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Old 05-29-2015, 08:53 AM   #1
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Help calculating electrical?

I've been reading an awful lot about dry-camping, inverters, batteries and residential fridges and frankly, I'm more confused now than ever. Our RV fridge is acting like it's on it's last legs and we are considering a residential replacement, but we don't want to be unable to dry-camp because of it. And I can't seem to wrap my head around the calculations to see if it's feasible. Care to help?

We have 4 6-volt 240AH batteries, and a 2000 watt inverter. We can recharge using our 7500watt generator, and I'd be willing to consider some solar to supplement, if it makes sense. The fridges we are looking at run about 650kWh/year or about 75watts per hour (at 120v, of course). My EMS says that we currently run about 2a draw when dry-camping between lights, water pump, a little tv watching, and just general overhead (converter inefficiency, standby modes, etc). I assume the 2a is 120v since it's the same readout we get when plugged into shore. Beyond that I am really lost. Do we have enough juice to run our current loads and a residential fridge? If so, for how long, roughly? Any help is much appreciated.
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Old 05-29-2015, 09:17 AM   #2
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Remember that watts are watts, whether 12V or 120V. So, let's determine how many amps the fridge will draw:

Power (watts) = voltage x current

Current = watts/volts. For your fridge that is 6.25 amps ( call it 7 with losses)

Over a 24 hour period you will need to provide 168 amp-hrs to run the fridge.

You don't want to run your batteries lower than 50% capacity, so divide your total battery amp-hr capacity by 2.

Try the same calculation for solar panels. 100 watt panel in 6 hrs of optimum sun yields 50 amp-hrs replenishment.

So, realistically you are looking at maybe 500 amp-hr batteries and 400+ watts solar

You need to look at the size of the battery charger to compute how much running gen set will provide.
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Old 05-29-2015, 10:53 AM   #3
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I think you could get by using your current batteries and 400 watts of solar, supplemented by your generator. You will want an efficient converter/charger to charge as fast and completely as possible. Interesting that they give the KW per year figure, that makes it relatively easy to calculate the power required by the refrigerator. If it really uses 75 watts/hr, then that is 144 amps per day. That seems high compared to what most are using for residential refrigerators in RVs. What make and model are you looking at?
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Old 05-29-2015, 01:37 PM   #4
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One other rather pricy option (but these days all the options are pricy) is a new type of compressor Fridg of which both Dometic and Norcold have models.. It uses 12/120 volt No propane at all

LESS THAN 50 WATTS when it is running. If the link works,, here is one of em

http://www.americanrvcompany.com/Nor...iler-Camper-RV
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Old 05-30-2015, 07:32 AM   #5
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We have very few options that will fit our fridge cabinet, so this is not the most efficient fridge available at 75watts, I have seen some as low as 55watts.

I'm still fuzzy on the math. If I have 240aH of 12v available (i.e. not discharging beyond 50%) that's 2880 watts. The fridge draws an average of 75 watts an hour at 120, so that's 750 watts at 12v, right? So 2880 divided by 750 is less than 4 hours before I start over-tapping my batteries?! That can't be right, can it?

I ran an experiment yesterday, running an extension cord into the house from the moho so I could plug in my existing home fridge. It ran a good 9-10 hours with the Xantrex reporting the battery state as normal, and ran it to 11 hours, into the "low" state, before plugging back in. I was curious to see when the safeguard would kick in and shutdown the system, but some Facebook comments got me worried about ruining my batteries so I didn't let it get that far. Anyhow, if the numbers above are right, I wouldn't be able to get 11 hours even if the inverter were 100% efficient and there were no other drains and the batteries were completely discharged. So I don't get where I'm going wrong.
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Old 05-30-2015, 07:39 AM   #6
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650kwh/365 days = 1800Wh per 24 hr cycle
1800 Wh/12v /0.85 = 176Ah per 24hr cycle. I'm assuming 85%efficiency for a high quality true sine wave inverter. You'll needed 400-600 Watts of solar to keep up depending on direct sunlight hours.

Do you have AGM batteries to support the high current draw from the inverter? Look inside the fridge to find the rated current draw.

What's wrong with your RV fridge?
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Old 05-30-2015, 08:05 AM   #7
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If the fridge draws 75 watts, it draws 75 watts. There is no conversion for voltage.
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Old 05-30-2015, 08:45 AM   #8
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There is no way a household fridge draws 75watts while the compressor is running. Most fridges pull 10-12A at 120vac, which is in the neighbourhood of 1300Watts. That's why in residential wiring you need a plug and circuit just for the fridge.

A 1300 W load on the inverter would pull about 130A from the batteries. A pair of flooded batteries can't support that load and would suffer from a significant voltage drop, increasing the current. You either need a very large flooded battery bank or use AGM batteries which have much lower resistance and can deliver move current and suffer repeated deep discharges.

https://handybobsolar.wordpress.com
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Old 05-30-2015, 09:11 AM   #9
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You are contradicting what the OP just tested.

The 75 watt figure is usage per hour. The fridge does not run steady.

Most fridges use 3 amps or less, running. The start current may be 3 times that, still under the surge capacity of a 1000 watt inverter.

I actually run a 7.6 c.f., household fridge in my boat. The start amps is 35 amps DC and the running amps are 8 DC, thru my inverter
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Old 05-30-2015, 12:00 PM   #10
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Thanks for the feedback, guys. I'll try to respond to the new questions.

What's wrong with your stock fridge? Well, a few things. Most important is it doesn't work very well. On a warm day it cannot get down into the safe-zone, so I'm really afraid of how it will respond to real summer heat (we've only owned the coach for a couple of months). I've had a tech look at it and he said everything is working and the only way to improve it is a cooling unit swap, nearly $2000. And even then, the design of the fridge compartment (in slide, sidewall vent, under awning = poor air circulation) may not allow sufficient cooling. Also, my wife is very concerned about the fire risk. And finally, it's not very big for the amount of space it takes up.

Are your batteries able to handle that current load? Honestly, I didn't even think about something like that. The four 6v batteries and 2K inverter are part of the stock system in the coach, so I just assumed they knew what they were doing when they installed it. I assume the inverter won't call for more juice than the batteries can give, or the cables conduct. Maybe I'm being naive. As I mentioned above, though, I did run an even bigger fridge for 11 hours off of the current setup.

There is no way a household fridge draws 75watts while the compressor is running. That may very well be. I'm getting my info off of the energy certification label (the big yellow ones required on major appliances) that says this will cost you $XX per year to operate, and then just below that it says 650 kWh/year. So 650,000/365/24 = watt hours. I think. 75 watts average per hour.

So just to sum up, if Marcham's calcs are right, the fridge I'm considering will consume 176 amp hours per day of 240 available amp hours, leaving me 64aH for other uses (lights, water pump, inverter loss, etc), without dipping into my 50% reserve. Assuming my batteries are capable of delivering their rated 240aH. Is that right?

Thanks again for the help.
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Old 05-30-2015, 12:41 PM   #11
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You got it.

One other point, if you run your generator for 3 or 4 hours a day, you are only using you batteries 20 to 21 hours, leaving a bit more in reserve.
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Old 06-04-2015, 11:34 AM   #12
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I have now done several rounds of depleting, charging then equalization. Specific gravity shows that the front two batteries (easiest to maintain) are good, the back ones are near dead and plumb dead! I've heard that you should always replace all 4 at once, but does that make real sense, or is it a myth, like storing batteries on a concrete floor? If the front two are taking a full charge how can they drag down two new ones? TIA
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Old 06-04-2015, 11:40 AM   #13
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Quote:
Originally Posted by twinboat View Post
You got it.

One other point, if you run your generator for 3 or 4 hours a day, you are only using you batteries 20 to 21 hours, leaving a bit more in reserve.
The generator will also be replenishing the batteries while powering the refrigerator while it is running.
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Old 06-04-2015, 12:06 PM   #14
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It's odd that two are that much worse. I'd start by checking the connections between the batteries. If two are bad go for all 4 given that you want to change the refrigerator. You might want to just stick with 2 as long as you have the old refrigerator and get a sense of what you are setting yourself up for.

If you get into the habit of starting the generator for a couple of hours before breakfast and again before dinner so the preparation is covered by run time you greatly reduce the battery hit because your heavy draws are covered by the generator. Afternoons are also the peak temperature if you want AC. Batteries need hours to charge for the chemical processes to take place. Solar during the day will let you reduce the run time a bit unless you go big on the solar.
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