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Old 05-02-2016, 10:26 AM   #1
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How long should the house batteries last?

I know this kind of like asking how high is up but...

What is a reasonable expectation for the amount of time you can boondock on batteries with having to start any engines?

My rig is a 2015 Itasca Suncruiser with Residential Reefer. The battery bank is 4 12vs in parallel each with 200AMPs of reserve capacity (which I think means 328amp-hours).

I've kind of done some arithmetic assuming moderate use of the reefer, the lights (mostly leds) etc and I think I should doing a whole lot better than I'm doing.

But there is nothing like real-world. Anybody out there with similar equipment and experience? How long did you go before you lost the inverter at LBCO?

Thanks,
Jerald
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Old 05-02-2016, 02:11 PM   #2
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A lot of it will depend on your usage (known and unknown) ... really, you're going to have to dig it and do some testing ... this is a good video on how to get started:

RV Residential Refrigerator - How Much Power Does It Use

So many factors .. nobody here will be able to give you a reasonably accurate answer ..
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Old 05-02-2016, 05:08 PM   #3
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Quote:
Originally Posted by jrpend View Post
I know this kind of like asking how high is up but...

What is a reasonable expectation for the amount of time you can boondock on batteries with having to start any engines?

My rig is a 2015 Itasca Suncruiser with Residential Reefer. The battery bank is 4 12vs in parallel each with 200AMPs of reserve capacity (which I think means 328amp-hours).

I've kind of done some arithmetic assuming moderate use of the reefer, the lights (mostly leds) etc and I think I should doing a whole lot better than I'm doing.

But there is nothing like real-world. Anybody out there with similar equipment and experience? How long did you go before you lost the inverter at LBCO?

Thanks,
Jerald
If you do a search there are several threads on RR and boondocking...without a lot of solar and more batteries you will more than likely be running your generator several times a day to maintain your battery bank at 50% + capacity.
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Old 05-02-2016, 06:21 PM   #4
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Jerald,
Please double check those battery capacities. Reserve capacity is usually expressed in minutes at a certain Ampere draw. Also 12 volt batteries usually are about 100 Ampere hours, usually measured over 20 hours. This is called the 20 hour rate. I'm not saying you're wrong, but your numbers don't seem normal to me
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Old 05-03-2016, 01:29 PM   #5
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Every appliance has a data-plate that shows the ampere draw per hr. Locate all those and record the ampere draw. Since you don't want to shorten the life-cycle of your battery bank figure on not drawing them below 50% of total capacity.
As to amp-hr compared to reserve capacity, this website explains everything Understanding Battery Ratings | thesolarpowerexpert.com
As you see it's not as simple as it sounds.
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Old 05-03-2016, 02:33 PM   #6
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Residential Reefer:

I am a boater in the winter with a 12 volt refrigerator and a separate 12 volt freezer. Those are my most significant loads on the batteries at night (solar in the daylight makes it a non-issue).

Are you running an inverter with a 120 volt refrigerator which is what residential reefer means to me?
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Old 05-03-2016, 04:05 PM   #7
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Originally Posted by scgator2001 View Post
Residential Reefer:

I am a boater in the winter with a 12 volt refrigerator and a separate 12 volt freezer. Those are my most significant loads on the batteries at night (solar in the daylight makes it a non-issue).

Are you running an inverter with a 120 volt refrigerator which is what residential reefer means to me?
Yes - A three door residential reefer on 120VAC with the stock Magnum M2xxx pure Sine wave inverter/converter. The battery bank is 4 NAPA 9831DTs 12, 200RC AGM batteries.

Thanks,
J
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Old 05-03-2016, 04:59 PM   #8
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Jerald,
Please double check those battery capacities. Reserve capacity is usually expressed in minutes at a certain Ampere draw. Also 12 volt batteries usually are about 100 Ampere hours, usually measured over 20 hours. This is called the 20 hour rate. I'm not saying you're wrong, but your numbers don't seem normal to me
I redid my numbers and came up with the same answer. I'll show how I got there and perhaps someone can tell me where I am full of <some substance>:

The batteries are NAPA 9831DTs which are rated by NAPA with a Reserve Capacity of 200. Reserve Capacity is defined at the number of minutes you can draw 25 amps. So the conversion factor from Reserve Capacity to Amp-hours is 25/60 or .41 (maybe .42 if you want to round).

So 200 x .41 = 82 amp-hours per battery. I have 4 of these critters so it comes 82*4 = 328 amp hours.

So, I think that's right.

Thanks,
Jerald
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Old 05-03-2016, 05:25 PM   #9
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Not a math expert but found this.

Looks like you got it right.

Reserve Capacity*(RC) is the number of minutes a fully charged battery at 80 o F (26.7 o C) is discharged at 25 amps before the voltage falls below 10.5 volts. To*convert Reserve Capacity*(RC) to Ampere-Hours at the 25 amp rate, multiple*RC*by .4167. More ampere-hours (or*RC) are better in every case.
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Old 05-03-2016, 06:23 PM   #10
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How long should the house batteries last?

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Originally Posted by jrpend View Post
So 200 x .41 = 82 amp-hours per battery. I have 4 of these critters so it comes 82*4 = 328 amp hours.

You don't want to run the batteries to such a low value that the inverter cuts off. That WILL cause damage to the batteries and shorten their life considerably. So use 50% of your capacity as a safe number.....meaning you have 160 amp hours usable.

Most residential refrigerators use about 1.1 KWh per day. ( some up to 1.5 KWh)

160 * 12=1.9 KWh So with inverter losses, other loads, etc I would say you would be lucky to get 24 hours on battery power. If your other loads add up significantly more, then of course your mileage may vary.

How long are your batteries lasting?
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Old 05-04-2016, 06:40 PM   #11
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You don't want to run the batteries to such a low value that the inverter cuts off. That WILL cause damage to the batteries and shorten their life considerably. So use 50% of your capacity as a safe number.....meaning you have 160 amp hours usable.

Most residential refrigerators use about 1.1 KWh per day. ( some up to 1.5 KWh)

160 * 12=1.9 KWh So with inverter losses, other loads, etc I would say you would be lucky to get 24 hours on battery power. If your other loads add up significantly more, then of course your mileage may vary.

How long are your batteries lasting?
I'm getting around 4 hours right now. That is based on one night boondocking with some of the electrical running, but certainly not all of it.
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Old 05-04-2016, 07:25 PM   #12
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How long should the house batteries last?

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Originally Posted by jrpend View Post
I'm getting around 4 hours right now. That is based on one night boondocking with some of the electrical running, but certainly not all of it.

I agree, you should be doing better. How old are the batteries? Are you sure your charger is getting them to 100% charged?
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Old 05-04-2016, 08:30 PM   #13
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Check out this:
RV Residential Refrigerator - How Much Power Does It Use
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Old 05-04-2016, 08:43 PM   #14
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Our rig came with 4 12 V 8D AGM's. Each is around 240 amp hrs at the normal 20 hour rate. As to how long we can go? Don't know yet, can't get out due to DW's continuing health problems.
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Old 05-05-2016, 10:51 AM   #15
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Question

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I agree, you should be doing better. How old are the batteries? Are you sure your charger is getting them to 100% charged?
I think so - it is a 4 stage charger (Magnum) and goes to "float" mode. This weekend I'm going to measure open circuit voltages and see what those are.

The batteries are the original equipment - from what I read they should last 7-10 years and I can't think of anything I may have done to damage them.
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Old 05-05-2016, 12:16 PM   #16
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You may not have done anything to damage them but was it on a dealers lot, going dead and being jumped a bunch of times ?

I think the biggest killer is dealer abuse.
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Old 05-05-2016, 02:12 PM   #17
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Refrigerators suck the power; there is no getting around it. The article the Wynns generated was a good overview – albeit nearly technical.
The calculation for 4 batteries of 328 Amp-hours (or 3.9 kwh) seems correct to the manufacturer’s new battery ratings. Often but not always, the 25 Amp rating is for the battery to drop to 10.5 volts – but each manufacturer defines their own end of charge voltage.Also of note, the wire, electrical connections, and inverter – all combined add 20% to 25% inefficiencies (the Wynns claimed 38% inefficiencies).So a 600 watt residential refrigerator (with 25% inefficiency) actually draws 750 watts from the batteries when the frig compressor is running.750 watts (at 12V) draws 62 amps from the batteries.At 62 Amps, with a 12V fuse, 12V battery disconnect switch, and wire - the voltage drop from the batteries to the inverter can be 1 Volt.So even though the batteries may be at 11.4 volts, the inverter would only see 10.4 to 10.5 volts which is likely where the inverter will drop offline.With a 750 watt continuous load, the batteries could draw down to 11.4V in less than 3 hours for this refrigerator.You could assume 30% to 40% compressor utilization based on the power load of your refrigerator, refrigerator temp, ambient temp and keeping door closed to extend operation 2 to 4 times that.Long winded way of saying: refrigerators will suck the life out of your batteries, even new batteries.
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Old 05-05-2016, 02:59 PM   #18
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Refrigerators suck the power; there is no getting around it. The article the Wynns generated was a good overview – albeit nearly technical.
The calculation for 4 batteries of 328 Amp-hours (or 3.9 kwh) seems correct to the manufacturer’s new battery ratings. Often but not always, the 25 Amp rating is for the battery to drop to 10.5 volts – but each manufacturer defines their own end of charge voltage.Also of note, the wire, electrical connections, and inverter – all combined add 20% to 25% inefficiencies (the Wynns claimed 38% inefficiencies).So a 600 watt residential refrigerator (with 25% inefficiency) actually draws 750 watts from the batteries when the frig compressor is running.750 watts (at 12V) draws 62 amps from the batteries.At 62 Amps, with a 12V fuse, 12V battery disconnect switch, and wire - the voltage drop from the batteries to the inverter can be 1 Volt.So even though the batteries may be at 11.4 volts, the inverter would only see 10.4 to 10.5 volts which is likely where the inverter will drop offline.With a 750 watt continuous load, the batteries could draw down to 11.4V in less than 3 hours for this refrigerator.You could assume 30% to 40% compressor utilization based on the power load of your refrigerator, refrigerator temp, ambient temp and keeping door closed to extend operation 2 to 4 times that.Long winded way of saying: refrigerators will suck the life out of your batteries, even new batteries.
The calculations you are offering up are not what I found with my residential refrigerator. I do understand that they are not yours.

In actual use, my fridge draws 9 [email protected], DC, as displayed on my battery monitor. That works out to 108 watts.

As a matter of fact, my absorbsion fridge only draws 350 watts.

Others on this forum have found comparable numbers to mine.
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Old 05-06-2016, 11:33 AM   #19
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Playing some more numbers games...


On the Kitchen Aide Residential the rating plate states the draw is 7.1 amps or 852 watts. I think that is a worst case and pretty unrealistic. The power consumption sticker says it will use 451kwh on an annual basis.


Ok, very gross number, but supposedly based on moderate usage. That comes out to about 1.25 kwh/day. Dividing out 12volts the reefer should be using 104 amp hours per day. Taking into account the Winns inefficiency factor of 38% (I've heard 25%) gives 144amp-hours per day.


[I'm pontificating a bit to see if anyone sees a problem with my assumptions/calculations]


So I supposedly have 328 amp-hours total battery capacity or 164 to the 50% level.


I think I should be getting at least a day out of those batteries. Granted we have some additional draw for lights (LEDs) and some vampire devices (alarm clocks etc.) but that shouldn't be that much.


So I'm going to do open circuit testing on the batteries this weekend. I've been charging them for the last 2 days and I'm going to disconnect them and let them sit for about 4 hours. I will then measure the voltage with my trusty volt-ohm meter and determine state of charge. I unfortunately do not have a load tester.


Thanks,
Jerald
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Old 05-06-2016, 12:46 PM   #20
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If you had a battery monitor like this one you would have a very clear picture of how healthy your batteries are at all times.

Click image for larger version

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