RV News RVBusiness 2021 Top 10 RVs of the Year, plus 56 additional debuts and must-see units → ×
 

Go Back   Thor Forums > Thor Tech Forums > Motorhome Tech Topics
Click Here to Login
Reply
 
Thread Tools Display Modes
 
Old 01-06-2021, 02:30 PM   #1
Member
 
Brand: Thor Motor Coach
State: Massachusetts
Posts: 33
THOR #21704
Residential fridge power questions off the grid

I have a 2 battery set up 12V cab set up, auto generator start.

Question:

Anyone that has the 16ft residential fridge/freezer know how many hours on average or how long the cab batteries should provide power for the fridge through the inverter (1800 watt) before the generator starts

I have a Thor ws31- double cab batteries and both are the correct deep cycle. Standard wet cell each 750 amp

Thanks
__________________

__________________
quantum is offline   Reply With Quote
Old 01-06-2021, 03:05 PM   #2
Site Team
 
16ACE27's Avatar
 
Brand: Thor Motor Coach
Model: ACE 27.1
State: Florida
Posts: 5,386
THOR #7035
Quote:
Originally Posted by quantum View Post
I have a 2 battery set up 12V cab set up, auto generator start.

Question:

Anyone that has the 16ft residential fridge/freezer know how many hours on average or how long the cab batteries should provide power for the fridge through the inverter (1800 watt) before the generator starts

I have a Thor ws31- double cab batteries and both are the correct deep cycle. Standard wet cell each 750 amp

Thanks
I assume the "750 amp" is cranking or cold cranking amps? If so, you don't really have depp cycle batteries, you have combination starting/deep cycle batteries.

Battery capacity is measured in amp-hours (AH) usually at the 20 amp rate. So a 100 amp-hour battery can supply 20 amps for 5 hours before it is dead (0% charged). But lead-acid batteries permanently die a little if discharged below 50% so a 100 AH lead-acid battery really only has 50 AH that are usable.

The state of charge of a lead-acid battery can be approximately measured by its no load voltage.


But the voltage under load will be lower depending on the load. Other factors affect the battery voltage as well such as temperature.

When your AGS starts the generator depends on what voltage threshold it is set at. When it reaches that threshold depends on the load the house batteries are supplying and their actual capacity which varies with age as well as state of charge.

How much energy your residential fridge uses varies with many conditions such as ambient temperature, insulation, temperature of the stuff inside, etc.
If you just threw a couple of two liter sodas in there and filled the ice trays it will use more energy than if everything was already at a cooled temperature. Also being Energy Star rated makes a difference.


BTW, a 16 Cubic Foot Fridge is might large for an RV but lets say it uses 500 watts on the average; and it has an average duty cycle of 50%, so 250 watts average running power or 21 DC amps in a 12 VDC system. Say 10% inverter ineffeciency and we're at 23 DC amps average.

If you have two 100 AH house batteries (100 AH usable) you would be at 50% capacity in a little over 4 hours and that's where you would want your AGS to kick in.

But 4 hours is just an educated WAG because there are so many variables and unknowns.
__________________

__________________
Ted & Melinda
2016 ACE 27.1
2016 Chevy Sonic Toad
2020 Chevy Colorado Z71 Trail Runner Alternate Toad
16ACE27 is online now   Reply With Quote
Old 01-06-2021, 03:27 PM   #3
Senior Member
 
Brand: Thor Motor Coach
Model: Axis 24.1
State: Connecticut
Posts: 210
THOR #20289
As Ted notes above, your batteries are not true deep cycle. Also we don't know at what DC voltage triggers the AGS to start. Let's assume it starts at 12.0 volts but as he notes that will be driven lower if there is a significant DC current draw.

From what I have seen a residential type AC powered fridge of about 16 cu ft draws 150-200 amp hours DC daily to supply the inverter. I suspect you have two Group 27 batteries which have about 75 amp hours of capacity each. So I would make an educated guess that your AGS would start after 12 hours, ie after 75-100 Ahs of DC draw which should bring your batteries down to 12.0 volts or below. But since the current draw will bring that down temporarily, it probably will start much sooner.

Residential fridges and dry camping is a tough combination to make work, particularly with no generator running. Many if not most dry camping sites prohibit generator usage except in the middle of the day. To prevent your generator from kicking on at night which is maybe the root of your question, you would have to double your battery capacity or accept running them down below 50%. Running below 50% is not good long term but can be done occasionally with little harm.

David
__________________
DavidEM is offline   Reply With Quote
Old 01-06-2021, 05:12 PM   #4
Member
 
Brand: Thor Motor Coach
State: Massachusetts
Posts: 33
THOR #21704
Sorry I do have the two Deep cycle. 2 -100 amp hour as you say.

My engine is the cold crank
__________________
quantum is offline   Reply With Quote
Old 01-07-2021, 04:33 AM   #5
Senior Member
 
Brand: Thor Motor Coach
Model: Aria 3901
State: Idaho
Posts: 476
THOR #9660
When we had 4 220AH 6V batteries (for a total of 440AH @ 12V), our batteries would last all night after a full charge, then watching a couple of TV's for a few hours, a few lights, and running the same 16 cf fridge. They would still be at about 12.3 volts. That was when they were new and it great shape. But a few minutes after the coffee maker started, they would dip to 12.1 and the gen would start.
__________________
Vicki and Mike, Aria 3901- The Wined Up Bus
m1noel is offline   Reply With Quote
Old 01-07-2021, 12:56 PM   #6
Senior Member
 
Brand: Thor Motor Coach
State: Missouri
Posts: 1,069
THOR #6903
We dry camp with fridge and heat and can make coffee prior to generator run time

4 Trojan T105 AGM
__________________
lwmcguire is offline   Reply With Quote
Old 01-07-2021, 02:21 PM   #7
Member
 
Brand: Thor Motor Coach
State: Massachusetts
Posts: 33
THOR #21704
Thanks...Good feedback. Where as I have 2-12v at 100 I believe I have 240AH. Being a year old I guess I would be in good shape for 1/2 the time. That is adequate for our needs. Also if the fridge only runs on occasion, we watch i tv or use our computers we should be fine for a good night sleep.

Thanks for the feedback..

This trip will help me decide if I want to add additional AGM batteries for addition time and less maintenance. If the fridge is well insulated I see no issues of anything frozen defrosting or anything in the fridge getting warm. We run the inside fridge at about 38/40 degrees.
__________________
quantum is offline   Reply With Quote
Old 01-07-2021, 02:26 PM   #8
Site Team
 
16ACE27's Avatar
 
Brand: Thor Motor Coach
Model: ACE 27.1
State: Florida
Posts: 5,386
THOR #7035
Quote:
Originally Posted by quantum View Post
Thanks...Good feedback. Where as I have 2-12v at 100 I believe I have 240AH. Being a year old I guess I would be in good shape for 1/2 the time. That is adequate for our needs. Also if the fridge only runs on occasion, we watch i tv or use our computers we should be fine for a good night sleep.

Thanks for the feedback..

This trip will help me decide if I want to add additional AGM batteries for addition time and less maintenance. If the fridge is well insulated I see no issues of anything frozen defrosting or anything in the fridge getting warm. We run the inside fridge at about 38/40 degrees.
I don't understand your math.

Two 100 AH batteries in parallel provide 200 AH. Where does "240AH" come from?
__________________
Ted & Melinda
2016 ACE 27.1
2016 Chevy Sonic Toad
2020 Chevy Colorado Z71 Trail Runner Alternate Toad
16ACE27 is online now   Reply With Quote
Old 01-07-2021, 10:50 PM   #9
Senior Member
 
Brand: Thor Motor Coach
Model: Palazzo 33.2
State: Georgia
Posts: 225
THOR #8078
I think you will find two batteries are not enough to be practical. As noted above it depends on the weather, your battery age, fridge use, etc. We have a 4 battery system for our residential fridge. If the weather is <90 degrees we are OK if we run the generator for 1.5-2 hours in the AM and 1-2 hrs in the late afternoon. We use that same time to run the microwave, shower, and other large power needs. Overnight our voltage drops to 11.9- 12.0
__________________
rudyhehn is offline   Reply With Quote
Old 01-07-2021, 11:41 PM   #10
Member
 
Brand: Thor Motor Coach
State: Massachusetts
Posts: 33
THOR #21704
Here ya go. My math....As I thought I had read. I had thought the 12v battery times the 100ah = 120 x 2 = 240. I am new at this thus the questions and I am sure you are correct... bad brain day..LOL

My reply was based on what I read below/?
This is the online version I used :The key is to use the watts you know to calculate the amps at the battery voltage . For example, say you want to run a 250 watt 110VAC light bulb from an inverter for 5 hours. Amp-hours (at 12 volts) = watt-hours / 12 volts = 1470 / 12 = 122.5 amp-hours.

Thanks for the info on the generator. Our use will be small other than some LED lights and possible, TV for a bit. My concern was the heater fan and the fridge till the generator kicks back on.

Be well
__________________
quantum is offline   Reply With Quote
Old 01-07-2021, 11:49 PM   #11
Member
 
Brand: Thor Motor Coach
State: Massachusetts
Posts: 33
THOR #21704
You seem well educated in this area. Any recommendations when it's time to replace the batteries if they do not power as long as I would like. My battery storage is under the steps coming in, I'm a class c E450. Lat thing I want for my family is to be cold because I was battery stupid....

The battery area may be able to hold 3 smaller batteries that the current ones I have. I have always thought they would be the most important investment to be comfortable when stopped. So many choices but I do not want to trust a battery shop or store trying to upsell me something I will need to again replace. Thanks in advance....You have been a good source for my power concerns.
__________________
quantum is offline   Reply With Quote
Old 01-08-2021, 02:50 AM   #12
Senior Member
 
Brand: Still Looking
State: Texas
Posts: 4,829
THOR #2121
Quote:
Originally Posted by quantum View Post
Here ya go. My math....As I thought I had read. I had thought the 12v battery times the 100ah = 120 x 2 = 240. I am new at this thus the questions and I am sure you are correct... bad brain day..LOL


My reply was based on what I read below/?
This is the online version I used :The key is to use the watts you know to calculate the amps at the battery voltage . For example, say you want to run a 250 watt 110VAC light bulb from an inverter for 5 hours. Amp-hours (at 12 volts) = watt-hours / 12 volts = 1470 / 12 = 122.5 amp-hours.

Thanks for the info on the generator. Our use will be small other than some LED lights and possible, TV for a bit. My concern was the heater fan and the fridge till the generator kicks back on.

Be well
Your math is off.

100 Ah X 12 Volt = 1,200 Watt-hours (per battery)

2 batteries is therefore 2,400 Watt-hours

Useable capacity at 50% state of charge:

2,400 X 0.50 = 1,200 Watt hours

Large residential refrigerators use ďaboutĒ 2,000 Watt-hours daily (in a house setting where itís cool), so your two batteries will need charging multiple times per day. The more you use other electrical loads, the more often youíll need to charge the two batteries.
__________________
Chance is offline   Reply With Quote
Old 01-08-2021, 05:53 PM   #13
Member
 
Brand: Thor Motor Coach
State: Massachusetts
Posts: 33
THOR #21704
Thanks, I a bit misled now as below was my response much earlier in the thread and I was told I was incorrect with my math?

post #15----Thanks...Good feedback. Where as I have 2-12v at 100 I believe I have 240AH. Being a year old I guess I would be in good shape for 1/2 the time. That is adequate for our needs. Also if the fridge only runs on occasion, we watch i tv or use our computers we should be fine for a good night sleep.

I want to ask which one is correct my first one stating how you did 12v x 100 Amp =1200 x 2 = 2400 ??

I do appreciate your seeing this so I can figure how to do the correct math. LOL

Just a note and I want to thank you for the estimated fridge usage. I will be driving south and dry camping ( harvest Hosts locations) for about 8 to 10hours each night of the 3 day drive. So while traveling batteries will be charges, fridge will be ok as well as my heat. Those are my most important 2 concerns. If the generator only starts one or twice, I think I'll be fine. Your thoughts/

Also I want to thank everyone's feedback. It has surely removed many of my concerns
__________________
quantum is offline   Reply With Quote
Old 01-08-2021, 06:03 PM   #14
Member
 
Brand: Thor Motor Coach
State: Massachusetts
Posts: 33
THOR #21704
Correction-------I meant to say I was informed by someone else, not you, my math was wrong so I included the post I first was told
__________________
quantum is offline   Reply With Quote
Old 01-08-2021, 06:09 PM   #15
Site Team
 
16ACE27's Avatar
 
Brand: Thor Motor Coach
Model: ACE 27.1
State: Florida
Posts: 5,386
THOR #7035
Quote:
Originally Posted by quantum View Post
Thanks, I a bit misled now as below was my response much earlier in the thread and I was told I was incorrect with my math?

post #15----Thanks...Good feedback. Where as I have 2-12v at 100 I believe I have 240AH. Being a year old I guess I would be in good shape for 1/2 the time. That is adequate for our needs. Also if the fridge only runs on occasion, we watch i tv or use our computers we should be fine for a good night sleep.

I want to ask which one is correct my first one stating how you did 12v x 100 Amp =1200 x 2 = 2400 ??

I do appreciate your seeing this so I can figure how to do the correct math. LOL

Just a note and I want to thank you for the estimated fridge usage. I will be driving south and dry camping ( harvest Hosts locations) for about 8 to 10hours each night of the 3 day drive. So while traveling batteries will be charges, fridge will be ok as well as my heat. Those are my most important 2 concerns. If the generator only starts one or twice, I think I'll be fine. Your thoughts/

Also I want to thank everyone's feedback. It has surely removed many of my concerns

The problem is you are confusing watt-hrs with amp-hours: Not the same thing. Numbers mean nothing without units.

If you have (2) 12 volt batteries rated at 100 AH each, you have a total of 200 AHs stored energy at 12 volts. But since only half of that is usable, you have 100 AHs of usable energy at 12 volts.

Watt-hours is a measured of the stored energy independent of the applied voltage.
Your same 100 AH battery at 12 volts has 12 volts * 100 AH = 1200 Watt-Hours of stored energy. The watt is a product of voltage times current; 1 volt times 1 amp = 1 watt.

Also, if you don't quote the person you are directing a response to, nobody knows who you are talking to.
__________________
Ted & Melinda
2016 ACE 27.1
2016 Chevy Sonic Toad
2020 Chevy Colorado Z71 Trail Runner Alternate Toad
16ACE27 is online now   Reply With Quote
Old 01-08-2021, 08:23 PM   #16
Senior Member
 
Travelin' Texans's Avatar
 
Brand: Redwood
State: Texas
Posts: 1,280
THOR #3610
One other concern is if your batteries by morning have depleted to 50% the minimal charging they do while traveling with the alternator will be very slight. If you run the generator while traveling would do much better job of charging than the alternator.
__________________
Danny & Linda
Sold our '13 Thor Redwood 36 FB
Currently RV & truckless
Replacement undetermined
Travelin' Texans is offline   Reply With Quote
Old 01-08-2021, 08:30 PM   #17
Site Team
 
16ACE27's Avatar
 
Brand: Thor Motor Coach
Model: ACE 27.1
State: Florida
Posts: 5,386
THOR #7035
Quote:
Originally Posted by Travelin' Texans View Post
One other concern is if your batteries by morning have depleted to 50% the minimal charging they do while traveling with the alternator will be very slight. If you run the generator while traveling would do much better job of charging than the alternator.
Care to expound on that?

45 Amp converter vs 160 Amp alternator?
__________________
Ted & Melinda
2016 ACE 27.1
2016 Chevy Sonic Toad
2020 Chevy Colorado Z71 Trail Runner Alternate Toad
16ACE27 is online now   Reply With Quote
Old 01-08-2021, 08:37 PM   #18
Senior Member
 
Brand: Thor Motor Coach
Model: Axis 24.1
State: Connecticut
Posts: 210
THOR #20289
Quote:
Originally Posted by 16ACE27 View Post
Care to expound on that?

45 Amp converter vs 160 Amp alternator?
It all depends on how you are set up: lithium or lead acid batteries, any DC to DC converter, etc.

For a standard lead acid coach battery setup the fixed voltage output of the chassis alternator won't get any where near 160 amps, maybe 50 amps if the battery is well discharged, decreasing as it charges.

David
__________________
DavidEM is offline   Reply With Quote
Old 01-08-2021, 08:43 PM   #19
Site Team
 
16ACE27's Avatar
 
Brand: Thor Motor Coach
Model: ACE 27.1
State: Florida
Posts: 5,386
THOR #7035
Quote:
Originally Posted by DavidEM View Post
It all depends on how you are set up: lithium or lead acid batteries, any DC to DC converter, etc.

For a standard lead acid coach battery setup the fixed voltage output of the chassis alternator won't get any where near 160 amps, maybe 50 amps if the battery is well discharged, decreasing as it charges.

David
Well sure, the output of the alternator depends on the voltage difference between it's output (14.5 VDC) and the average load voltage/resistance. The 160 amps is there for all the other DC loads as well.

So if the converter is putting out 14.5 VDC and the alternator is putting out 14.5 VDC why would there be an advantage to using the converter to the same battery at the same state of charge?
__________________
Ted & Melinda
2016 ACE 27.1
2016 Chevy Sonic Toad
2020 Chevy Colorado Z71 Trail Runner Alternate Toad
16ACE27 is online now   Reply With Quote
Old 01-08-2021, 08:48 PM   #20
Member
 
Brand: Thor Motor Coach
State: Massachusetts
Posts: 33
THOR #21704
Hey there,

This is Quantum.

The last note was a alternator vs the generator while driving?? New and confused still on the battery use and now on the recharge while driving.

I pretty good with most everything not electrical related. So I ask a lot of questions to try to learn more. But now I'm
__________________

__________________
quantum is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


» Featured Campgrounds

Reviews provided by

Disclaimer:

This website is not affiliated with or endorsed by Thor Industries or any of its affiliates. This is an independent, unofficial site.




All times are GMT. The time now is 02:22 AM.


Powered by vBulletin® Version 3.8.8
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Content Relevant URLs by vBSEO 3.6.0 PL2
×