Calculating Dometic fridge daily AH usage

  • Thread starter Thread starter 82125
  • Start date Start date
8

82125

Deleted User
Im only after a rough idea of what my Dometic 8505 3-way fridge uses when on 12v (wondering whether Ill be able to run it from solar in the uk summer rather than gas).

The fridge rating plate saying 170w @ 12v (so just over 14amps), but ive no idea how often its actually running (assuming it cycles on and off to regulate the temperature?)

Any ideas what its daily AH might be please?


I have two new lead-acid 105ah batteries with a 495w solar panel, 40amp MPPT (even now in late November 9:50am on a very cloudy UK grey day I'm getting 3.8amps from my solar).

**i just noticed the manual states consumption electricity/gas as "c.a. 3,2 KWh / 380g", not sure how to apply that to the 105ah usable battery capacity (and solar) I have though
 
Last edited by a moderator:
Sort by date Sort by votes
Since you said "rough idea" I'll jump in. From reading this forum for quite a while the general opinion is that 3way fridges on 12V only are only good at maintaining temperature and not good at reducing it. It's an inefficient process. If that is true then your fridge would use 340Ahr a day. Your battery bank has 105AHr available if you want to not go beyond 50%DOD. I would think the batteries would struggle to last the night if they were fully charged at sundown.

During the day. With 495W you could produce 2970Wh to 3465WHr on a summers day so therefore 247Ahr to 288Ahr. That is based on figures from https://www.campingandcaravanningclub.co.uk/helpandadvice/technicalhelp/solar-power/ and is for an average day so on a really sunny day you could get quite a bit more. Still I would think on most days you would not get enought to fill the batteries and even if you did they would likely be flat by morning.

I actually thought you would be OK before doing the maths. I am a relative newb so I will stand corrected!!
 
Upvote 0
Surely the only way you could run the fridge without the vehicle engine on the go would be via an inverter from your hab batteries. Even if you had a decent lithium battery bank, the inverter would be sucking around 30-35A out of it. Defray that draw with a good bit of sun might give you around 15A from say 10-4. So even during the bulk of the day you would still be demanding 20A from the batteries. If you were low on gas, it might be a bit of a stop gap solution during the day to reduce gas requirements but not really a good idea.
 
Upvote 0
Finger in the air....
In Scotland last year half decent sun, mine was using 14 a ish when it was running day and night when switching itself on and off via an inverter auto switch over.
min....90ah per 24 hours ish.
If not continuous sun on consecutive days or driving using the b2b....id use gas. I had 300w solar, 200ah lithium
 
Last edited:
Upvote 0
I think 3-way fridges use Peltier thermo-electric plates for cooling on 12v and 240v. The Peltier effect is ridiculously inefficient and wastes huge amounts of energy as heat. But it is compact and has no moving parts and is quite easy to integrate with gas cooled elements.

To run a 3-way on battery, you'd need masses of solar (which would only work in the summer months) and loads of battery just to keep the fridge alive. Either convert to a 2-way compressor fridge, or continue to use gas.
 
Upvote 0
I have a Votronic solar regulator that has an AES output to turn the fridge over to 12v when the batteries are charged and there is enough solar, in mid summer it may run on 12v for 2 to 3 hours max. The controller switches in 30 min bursts so after 30min on 12v it will switch back to gas for 30 min to minimize battery drain.

My fridge is an 8505 the 12v element is 170W compared to the mains one at 250 watt so not so good on 12v. Not that good if it's 35 deg otherwise works quite well on 12v. The old wives tale about them only maintaining temperature on 12v is only true of very old fridges getting on for 20 years old.

The 3.2kw power figure is for gas.
The electric elements are standard resistive elements so 100% efficent. At a guess if you said a 75% duty cycle on 12v it would use 10.5 amps per hour so 24 hours would be 252 ah and a 50% duty cycle 168ah for 24 hours. The duty cycle will depend on both inside & outside temperature.
Conclusion only viable to run it for a few hours a day on bright sunny days in summer.
 
Upvote 0
perfect, exactly what i was looking for thanks Lenny (y)
(not viable)

Time to plan another power upgrade (not to run the fridge, but because I need a little more anyway).

Subscribers  do not see these advertisements

 
Last edited by a moderator:
Upvote 1
Richard n Ann said:
Buy a compressor fridge if you want to power it with 12v off grid 👍
Click to expand...
thanks, but I just wanted to understand the situation of the equipment I have currently.
cyberyacht said:
Surely the only way you could run the fridge without the vehicle engine on the go would be via an inverter from your hab batteries. Even if you had a decent lithium battery bank, the inverter would be sucking around 30-35A out of it. Defray that draw with a good bit of sun might give you around 15A from say 10-4. So even during the bulk of the day you would still be demanding 20A from the batteries. If you were low on gas, it might be a bit of a stop gap solution during the day to reduce gas requirements but not really a good idea.
Click to expand...
mine runs 12v stationary engine off
 
Upvote 0
Guigsy said:
I think 3-way fridges use Peltier thermo-electric plates for cooling on 12v and 240v. The Peltier effect is ridiculously inefficient and wastes huge amounts of energy as heat. But it is compact and has no moving parts and is quite easy to integrate with gas cooled elements.

To run a 3-way on battery, you'd need masses of solar (which would only work in the summer months) and loads of battery just to keep the fridge alive. Either convert to a 2-way compressor fridge, or continue to use gas.
Click to expand...

3 way fridges do not use Peltier. They use the same absorbtion process that they use on gas. This cycle involves applying heat (hence this works well for gas). The electric operation simply replaces the gas burner with electric heating elements. However, it is still a very inefficient process. You can expect energy consumption to be up to around 10 times less efficient than a compressor based cycle.
 
Upvote 0
Calm Seas said:
i just noticed the manual states consumption electricity/gas as "c.a. 3,2 KWh / 380g", not sure how to apply that to the 105ah usable battery capacity (and solar) I have though
Click to expand...
3.2kWh is a quantity of energy, equivalent to 3200/12 = 267 Ah from a 12 volt battery.

A 15A element on continuously for 24 hours would use 15 x 24 = 360Ah, so it looks like the thermostat is keeping the element on about 75% of the time. If you were in special circumstances like on a ferry crossing, where the fridge is shut all the time, then that might reduce a bit, but I'd say that was about right for normal usage in average to hot weather. It's the reason compressor fridges are popular now that solar is a viable option.
 
Upvote 0
cmcardle75 said:
3 way fridges do not use Peltier. They use the same absorbtion process that they use on gas. This cycle involves applying heat (hence this works well for gas). The electric operation simply replaces the gas burner with electric heating elements. However, it is still a very inefficient process. You can expect energy consumption to be up to around 10 times less efficient than a compressor based cycle.
Click to expand...
I guess that makes sense, just tap into the absorption gas cycle. I assumed that it was a Peltier like the cheap 12v fridges you can buy.

www.ferrellgas.com

How propane powers refrigeration | Ferrellgas | Fuel Life Simply

Did you know propane can do more than just heat your home? Read our latest blog to learn how propane powers refrigeration.
www.ferrellgas.com
 
Upvote 0
autorouter said:
3.2kWh is a quantity of energy, equivalent to 3200/12 = 267 Ah from a 12 volt battery.

A 15A element on continuously for 24 hours would use 15 x 24 = 360Ah, so it looks like the thermostat is keeping the element on about 75% of the time. If you were in special circumstances like on a ferry crossing, where the fridge is shut all the time, then that might reduce a bit, but I'd say that was about right for normal usage in average to hot weather. It's the reason compressor fridges are popular now that solar is a viable option.
Click to expand...
so in theory, in summer, if im seeing around 20amps or more coming in from my solar, during the day, I could manually switch the fridge over to 12v (assuming my batteries arent in need of a major charge).
Although by the sound of it, the gas saving isnt really much to write home about anyway, but its useful to understand what's going on thanks.

Subscribers  do not see these advertisements

 
Upvote 0
Calm Seas said:
perfect, exactly what i was looking for thanks Lenny (y)
(not viable)

Time to plan another power upgrade (not to run the fridge, but because I need a little more anyway).
Click to expand...
Agreed, I think a little bit more battery power would give you more options as you have a decent amount of solar, we have 480ah lithium but still consider it not really worthwhile running the fridge on 12v as we have in our view more important things to do with that power, electric kettle, coffee machine, hair dryer and induction hob outside, all of these use at least 1kw from the inverter so minimum 100amps from the batteries which 2x110ah may not be too happy with.
 
Upvote 0
Calm Seas said:
so in theory, in summer, if im seeing around 20amps or more coming in from my solar, during the day, I could manually switch the fridge over to 12v (assuming my batteries arent in need of a major charge).
Although by the sound of it, the gas saving isnt really much to write home about anyway, but its useful to understand what's going on thanks.
Click to expand...
If you have a Votronic MPPT solar regulator they have a AES out signal which can be used to power the AES fridge when the batteries are fully charged and the solar is going spare, I did wire our previous van to use this feature but not sure how much gas it saved, when we were in Spain I used to turn on the inverter in the morning to do coffee and the usual hair drying duties then I would leave it on running the fridge till after lunch time (not all vans are wires the same) then we would go out and I plugged the site electric back in just in case the sun went in, I will add that this exercise was designed at staying within the included electric allowance on the site.
 
Upvote 0
funflair said:
If you have a Votronic MPPT solar regulator they have a AES out signal which can be used to power the AES fridge when the batteries are fully charged and the solar is going spare,
Click to expand...
Note, this AES output isn't a power output, it's a 'signal' voltage which goes to the 'S+' input on the fridge control board. It tells the fridge to switch from gas/240V to 12V, and switches it back again if the solar power falls off.
 
Upvote 0
The amount of gas that a fridge uses is pretty tiny. It pales into insignificance compared to heating.
 
Upvote 0
Guigsy said:
The amount of gas that a fridge uses is pretty tiny. It pales into insignificance compared to heating.
Click to expand...
Over a year I would say the fridge is the bulk of our gas usage as it's running 24/7 when we are using the van.

Subscribers  do not see these advertisements

 
Upvote 0
Lenny HB said:
Over a year I would say the fridge is the bulk of our gas usage as it's running 24/7 when we are using the van.
Click to expand...
In 2019, we drove around Europe for 6 months in our campervan. We were off-grid almost the whole time. The fridge running 24/7 and was either running on gas or 12v for the couple of hours each day we were driving. We only had a 20 litre underslung gas tank that we only filled up four times, and it was never completely empty. The gas was also used to heat water for the dishes and the shower as well as a bit of cooking.

This year we got a new van. I've not managed to get an underslung yet, so I'm using a single 11kg Gaslow. We've used it for about a month of overnights so far and we've already needed to refill the gas. It's been colder on quite a few of the times we've been away, so we've been using the air heating more. The gauge seems to drop much quicker with heating and cooking than from the fridge.
 
Upvote 0
autorouter said:
Note, this AES output isn't a power output, it's a 'signal' voltage which goes to the 'S+' input on the fridge control board. It tells the fridge to switch from gas/240V to 12V, and switches it back again if the solar power falls off.
Click to expand...
Correct 👍 but it can easily be connected to run a non S+ AES by using a diode on the D+, it worked for me anyway.
 
Upvote 0
I thought on 3 way fridges, the 12v option was only for when the engine is running, and all the caravans/motorhomes I've had are wired so that they will only work this way.

Running off 12v will keep an already cold fridge cool, it is designed to provide some sort of cooling whilst travelling, but not work anything like as efficiently as running off 240v or gas.
 
Upvote 0
Old Gibber said:
Running off 12v will keep an already cold fridge cool, it is designed to provide some sort of cooling whilst travelling, but not work anything like as efficiently as running off 240v or gas.
Click to expand...
It depends on the wattage of the heater elements. If for example the 240V element is 120W, then the designers might have decided that a 90W heater will keep the fridge cold if it's already cold. However on some fridges they might decide to fit a 120W element, in which case the fridge will work just as well on 12V as 240V.

It's also likely that some fridges will have a 120W element, but the long thin wiring from the starter/alternator might have so much voltage drop that it's only producing 90W of heat anyway.
 
Upvote 0
Old Gibber said:
I thought on 3 way fridges, the 12v option was only for when the engine is running, and all the caravans/motorhomes I've had are wired so that they will only work this way.
Click to expand...
Mine appears to allow me to select 12v with the engine off (off-grid and doesnt alarm in any way). Indeed I seem to recall when left to Auto, if the gas is off it will select 12v by itself I think.

Subscribers  do not see these advertisements

 
Upvote 0
Calm Seas said:
Mine appears to allow me to select 12v with the engine off (off-grid and doesnt alarm in any way). Indeed I seem to recall when left to Auto, if the gas is off it will select 12v by itself I think.
Click to expand...
Our first Hymer did that you can stop it by pulling the AES fuse it will still work on 12v with the engine running.
Last two vans stay on 12v for 15min when you turn the engine off. And if you select 12v manually it will run on 12v.
 
Upvote 0
Yeah I think the previous owner may have done that, I found a blown and too small fuse in that holder which at the time i thought must have been a mistake, but maybe he put a blown one in deliberately. I replaced it with the correct one as Im happy to have it active because i leave the gas on even when driving.

Im hoping my fridge might be a bit more efficient now Ive defrosted all the ice from the freezer (dont know if it makes much difference but may help).
 
Upvote 0
I run my fridge off >450W of solar in the day during sunny days in the summer, I setup a relay with timer and momentary push switch to send a signal to S+ for 360 min, as it's on AES it then reverts to gas. If it gets cloudy I can cancel by pressing the push switch again.

Hope this helps.

Cheers
Red
 
Upvote 0

Join us or log in to post a reply.

To join in you must be a member of MotorhomeFun

Join MotorhomeFun

Join us, it quick and easy!

Join us

Log in

Already a member? Log in here.

Latest journal entries

Back
Top