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Old 08-19-2015, 12:31 PM   #11
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New thought... Just set up for plugin only and worry about batteries later.

Somebody was saying two 105ah 500cca batteries weren't going to power a 1040w microwave. Their maths didn't coincide with mine!
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Old 08-19-2015, 12:40 PM   #12
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It probably won't power the microwave for very long. The problem is likely going to be voltage drop of the batteries.

Voltages temporarily drop in flooded lead acid (FLA) batteries when they're under load. After they sit for a while the voltage while rise again.

To power that microwave you'll be pulling over 86amps from the batteries. That's over 43amps per battery. Without the battery specs I can't say for sure, but pulling that many amps steady may drop the system voltage below the safe operating level of the inverter, causing it to turn off. Which is a good thing. It keeps the battery cables from starting your bus on fire.

I bet you could do it with 4 of those 105Ah batteries, but, again, we can't be sure without having the battery specs.
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Old 08-19-2015, 12:44 PM   #13
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Quote:
Originally Posted by Zephod_beeblebrox2 View Post
Somebody was saying two 105ah 500cca batteries weren't going to power a 1040w microwave. Their maths didn't coincide with mine!
500cca? Are those deep cycle FLAs?
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Old 08-19-2015, 12:48 PM   #14
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I'm fairly new to electricity for RVs and from what I gathered, DC current isn't = as AC current. In fact: DC current * 10 = AC current.

So if his microwave is 1040W (on a 120V plug) it would pull 1040/120 = roughly 9 AC amps, am I wrong?

So he would need 90 DC amps to make it work, make it 100 DC amps to make it safe.

Is that too close and that's why you say it's not ok (@jazty)? What am I missing?

I'm learning all this so I figured I'd use Zephod_beeblebrox2's situation to understand better. Sorry to thread jack!
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Old 08-19-2015, 01:06 PM   #15
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I'm fairly new to electricity for RVs and from what I gathered, DC current isn't = as AC current. In fact: DC current * 10 = AC current.
^ That's not a proper relationship. For these calculations you can ignore the fact that one is DC or AC. What you want is the voltage. 12 volts and 120 volts in this case.

AC and DC describes the flow of the electricity. You can have any combination of volts and amps in AC or DC. There are high voltage DC power lines in excess of 100,000 volts. It's also not uncommon to see a power adapter around the house that is 12 volts AC.

Quote:
Originally Posted by mrblah View Post
So if his microwave is 1040W (on a 120V plug) it would pull 1040/120 = roughly 9 AC amps, am I wrong?

So he would need 90 DC amps to make it work, make it 100 DC amps to make it safe.

Is that too close and that's why you say it's not ok (@jazty)? What am I missing?

I'm learning all this so I figured I'd use Zephod_beeblebrox2's situation to understand better. Sorry to thread jack!
You've got the right idea here for the most part. You just need to represent the data correctly.
"9 AC amps" should be "9 amps @ 120 volts AC"
"90 DC amps" should be "90 amps @ 12volts DC".

Amps = Watts / Volts

Therefore
Watts = Amps * Volts

and
Volts = Watts / Amps

1040w / 120v = 8.66a @ 120v
1040w / 12v = 86.6a @ 12v

So 86.6amps divided by two batteries = 43.3amps per battery (in a perfect world).

Your calculations are right, I just used more precise decimal places.

Also, here's a simple calculator for doing any of the above calculations: http://www.supercircuits.com/resourc...amps-converter

This is also a handy calculator for sizing cables for your DC install:
http://www.solar-wind.co.uk/cable-sizing-DC-cables.html
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Old 08-19-2015, 01:38 PM   #16
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My question is still complete: he states he would have 105 amp @ 12V DC. wouldn't this be enough or it's too close to maximum amperage?
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Old 08-19-2015, 01:43 PM   #17
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500cca? Are those deep cycle FLAs?
Yes. The type 27 marine deep cycle $89 battery from tractor supply.

I'm not too worried about whether it lasts beyond a year. I can replace it and the key is its chgeap!
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Old 08-19-2015, 01:45 PM   #18
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How you set up your electrical system often depends on how your bus will be used. People who will have access to shore power usually power most everything off 120v AC and use battery power and an inverter for the odd times when they can't plug in. Those who boondock a lot primarily use 12v DC and propane and may use solar panels or a generator to charge the batteries on long boondocks.

Of course, the shore power people tend to use a lot more energy than the boondockers.
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Old 08-19-2015, 01:48 PM   #19
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I'm not 100% sure how I'll be using it. I don't want to do propane. I had a look at generators but the diesel generators seemed outrageous. The harbor freight $100 generator seemed the most practical though it might take a while to charge the batteries.
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Old 08-19-2015, 01:55 PM   #20
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Originally Posted by mrblah View Post
My question is still complete: he states he would have 105 amp @ 12V DC. wouldn't this be enough or it's too close to maximum amperage?
FLA batteries are typically rated as Amp Hours @ 20 hours. In this case we're probably looking at 105Ah @ 20 Hours. That's very different than being able to provide 105 amps continuously.

What that measurement means is that the battery (when new) is capable of providing 5.25 amps @ ~12v for 20 hours before being depleted (105Ah / 20 hours = 5.25 amps).

Usually there will be a chart online for FLAs to show how long they will last under different circumstances. It's not linear. As in, you can't say that since the battery can provide 5.25 amps for 20 hours that it can provide 52.5 amps for 2 hours. That much amperage would probably only be provided for minutes before the voltage dropped too low. Also, as noted before, you don't want to deplete the batteries beyond 50%.

If you want things to be simpler go get a pre-built LiFePO4 battery pack! They can be drained lower than FLAs. I'm not sure about how voltage drop affects them, though.
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