Compressor fridge calculation

[Nabsim]

There are too many makes and models about to compare these days but from my experience and experience of friends I concluded the following:
12V compressor fridge would use approx 24A in a 24 hour period
12V compressore fridge/Freezer would use approx 48A in a 24 hour period

When David did his tests with the 240V fridges with an inverter it was almost the same as I was finding

Can't say I notice much difference if it's a hot day in Wiltshire or freezing up on Cairngorm. Not saying usage doesn't change just I haven't noticed. I usually just react to battery S0C

 

[r4dent]

We have 300w of solar on the roof, and 2x110ah lead carbon gel batteries. We usually watch an hour or two of TV in the evening. We do use the van in the winter, but at that time of year we would normally go for ehu so that we can charge our ebike batteries.
I could add more solar but I would have to upgrade the controller to do it.
Any guidance would be welcome

You need to know three things
a -How much power do you use each day? (amp hours )
The fridge specification should tell you how much power it uses in 24 hours.
If it is given in watt hours multiply by 12 to convert to amp hours
Do NOT use the power consumption (amperage) or power (watts) figures as the fridge switches on /off in use.
A typical 12v compressor fridge will use 50ah per day.
Plus 10 ah for your TV and another 10 ah for lights / pump / etc
So say 70 ah per day.



b- How much power does your Solar Generate (amp hours)
This depends on how much sunshine there is (i.e. sun shines for longer in June than it does in January)
how high in the sky the sun is (i.e. where in the world you are, sun is higher nearer the equator)
The table is an indication for a 150 w panel. For your 300w panel double the values.
e.g in Brighton in March = 40 x 2 = 80 ah per day

c- How much capacity does your battery hold.(amp hours)
For your battery set up the usable capacity is half the nominal capacity, i.e. (110 + 110) / 2 = 110 ah

Once you have these values you can answer your question.

Q1 -Do I produce enough power each day to ensure the battery isn't flattened?.
If ‘a’ is greater then ‘b’ then NO, otherwise YES
70 is NOT greater than 80 so your charging is greater than your usage

Q2 - How many days will my battery last without the sun shining.
Two consecutive days without sun will be a problem.

 

[YeoBlade]

If you drive more and stop over less you’ll not look back, but I guess if you go to Northern Scotland in the winter, drive less and stopover more it may not do so well.

Just put it outside then, fridge not needed.

 

[colinm]

At 1 amp every 4 hours Trev so 6 amps a day then that would mean it would run for 15 days on a 90ah battery, or am I calculating it wrongly?

It's a Skoda fridge.

 

[Robmac]

It's a Skoda fridge.

...running on Linux.

 

[r4dent]

At 1 amp every 4 hours Trev so 6 amps a day then that would mean it would run for 15 days on a 90ah battery, or am I calculating it wrongly?

You are correct ..... the calculation is very wrong

a) Amps is a rate of consumption, amp hours is the measure of quantity
Using a bucket of water analogy
How long will water run out of the bucket depends on the size of the bucket and how fast the water runs out.s
Amp hours is the size of the bucket (16 pints)
Amps is the rate the water runs out of a bucket (2 pints a a minute)
Answer = 8 minutes..

Assuming that you mean

At 1 amp hour every 4 hours Trev so 6 amp hours a day then that would mean it would run for 15 days on a 90ah battery, or am I calculating it wrongly?

This doesn't look correct as a typical compression fridge uses a lot more that 6 ah / day, 50 ah / day is a more realistic figure.
So if we use that value

50 amp hours a day then that would mean it would run for 1.8 days on a 90ah battery, or am I calculating it wrongly?

Still wrong because a 90 ah lead battery is "flat" after providing 45 ah.

Using the bucket of water analogy
The hole is half way down the bucket so only half the water runes out of the bucket.
Answer = 4 minutes..


So @ 50 ah / day a 90 ah battery lasts less than a day.

 

[Robmac]

You are right ..... the calculation is very wong

a) Amps is a rate of consumption, the measure of quanttity is amp hours.

So assuming you mean


This doesn't look correct as a typical compression fridge use al lot more that 6 ah / day, 50 is a mire realistic figure. So if we use that value

I took it from Trevs post which states "My small fridge uses a qtr of a amp an hr, 250mA, its a tripple a rated fridge and sealed"

This is at 240V via a 600W Inverter powered by a 90AH LA Battery. I'm not saying his figures are wrong but if they are right I want one.

 

[Pudsey Bear]

I took it from Trevs post

That was your first mistake

 

[Robmac]

That was your first mistake

It's all black magic to me Kev, but I'd like to know.

 

[r4dent]

I took it from Trevs post which states "My small fridge uses a qtr of a amp an hr, 250mA, its a tripple a rated fridge and sealed"

This is at 240V via a 600W Inverter powered by a 90AH LA Battery. I'm not saying his figures are wrong but if they are right I want one.

The import bit that you didn't include in your calculation is "This is at 240V"
Which means the draw from the battery is 0.25 amp * (230 / 12) = 5 amp.
Because compressors switch on/off this is the same as the 50 ah per day that I used.

So based on Trev's experience a 90ah battery would not last a day.

 

[Robmac]

The import bit that you didn't include in your calculation is "This is at 240V"
Which means the draw from the battery is 0.25 amp * (230 / 12) = 5 amp.
Because compressors switch on/off this is the same as the 50 ah per day that I used.

So based on Trev's experience a 90ah battery would not last a day.

Sorry no I didn't because I was replying to Trev and I knew from previous posts he used a 240v fridge, but yes I should have included it in my post.

 

[Robmac]

Still wrong because a 90 ah lead battery is "flat" after providing 45 ah.

Yes I did concede that in the next post.

 

[Rob H]

During our present stay in France and Spain our old 3 way fridge has not been getting anywhere near cold enough, especially on gas. I had decided to give it a service when we return to blighty (or pay someone to do it!), but today one of the push fit pipes on the kitchen tap decided to disengage itself, so I had to spend part of the day dragging the fridge out to get to the underside of the tap. While the fridge was out I came to the conclusion that it was past rescue and the time has come to go with a compressor type.
It looks as though the price difference between the 12v and 230v types is not as great as it was, so I'm looking at a 12v.
I would appreciate some guidance as to whether the present power setup is suitable.
We have 300w of solar on the roof, and 2x110ah lead carbon gel batteries. We usually watch an hour or two of TV in the evening. We do use the van in the winter, but at that time of year we would normally go for ehu so that we can charge our ebike batteries.
I could add more solar but I would have to upgrade the controller to do it.
Any guidance would be welcome

I get by summer and autumn with 200w solar roof plus charge from the vehicle alternator when on the move, into 2 x 80ah lead acid, running a Dometic 12v chest style compressor fridge with diy computer fan for additional heat exchanger airflow, I installed in 2012 system still working fine.

 

[Nesting Zombie]

The important bit must be -
Is the Cider cold ?

If YES then all is good

 

[Robmac]

The important bit must be -
Is the Cider cold ?

If YES then all is good

I drink Bitter though Paul, don't like it too cold!

Having said that a nice cold pint of Cider would have gone down well in recent weeks.

 

[Nesting Zombie]

BITTER !
(Heathen)

 

[Nesting Zombie]

Although maybe they are secretly’Smart Fridges’ & there’s a direct correlation of Fridge temperature to what the favourite Gulpa is !.
White wines, Sparkling Wines Cider = COLDER fridge.
Bitter, Ales & Other ‘Stuff’ Don’t even plug your fridge in just use it as a Cupboard .

Right, On to Climate change & World peace.

 

[Robmac]

Although maybe there’s a direct correlation of Fridge temperature to what the favourite Gulpa is !.
White wines, Sparkling Wines Cider = COLDER fridge.
Bitter, Ales & Other ‘Stuff’ Don’t even plug your fridge in just use it as a Cupboard .

Right, On to Climate change & World peace.

See Paul, us Bitter drinkers are obviously more concerned about using less electricity and halting climate change.

 

[Nesting Zombie]

I’m putting in an ‘Exemption’ clause for Cider & Wine drinkers if born before 2055 !.

 

[trevskoda]

At 1 amp every 4 hours Trev so 6 amps a day then that would mean it would run for 15 days on a 90ah battery, or am I calculating it wrongly?

I wondered that two, its says on the back plate 250mA. i dont think thats correct as it only ran 24 hrs before the battery was well down.
Got me thinking now.
This all i can find online.