fridge maths

[mboat01]

so Ive been looking at a 12v dometic fridge and it says "Energy consumption (DC@5/25°C) 2.64 kWh/24h" , which surely means 2640/12= 220 Ah /24hr ????

Surely that cant be right ? - Thats pants !

If I get a cheap 230v rated at 108kwh/yr = 108000/365 = 295 wh/24hrs = 24.5 Ah/24hrs, say even only 75% inverter efficiency = 32Ah/24hrs 

Wut am I missing here ???? 

[mrsmelly]

3 minutes ago, Sassy Lass said:

so Ive been looking at a 12v dometic fridge and it says "Energy consumption (DC@5/25°C) 2.64 kWh/24h" , which surely means 2640/12= 220 Ah /24hr ????

Surely that cant be right ? - Thats pants !

If I get a cheap 230v rated at 108kwh/yr = 108000/365 = 295 wh/24hrs = 24.5 Ah/24hrs, say even only 75% inverter efficiency = 32Ah/24hrs 

Wut am I missing here ???? 

I dont understand your gobbledygook ? In a nutshell 12 volt and modern mains fridges use a similar amount of power. Mains fridges huge choice and range

[WotEver]

8 minutes ago, Sassy Lass said:

... which surely means 2640/12= 220 Ah /24hr ????

Surely that cant be right ? 

Yes it’s right. It’s a Absorption fridge, not a compressor fridge. The 12V is only designed to be active while the car is dragging the caravan to the site. 

 

 

Edited March 11, 2019 by WotEver

[peterboat]

Just now, WotEver said:

No it can’t be. Nothing wrong with your maths though. I suspect that someone misplaced a decimal point and that 2.64kWh should have been 0.264kWh. 

Exactly. my shoreline is 48ah daily and that is a fridge freezer, did I tell you about when my old 240 volt fridge defrosted because the inverter failed, just another thing to go wrong init

[WotEver]

Just now, peterboat said:

Exactly. my shoreline is 48ah daily and that is a fridge freezer, did I tell you about when my old 240 volt fridge defrosted because the inverter failed, just another thing to go wrong init

See my edited post. 

[peterboat]

1 minute ago, WotEver said:

See my edited post. 

That explains everything init

[mboat01]

thanks. I just needed a sanity check. maybe its just dometic that are pants - equiv compressor to el cheapo 230v I was looking at (£99) is £1265 !!!!!!!!!! and uses "0.36 kWh/24h" so 360/12 = 30Ah/24hrs, i.e., the same juice . ok, so its probably a bit nicer unit, but I could get x12 of the others for the same price ?

[Boater Sam]

Its all due to production quantities and the country of origin. The "12v" Danfoss compressor package is a Danish component I think. Denmark is a country with a very high cost of living so its expensive.

Then the fridge has to be produced in quite small quantities compared with the flood from Italy of cheap makes.

It has always been the same.

[roland elsdon]

Most if our campervan  equipment is dometic .water heater cooker shower compartment fittings,  aircon system.

it was ex hire before us ( built in 2005)

no dometic parts have ever gone wrong.

i think this issue is with the maths not the equipment.

also i assume 5/25 is abut steep for uk

try 5/18 at best and 5/10 half the time.

the dometic fridge does 2 days on our 100 anp hr fridge frequently at 30 degrees, before the 12.05 battery cuttoff comes in, usually if the water pump and the fridge are going concurrently.

100 amp hr battery sorry 

[Sea Dog]

9 hours ago, peterboat said:

Exactly. my shoreline is 48ah daily and that is a fridge freezer, did I tell you about when my old 240 volt fridge defrosted because the inverter failed, just another thing to go wrong init

@mrsmelly has a spare inverter.

 

...Probably a spare fridge too! 

 

There's a pretty finely balanced arguement for the choice between a 230v fridge and a 12v fridge, but it's much more clear cut in favour of a compressor fridge in our application.

Edited March 12, 2019 by Sea Dog
Sentence construction

[Alan de Enfield]

5 hours ago, roland elsdon said:

i think this issue is with the maths not the equipment.

No, it is the equipment.

 

A 12v Absorption fridge does not have a thermostat and is running continuously - They are designed for caravan use and simply as a 'temporary' method of keeping food whilst in transit.

if you read the manuals for them they will state (words to the effect of)

 

"Before departure, run the fridge on a main lead overnight to get the fridge down to temperature, when towing DO NOT leave the fridge switched on if going into the 'services' for any length of time as it will flatten the battery, plug the fridge into the mains supply immediately on arriving at the camp-site"

 

Edit :

 

Found it :

 

Electric operation

The dual voltage electric equipment is for operation from the main

12 volt battery in the car when the caravan is on tow, or from mains

voltage electricity when a 220 to 240V a.c. supply, with satisfactory

earthing, is available on a site. Before using the refrigerator on electricity

make sure that the electricity supply is suitable for the refrigerator.

It is important to understand that 12 volt operation is only intended

to be used while the car engine is running and is charging the battery,

otherwise the battery may be discharged to a point where it will not

restart the engine. (The current drain at 12V is 8 amps minimum). When

at rest for more than a short period, the caravan should be levelled and

the refrigerator switched over to mains voltage, if available, or the 12V

supply switched off and the refrigerator started up on bottled gas.

Before connecting to a mains voltage supply, it is important to

make certain that the circuit to, and in, the caravan is properly and

effectively earthed.

When operating on mains voltage, the temperature in the refrigerator

is thermostatically controlled and can be adjusted by means of the

knob (6) of the thermostat. The 12V circuit is not thermostatically

controlled and the cooling unit will operate all the time the refrigerator

is connected to 12V and switched on. 12V operation is, therefore,

only intended to be used for relatively short periods, i.e. when the

caravan is on tow. It is not intended for extended periods of use from

a continuous 12V supply, otherwise the fresh food compartment may

become too cold for the satisfactory storage of fresh foods and drinks.

For connection to the 12V supply, a two-way terminal block is

located behind the right-hand end of the control panel (1, fig 1) at the

top of the refrigerator.

For connection to a 220-240V electricity supply, the refrigerator

is provided with a 3-core mains lead which is intended for connection

to a properly earthed plug and socket outlet. In the United Kingdom,

the following plug connection instructions must be observed.

IMPORTANT: The wires in the mains lead of this appliance are

coloured in accordance with the following code.

GREEN-AND-YELLOW : E A R T H ,

BLUE : N E U T R A L ,

BROWN : LIVE

As the colours of the wires may not correspond with the coloured

markings identifying the terminals in your plug, proceed as follows :

The wire which is coloured GREEN-AND-YELLOW must be con

nected to the terminal in the plug which is marked with the letter E

or by the earth symbol -=L- or coloured green or green-and-yellow.

The wire which is coloured BLUE must be connected to the terminal

which is marked with the letter N or coloured black.

The wire which is coloured BROWN must be connected to the

terminal which is marked with the letter L or coloured red.

W ARNING -T HIS APPLIA NCE MUS T BE EART HED.

In the United Kingdom, the plug or circuit to the refrigerator must

be fitted with a fuse not greater than 5 amps. If a 13 amp. (B.S.1363)

fused-plug is used, it should be fitted with a 3 amp fuse. In other

countries, the fuse rating will depend on local practice.

During installation, a suitable socket outlet for the mains voltage

supply should have been fitted in the caravan, near the refrigerator,

in a position readily accessible to the user. In the United Kingdom,

all mains voltage wiring in the caravan must be installed in accord

ance with I.E.E. regulations, including the use of an outlet and coupler

to BS 4343/CE E17.

Voltage Selector Switch

The voltage selector switch (7) can be set to three different positions,

identified by the following symbols :

To start the refrigerator on electricity, see that the gas control knob (5)

is at '0' (off), set the voltage selector switch (7) to the voltage required,

then connect the refrigerator to the appropriate voltage supply.

If on mains voltage (220-240), turn the thermostat knob (6) to

or

setting No. 3

4.

Edited March 12, 2019 by Alan de Enfield

[roland elsdon]

Clearly our dometic fridge ( in australia) is something else rebranded. It defo has a thermostat you can hear it cycling on and off, and it isnt gas.

 

however i still say our dometic stuff is ok, compared with some of stuff on boat in uk

[alan_fincher]

Sme Dometic / Electrolux 3-way Absorption fridges do have a thermosta when run on 12 volts.

 

None-the-less they will still be complete battery killers compared to a compressor fridge.

 

It's not an option I would chose to ever use on a boat.  Ours is "plumbed in" to LPG and 230v, but the 12v connection is unused.

[Alan de Enfield]

3 minutes ago, roland elsdon said:

Clearly our dometic fridge ( in australia) is something else rebranded. It defo has a thermostat you can hear it cycling on and off, and it isnt gas.

 

however i still say our dometic stuff is ok, compared with some of stuff on boat in uk

Dometic is 'good-stuff' but you are not comparing like-for-like.

 

There are two 'types' of electrical fridges :

 

1) Absorption

2) Compressor

 

They work in very different ways to achieve the 'same' result.

 

Fridges on boats should be (unless the installer is ignorant of the differences) Compressor types. - more efficient and fitted with a thermostat.

1 minute ago, alan_fincher said:

Sme Dometic / Electrolux 3-way Absorption fridges do have a thermosta when run on 12 volts.

The extract I quoted in post #11 is from the Electrolux 3-way fridge manual

 

25 minutes ago, Alan de Enfield said:

The 12V circuit is not thermostatically

controlled and the cooling unit will operate all the time the refrigerator

is connected to 12V and switched on.

 

[alan_fincher]

42 minutes ago, Alan de Enfield said:

The extract I quoted in post #11 is from the Electrolux 3-way fridge manual

 

Yes I read that.

 

It's actually from an Electrolux 3 way fridge manual.

 

I believe they are not all the same.

[roland elsdon]

No you are right our fridge here has a compressor. We had a 12v absorption fridge on the boat when we first got it. It did the batteries overnight... first thing we swapped.

[Alan de Enfield]

1 hour ago, alan_fincher said:

 

Yes I read that.

 

It's actually from an Electrolux 3 way fridge manual.

 

I believe they are not all the same.

 

You may well be correct - I took it from the RM series manual (which is the one we had)

[mboat01]

You're quite right not really fair to compare a compressor to absorption - Things are moving on rapidly in 230v fridge world - this one is A+++ 61kwh/yr !!! = 18Ah/day (at 75% inverter efficiency) for £220....just wish it was  < 500mm deep ?