50% state of charge -- probably somewhat less as the 100 amp-hr rating of the battery is for a slow discharge, and the rating will be less for a fast discharge.

Gary
 

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Discussion Starter #7
Gee whiz....I was planning also to use a tea kettle rated at 1378 watts which is even more than the microwave draw. Guess that's not gonna work either. :| I remember when not plugged in to shore power when using the microwave in my old Class C RV we had to run the generator so I suppose this is the reason. It only had one house battery. Never really understood it. No microwave on my boat so never had to deal with anything with that amount of current draw. Trolling motor and bait tanks.

So what's the solution. 2 batteries? as I don't want to rely on running the van engine. Even with two batteries would it still depend on the rate at which the batteries were capable of delivering the current or would just adding 100 more amp hour capacity do the trick?

Even with 2 house batteries would I still need the addition of the van alternator juice?

Would a pure sine wave inverter make any difference? I may need to be rethinkin' some stuff. :| Thanks for the help. Most appreciated!
 

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In addition, the inverter that is required to run the microwave is very sensitive to lowered voltage. That big draw on a battery of that size will result in a voltage drop in the battery and the inverter is likely to shut down due to insufficient voltage. Larger battery sets can put out that 110 Amps and maintain a higher voltage so they work.
 

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Discussion Starter #10
Thanks guys for the info! I promised the admiral a microwave and tea kettle in the van so 2 batteries it will be. :D I may go with a 1500 watt pure sine wave and 2 or 4 gauge wiring. The extra cost later on won't matter, just be enjoying the conveniences.
 

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2016 136WB low roof diesel, converted to an RV by Sportsmobile, TX
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You can run loads like microwaves off batteries, for sufficiently large amounts of batteries.

What is sufficiently large? There are a few factors:
1) total energy capacity in watt-hours. If you want to run a 1000 watt microwave for an hour, you need at least 1000 watt-hours of battery storage. If you have a 12V battery with an ampacity of 100 amp-hours, you have, in theory, 1200 watt-hours of energy storage.
HOWEVER: you cannot use all of that stored energy. Deeply discharging a battery will kill it. What is "deeply discharging"? That depends upon battery chemistry; as a basic rule of thumb, lead acid batteries shouldn't be discharged to less than 50% capacity, lithium batteries shouldn't be discharged to less than 20% capacity. So if you have a 12V 100A-h lead-acid battery, you really can only use about 600 Watt-hours of energy.
2) Maximum discharge rate. There's only so fast you can pull energy out of a battery - try to pull it out faster and you will start losing energy due to what is called "internal resistance" of the battery; you will just heat it up. Again, different batteries have different rates.
A lead-acid starting battery is designed to be able to dump a LARGE amount of current for a short time - in theory, a car battery could run a 1000 watt microwave (about 120A of current at 12V) for a few seconds. After all, starting a car can take upwards of 600A to crank. But think about what happens if you keep cranking for more than a few tens of seconds: dead battery.
A deep cycle lead acid battery is designed to be able to discharge more deeply (hence the name: deep cycle), but at the cost of not being able to deliver as much current. A 100Ah deep cycle really shouldn't be asked to give up more than 50A for a sustained time. This is often referred to as the "C rate" - you may see a battery specified as having a maximum discharge rate of 0.5C - that means 0.5*(battery ampacity in A-hr)/1hr. In other words, you shouldn't discharge the battery any faster than 2 hours to zero (and being a lead-acid battery, don't go below 50%, so only 1 hour at that rate).
A lithium battery has a much greater ability to deliver current: usually better than 1C. I have lithium batteries in my van; they are rated at 3C - so I can theoretically pull all the juice out in under 20 minutes. So if you had a 12V 100Ah lithium battery, you could run a 1000W microwave for about 45 minutes give or take.

So, to take a concrete example: My van has 3 banks of LiMnFeO4 batteries, each 24V @ 100A-h, for a total of [email protected] or 7200 watt-hours. They can deliver over 300A sustained for about an hour. I can run my microwave, my AC, and my electric grill off my 3000W inverter - for a few tens of minutes. But there's no way I could run anything that makes heat (grill, oven, electric heat) for any real length of time - there's just no way given current battery technology to pack that much power into something portable. Even running my air conditioning for more than a couple of hours is pushing it.

tl;dr: anything power hungry isn't going to run very long on battery. Things that make heat are power hungry.
 

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Good explanation on why you need more battery. In fairness I must add you do not need Lithium batteries to do your job. 2 6volt golf cart batteries in AGM or better yet FLA will do fine and those can be had as 215 A-H for $85 each at Sam’s or Costco.
 

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Good explanation on why you need more battery. In fairness I must add you do not need Lithium batteries to do your job. 2 6volt golf cart batteries in AGM or better yet FLA will do fine and those can be had as 215 A-H for $85 each at Sam’s or Costco.
Maybe yes maybe no. I have 2 of the 215AH 6V FLA Sams Golf Cart batteries in series, 2AWG wire, about 3+ feet to HF 1500W MSW Inverter. So total of 215AH @12V and it won't run my 700W (1140 watt draw) Microwave. Start the engine and it runs perfectly. Without the engine running, the voltage drop is too much and Micrwave runs sluggishly and does not heat anything. Tested a 2000W Inverter and still wouldn't work. I needed the engine running to get it to work.
 

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Comparison,

2-FLA golf cart batteries, Sam's
cheap 2000w sine wave inverter
2/0 wire, 6 total feet
20 yr old microwave, wattage unk, label says 11.3 amps, between 1.1 and 1.3 cuft.
Ran normal
voltage drops to 11.4 while running.
Van off.
 

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This sort of stuff makes me glad I cook with butane or hot pot with 12 volts......... Caution Opinion Coming! ......... Microwaves suck when it comes to anything you might eat except popcorn, warming coffee, and softening ice cream. Warm your meal up? You have to open the MW and stir it about three times or it is hot one forkful and cold the next, bread products turn to hot cardboard products, stuff pops and spatters, packages of sucky food abound to put in them with pictures that make you think they will be great and they SUCK.......... Opinion over!...........

So we now need a $1000+ lithium set up to run one? There has to be a better solution.

Anyone know of if the folding Coleman oven works (looks doubtful to me) or is there a better one? How about a butane canister run oven rated for inside? I have seen a propane oven but it is rated for outside only.

Or for those who must have a microwave is there a 12 volt one? The draw would be big but it is to the inverter already! Oh there is one! http://www.roadtrucker.com/12-volt-microwave-wavebox/12-volt-dc-microwave.htm Who knew?
 

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I just took the microwave out of the house just to see what the issue is. It ran just fine on my cheap electrical setup, an unrefined version of yours. Microwave went back it the house.
 

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Uncle George on your wiring, 2 or4 gauge is small I note someone used 2/0 that is a big difference. Heavy 2/0 wire is only needed from the batteries to the inverter and that is usually only a short distance so that is no place to cut corners. Too many amps traveling in too small a wire creates heat and melted wires cause fires. Wire being oversized will never hurt you, just your wallet. Newer digital microwaves may only be happy running off of a pure sine wave inverter.
 

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Discussion Starter #19
Well after my wife and I saw the reality of the very high cost to only occasionally use a microwave while travelling just to heat up something to eat, reheat some coffee and pop some corn that it's just not worth it for us. We decided on a RoadPro 12 volt portable stove like the truckers use to heat some eats. Slower but much simpler and mucho cheaper. My uncle used one for years on the road as a cross country trucker. Just have to be smart about when to start heating up something just like the truckers do. Looking into something 12v to heat some water and that should take care of our traveling needs. Bags of tasty popcorn can be found practically anywhere there's food sold along the highways. Gives me another reason to stop and get out and stretch the legs, back, hips, neck, wrists, ankles, butt and eyelids.:D thanks for y'all's information and insight on this issue.
[ame]https://www.amazon.com/RoadPro-12-Volt-Portable-Stove-Black/dp/B00030DLEE/ref=sr_1_3?ie=UTF8&qid=1516587970&sr=8-3&keywords=roadpro+12+volt+portable+stove[/ame]
 

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When we were teenagers popcorn was made in a heavy saucepan with oil and the corn came from a jar you bought in the store. It was good. Why we can't pop it now that way on top of a butane cooktop I don’t know.

See: https://video.search.yahoo.com/sear...=c91a91aa76290ea50093b74e0f993c02&action=view

[ame]https://www.amazon.com/GS-1000-Portable-Automatic-Ignition-Carrying/dp/B01MYGMO6M/ref=sr_1_3?ie=UTF8&qid=1516595225&sr=8-3&keywords=butane+stove[/ame]
 
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