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Old 08-17-2015, 10:28 PM   #1
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110v ac

I've been looking at my options for power. How does this sound?

110v ac input at 15A

Goes via a charger into 12v batteries.

12v batteries feed a 12v system that is of no consequence to this topic .

12v batteries feed 4 x 110v sockets.

I found a 1500w inverter for $37 on eBay. That's 1500 normal, 2000 peak.

Looking at my electrics...
700w rival microwave (I'm a big spender) 1050w
Rival electric kettle 1200w
Rival rice cooker (6 cup,) - unknown
Rival quart crock pot - unknown
Rival coffee maker - unknown
Mini fridge - unknown though it says 315,kwh/year

I'm reckoning that as long as I don't double up on 110v appliances on sockets, the 1500w inverter should be fine at 1 per socket. That's 4 inverters each of which can fail so multiple redundancy.

The water heater is probably going to be a 5 to 10 gallon tank with a 12v element, mounted under the bus. I'll probably have electric hot and cold water pumps running off 12v

Lighting is going to be a battery powered hurricane style lamp. I just don't see the need to run excess cables!

110v ac from outside fills the batteries. Later when its economic to do so, I'll add solar panels.

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Old 08-18-2015, 10:00 AM   #2
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remember that you can only use about 50% of a batteries ah capacity before recharging,

even with that figure you can only draw about 10% off a batteries ah capacity at any one time which means if the battery is a 100ah deep cycle then you can only pull 10 amps out of the battery, if you do pull more than 10% then the volts may drop below 12v and the device and or inverter will stop working, some devices can take as low as 11.5 or even 11v before they stop working.

1 amp AC equals 10 amp DC is an important think go remember when deciding how big to make your battery bank.

Judging from the devices you list you will be needing a pretty big battery bank, probably 600ah at least if you only use one or large draw devices at at time,

The best and most efficient way to make a battery bank is to use bus bars, if you just hook the batteries together with cables from + to + and - to - they wont charge efficiently or quickly and the batteries at the end will get used/abused much more than the batteries in the middle.

bus bars make a quick and easy attachment point for your inverters, charge controllers and solar,
fuses are a necessity between the batteries and the bus bar but breakers are even better since you can use them to quickly disconnect power to the bus bar for maintenance and other stuff.

That 700 watt microwave probably pulls about 5 amps Ac which means it will pull 50 amps DC which means you will need at least a 500ah battery bank connected efficiently with big cables. That being said you probably wont run it for more than a few minutes at time so a slightly smaller bank may work but there is not way to know for sure without experimenting.
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Old 08-18-2015, 10:06 AM   #3
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The microwave and kettle are the biggest power hogs. At 1050w, the microwave uses close to 10a. It's dog slow so 15 minutes for a frozen entree is standard!

The kettle, maybe 4 minutes to boiling.

I'm trying to find if my microwave will work off a square wave because if it can then I can whip up a 6kw inverter in a few minutes.
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Old 08-18-2015, 12:25 PM   #4
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When your plugged in, bypass the battery's, and use the power strait from the source.

Also the AC power your talking about is 120 volts, not 110.

If all your getting is 110 volt, there is a voltage sag somewhere in your source power.

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Old 08-18-2015, 06:43 PM   #5
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To be honest I thought US voltage was 110v. This is why the US has to have such ridiculously large cables. In the UK where we have 240v, the cables are thinner because the higher voltage gets way less resistance drop.

It's an evolving idea.

The reason I'd feed mains through the battery is that I can have the batteries supping low amperage mains all day then have the capacity for heavy power usage if and when I need.
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Old 08-18-2015, 08:44 PM   #6
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I'm with Nat; if you're plugged in, power your AC plugs directly while charging the battery bank with a converter/charger. Leave the inverter for when shore power isn't available.
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Old 08-19-2015, 06:00 AM   #7
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240 volt is just two wires of 120.

There is not much savings on conductor size.

The only real benefit to 240 volt AC is that it becomes a balanced load for the transformer.

On this big land mass, we use offsets in the main panel to balance all the 120 volt loads.

After wiring a main breaker panel, I always come back a month later when the home owners are using the power under normal loads, and make sure the two 120 volt legs are balanced. If not, I move a large load breaker or two till it evens out. You don't want the common having to compensate to much.

One customer of mine had a DIY guy wire up his panel before calling me. In a year he burned on leg out of the main breaker on the power pole, just after the transformer. This caused only half his main breaker panel to work.

I rewired his main breaker panel, and now all is well.

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Old 08-19-2015, 06:07 AM   #8
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Should have seen a house I lived in here one time. All the lights flickered. Turned out all circuits ran through just 3 breakers!
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Old 08-19-2015, 10:42 AM   #9
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Quote:
Originally Posted by nat_ster View Post
240 volt is just two wires of 120.

There is not much savings on conductor size.

I think Zephod was referring to the 240vac/50Hz typical wall outlet current in the UK, which uses 3 wires in the same way as we do in North America: hot, neutral and ground. In the same place where you would use 12AWG (3.3mm˛) wiring for our 120vac system you could be using 15AWG (1.7mm˛) wiring for a 240vac system. I can definitely see the benefits. Light weight extension cords, cheaper due to less copper, easier to route...

^ Those wire sizes in mm˛ aren't real, by the way.. I think they'd be rounded up to the next full or half mm˛. It gets the idea across, though.

In North America we get 240vac by combining 120vac, as Nat mentioned.
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Old 08-19-2015, 10:53 AM   #10
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But, yes, bypass the batteries when you're plugged in. There are several benefits to this.

First, you'd be wasting electricity going through the batteries. You'd have a conversion of 120vac -> 12vdc -> 120vac. Those conversions all come with an efficiency loss. Additionally, some of your devices will be doing an internal conversion once again to DC. USB wall plugs will convert 120vac to 5vdc. Laptops will convert 120vac to ~20vdc. Small to medium sized LCD tvs will bring 120vac to ~12-24vdc. That's a lot of converting.

Another issue is that you'll be needlessly taxing your batteries. Batteries have a finite number of times they can be charged and discharged. You can cut this number down significantly by bypassing the batteries.

Lastly, you will be able to run more appliances at once while plugged in if you bypass the batteries. A 1500w inverter won't provide as much power as a direct plug into a 15 or 20amp outlet. You could also still kill the batteries while plugged in if you always went through the batteries. The charger might not put in as much power as you could potentially take out, so it's possible to drain the batteries with heavy use even while plugged in.
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Old 08-19-2015, 11:31 AM   #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, 11:40 AM   #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, 11:44 AM   #13
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Quote:
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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, 11:48 AM   #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, 12:06 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, 12:38 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, 12:43 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, 12:45 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, 12:48 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, 12:55 PM   #20
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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?
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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