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Old 08-23-2016, 02:37 PM   #1
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THOR #3852
Adding Coach batteries

I've been thinking of adding 2 coach batteries for a total of 4 to extend the time running on battery without recharging. My 2 batteries do OK but overnight they drain down just running the residential size refrigerator. Can anyone give me advice on how much adding 2 batteries will help? Can you run the AC units on battery?
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Old 08-23-2016, 04:22 PM   #2
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We added 2 additional house batteries to extend running time for the residential fridge and electronics. Even with 2 more batteries, you will not be able to run the AC on batteries since the AC units are not on the circuit that the inverter provides power to. Even if you did, you would need a lot of batteries just to start the AC unit(s) and it would be brutal on them.


Another option is to consider solar panels to keep your batteries charged during the day and ready for overnight. We added 400 watts of solar and it makes a huge difference when off the grid. But we still need to run the generator for AC, the solar charging can't keep up.
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Old 08-23-2016, 07:32 PM   #3
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Our friends have 4 216 Ah batteries and still need to run the gen set for 5 hours every day. Based on the amp draw I measured that is pretty close to what Onan comes up with based on their example. The refrig draw via the inverter was ~ 17 amps. When the gen set was running the inverter/charger was putting out 60 amps. 60 amps would be about 720 watts of solar. Their 432 Ah battery bank would require a minimum of 500 watts of solar.
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Old 08-25-2016, 01:46 PM   #4
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Originally Posted by BHoraist View Post
I've been thinking of adding 2 coach batteries for a total of 4 to extend the time running on battery without recharging. My 2 batteries do OK but overnight they drain down just running the residential size refrigerator. Can anyone give me advice on how much adding 2 batteries will help? Can you run the AC units on battery?
I am not familiar with your coach, but are you running your refrigerator on AC powered by the inverter when it drains your batteries overnight? Running the refrigerator on gas shouldn't drain the batteries overnight unless the batteries are not holding a charge.
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Old 08-25-2016, 02:05 PM   #5
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Quote:
Originally Posted by Metalman
Running the refrigerator on gas shouldn't drain the batteries overnight unless the batteries are not holding a charge.
He is talking about a residential fridge: There is no "running on gas" option.
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Old 08-25-2016, 02:11 PM   #6
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17 amps to run a residential refrig?
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Old 08-25-2016, 02:20 PM   #7
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He is talking about a residential fridge: There is no "running on gas" option.
He wrote "residential size" refrigerator, so I figured it was a large RV refrigerator not a residential unit. I can't imagine running a residential refrigerator on a bank of two fully charged deep cycle batteries over night
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Old 08-25-2016, 04:20 PM   #8
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I believe the Challenger has a residential fridge (similar to my Hurricane.) The two batteries will do a decent job overnight with the inverter, but the batteries would require a recharge prior to the next night's usage. 4 batteries increases the amp hours so the batteries will have far more charge left in the morning and maybe last another day or two if dry camping. That is why we added solar charging to our Hurricane with a 4 battery setup. We can dry camp for a while as long as we get some decent sunlight without having to run the generator.
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Old 08-25-2016, 05:15 PM   #9
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I am curious why some larger RVs have residential refrigerators. I understand the convenience when AC is available but wonder how batteries are able to power one overnight along with running the furnace, water pump, TVs, lights, etc. While running down the road will the engine alternator provide enough current to operate all DC electrical needs including the residential refrigerator?
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Old 08-25-2016, 06:12 PM   #10
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I am curious why some larger RVs have residential refrigerators. I understand the convenience when AC is available but wonder how batteries are able to power one overnight along with running the furnace, water pump, TVs, lights, etc. While running down the road will the engine alternator provide enough current to operate all DC electrical needs including the residential refrigerator?
I have never owned a motorhome until we bought our Four Winds a couple months ago. However I've had travel trailers or truck campers since the 1970's. In all that time I have never heard of a residential refrigerator in an RV unless it was a modification to a permanently moored trailer.

So don't feel bad. I was shocked when I had this same exchange in another thread a couple weeks ago. To me, It makes no sense whatsoever in any mobile RV that would not have shore power at all times.

Then, after I absorbed it, I said to myself, "I'd never do that, but I don't have to, so I don't need to understand why someone else would."

Ken

Sorry for the detour
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Old 08-25-2016, 08:11 PM   #11
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Any RV that has a residential fridge also almost always has an inverter. The inverter converts 12V DC into 120V AC. Not the most efficient use of power, but one that has its advantages.


A residential fridge of the same space actually has more storage inside than the equivalent absorption fridge. Most residential fridges also perform better in very warm weather where their absorption fridge brothers don't do so well (especially on the sunny side of the RV.) Residential fridges also cool much quicker when turned on. No need to pack them with ice or start them up a day in advance of your travel date.


So like most things RV, there is a tradeoff. I don't have one of the 21.5cf residential fridges with water and ice in the door, just a simple 10.1cf residential that is WAY better then my previous Norcold 6cf absorption fridge. Does it take more battery power to run? Yup. Does it have the option to run on LP? Nope. But I am OK with that. My ice cream stays frozen and my beer stays cold.


To each their own! Happy trails!
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Old 08-25-2016, 08:41 PM   #12
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Originally Posted by oldbird View Post
17 amps to run a residential refrig?
I would assume he meant 17 Amps at 12 Volts, so it only works out to about 200 watts of power going to inverter. The inverter output would likely be around 80 to 90 % of that, so around 160 to 180 watts going to fridge at 120 Volts.


Still, that's a lot of power for two standard-size batteries. If the fridge ran continuously (and I doubt it would), 17 Amps overnight would require close to 200 Amp-hours, and limiting batteries to 50% depth of discharge, it would take 4 batteries just to make it through the night.

If 17 Amps is correct, then it would go through 2 average-size (+/- 100 A-h at 12 Volt) batteries in a hurry unless the fridge's cycle duty is very low.
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Old 08-25-2016, 08:52 PM   #13
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Our friends have 4 216 Ah batteries and still need to run the gen set for 5 hours every day. Based on the amp draw I measured that is pretty close to what Onan comes up with based on their example. The refrig draw via the inverter was ~ 17 amps. When the gen set was running the inverter/charger was putting out 60 amps. 60 amps would be about 720 watts of solar. Their 432 Ah battery bank would require a minimum of 500 watts of solar.
Running a 4,000 or 5,500 watt generator for hours to charge at 720 watts seems inefficient if that's the main load. If they were going to run generator anyway to run air conditioner then it seems more efficient.

To me it seems clear that if owners add extra batteries, they should also increase their converter capacity proportionally (or higher) to avoid running generator at very low loads for hours at a time. I would just make sure not to exceed battery charging rate.
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Old 08-25-2016, 09:01 PM   #14
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I don't know about the 17 amps. If it is at 120 volts, it's way too much, considering refrigerators should work on a 15 amp circuit, and even when plugged into a 20 amp circuit 17 amps would be more the the standard 80% load. A refrigerator operates on a variety of amperages depending on what is running in addition to the compressor.
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Old 08-25-2016, 09:02 PM   #15
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Originally Posted by jpmihalk View Post

....cut....

So like most things RV, there is a tradeoff. I don't have one of the 21.5cf residential fridges with water and ice in the door, just a simple 10.1cf residential that is WAY better then my previous Norcold 6cf absorption fridge. Does it take more battery power to run? Yup. Does it have the option to run on LP? Nope. But I am OK with that. My ice cream stays frozen and my beer stays cold.


To each their own! Happy trails!
Agree -- personal preferences. I would prefer all-electric if not too expensive, so propane fridge would not be my first choice.

I'm curious what brand of fridge you have at 10.1 cf, and if you know how much power it uses. That seems like a good size that may fit in a standard RV fridge space. For us 10 cf would be plenty large.
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Old 08-25-2016, 09:08 PM   #16
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I don't know about the 17 amps. If it is at 120 volts, it's way too much, considering refrigerators should work on a 15 amp circuit, and even when plugged into a 20 amp circuit 17 amps would be more the the standard 80% load. A refrigerator operates on a variety of amperages depending on what is running in addition to the compressor.
If you look at any modern fridge's estimated power usage over an entire year, we can see that it can't be 17 Amps at 120 Volts on average -- not even close to that. This type of data is commonly published.

If I was going to put one in an RV to run off batteries, I would also make sure it was as efficient as possible.
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Old 08-25-2016, 09:23 PM   #17
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Originally Posted by BHoraist View Post
I've been thinking of adding 2 coach batteries for a total of 4 to extend the time running on battery without recharging. My 2 batteries do OK but overnight they drain down just running the residential size refrigerator. Can anyone give me advice on how much adding 2 batteries will help? Can you run the AC units on battery?
Doubling battery capacity will increase run time by a little more than double -- everything else being equal.

As to air conditioners, there are motorhomes set up to run smaller efficient air conditioners off batteries, but they won't run long. With enough "regular" lead batteries you may get an hour or two.

There are some Class Bs that offer huge lithium battery banks (up to 1600 Amp-hour) that can run much longer, but their cost is prohibitive (for my taste anyway). With enough money you can certainly run an AC overnight, but for now it's cheaper to run a generator.
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Old 08-25-2016, 11:00 PM   #18
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Power Draw of Residential Refrigerator

Attached is a graph of 120 volt AC power usage of the 11 cubic foot Whirlpool refrigerator that came standard in my Miramar. The left axis is watts, the vertical bars are hours. This test was run to see how much power the refrigerator draws and what the cycling pattern of that draw was. The numbers are most likely within 10%. This was from a warm start with a case of water in it for a heat load and the door was not opened during the test and no icemaker usage. During our first boondocking trip we noticed that the refrigerator began to make different noises, like it was having trouble starting, after about 6 hours with no 120 volt power.
As you can see it takes a couple of hours to cool down and then begins a cycle about once every 48 minutes. It uses about 200 watts of power during cool down, then around 170 watts during each cycle, but it never drops below about 70 watts. So using a factor of 10 this would translate into 20 amps, 17 amps and 7 amps respectively of DC power at 12 volts. Also worth noting is that the ice maker's heater draws 185 watts when it is on.

In answer to the question about load presented while going down the road the Ford V10 has a 175 amp alternator so that should not be an issue.
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Old 08-25-2016, 11:29 PM   #19
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Wrench, Thanks for posting the graph. You mentioned a "factor of 10". I don't understand why it was used in your calculation. The inefficiency of the inverter must be considered too. I didn't realize the Ford V-10 has a 175 amp alternator.
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Old 08-25-2016, 11:36 PM   #20
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Converting AC Watts to DC Amps

The reasoning is that, say, the inverter draws 10 amps at 12 v = 120 watts. But if you take off some (20/120 or about 16%) for losses in the inverter, you get only get 100 watts of AC out. So for ease in doing the math just say DC amps (at 12v) = AC watts (at 120v) divided by 10.
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