how much power do my fridge use

[dilly]

hi i have a moody 34 with a build in fridge /cooler box. and would like to know how long my batteries will last on full power and how long thy will last on low power .my batteries are new and are 2x 110hrs and one starting at 85 hrs so i would have a idear how lone thy will last uncharged . thank dilly

 

[sailorman]

hi i have a moody 34 with a build in fridge /cooler box. and would like to know how long my batteries will last on full power and how long thy will last on low power .my batteries are new and are 2x 110hrs and one starting at 85 hrs so i would have a idear how lone thy will last uncharged . thank dilly

the fridge will possibly pull 6 > 8Ah full chat

 

[dilly]

thanks

 

[dilly]

and how long would it take my alternator take to recharge them ?

 

[superheat6k]

Your useable charge will be approx 50% of a deep draw capacity battery, so you have 2x110x0.5=110ah. Assuming then it is an average of 8ah you have around 13 hours power at full chat.

Re-charge will depend on your alternator and charge control mode. Assuming say it is 50amps output max, the recharge of 100ah or so would take around 4 hours, possibly longer. You always have to put more in than you get out, and as the charge increases the charge rate decreases.

Battery charging is a very complex subject, but there are plenty if excellent books covering this subject to explain more fully the limitations of batteries and their charging.

 

[Momac]

I have just one leisure battery and one small fridge on our motor boat.
A 110Ah battery in good order has easlily been enough for 24 hour stop without running the engine. That's providing we are are careful to limit the use of interior lights.
Its best, of course, not to deeply discharge the battery.
I think some fridges will cut out if the voltage drops below a certain level but I am sure mine doesn't.
To conserve battery power perhaps it might help to put the fridge thermostat on high power while the engine is running and turn it down with the engine stopped.
Three hours with the engine running seems sufficient to provide an ample re-charge, even if river travelling at slow engine speed.

 

[srm]

Your fridge should not normally run all the time so you need to find out how much it takes out of the batteries in 24 hours. If you have a battery monitor such as the NASA unit this is an easy exercise, either temporarily change the wiring so only the fridge load is monitored, or switch everything else off for 24 hours.
I fitted a new fridge a couple of years ago, deliberately aiming for low power consumption with extra insulation and a keel cooler on the hull. Our consumption was 16 amps over 24 hours, but the old air cooled unit had been using two or three times that.
Also, the efficiency of a fridge varies between full and empty, being best when full. When the content of ours gets low I add bottles of water to keep the thermal mass. Most people keep theirs full of beer!

 

[David2452]

Fit a battery monitor then you will know the answer after a few days averaging out, same goes for the rest of the on board kit.

 

[AngusMcDoon]

hi i have a moody 34 with a build in fridge /cooler box. and would like to know how long my batteries will last on full power and how long thy will last on low power .my batteries are new and are 2x 110hrs and one starting at 85 hrs so i would have a idear how lone thy will last uncharged . thank dilly

A good start on this kind of question is to understand the difference between Ah and A, because all but one post on this thread uses wrong units, and that makes calculations meaningless. We haven't had A/h on this thread...yet...

 

[Rivers & creeks]

If its a cooler box it's about 4Ah, if its a three way gas/mains/12v it's about 8Ah and if its a decent Waeco type it's about 1.5Ah. Multiply by 24 to get a day's usage.

 

[dilly]

thanks guys i was wondering how meny keep there fridge on all the time or do thy just stop/ start and run them when needed. i have new batteries and hooked up to shore power and a good batterie charger and wanted to work my batteries rather than them just sitting there.

 

[VicS]

You have not told us what type and model your fridge /cooler is


If its is about a Peltier element based coolbox then it will draw about 4 amps continuously. That means it will take 4 Ah out of the battery for every hour that it is switched on..... in 24 hours it will take 24 x 4 Ah . That is 96 Ah anf almost half of your battery capacity.... You should not take more than half ... so that would be your limit.

If it is a proper thermostatically controlled, compressor driven fridge ( eg Waeco) then depending upon model it is likely to draw 3 to 6 amps while it is running and cooling down but once cooled it will control on the thermostat and the power consumption will be much less but will depend on the model and ambient temperature.

A small fridge may draw an average of as little as 0.3 amps, or 0.3 Ah per hour at 20C while a larger one could draw an average as much as 1.4 amps or 1.4 Ah per hour at higher temperatures

As you can see a "proper" compressor driven fridge is much much more economical on power consumption . Even at 1.4 Ah per hour you could power it for several days before running the battery bank down to half charge.


Impossible therefore to answer your question without knowing what type of fridge you have.

 

[William_H]

I agree about confusion of terms amps "A" amp hours "AH" then for a fridge you might need to talk about amp hours per day.
The current a fridge uses in amps "A" is typically about 4 amps on 12v system and is part of the design of the fridge.
The fridge has a thermostat which turns it off and on as necessary. A typical number might be for a compressor fridge might be on for a period and off for twice that period. 1 to 2 on off ration. This number is critical to OP question but depends on the insulation of the fridge the ambient temperature and the temperature you adjust the thermostat to. So can vary hugely.
It is this ratio that gives you amp hours (AH) used in any period. Often AH per day.

Recharge time from engine alternator again depends on the voltage setting of the regulator, size of batteries, degree of discharge of the batteries (and hence the amount of time into the recharge period). A smart regulator can increase voltage so increase charge current and keep it higher for a longer period. You need an amp meter or battery charge calculating meter to find if you have recharged enough or if your current engine running is being useful or not. (the actual current into the batteries tapers off after a time so though not fully recharged you might decide you have had enough engine running).

If your fridge is being used as a freezer you can usefully wind the thermostat to max cold when the engine is running.(or battery charging) The fridge running current will not detract from the current going into the batteries but the reduction in fridge temp especially if there is lots of water to be frozen will act like a storage of cold. So when you stop the engine and back off the thermostat the fridge will take some time before it turns on again. ie you have stored more energy from the engine run. Of course you may not want your fridge contents frozen solid.

In the end there are so many variables that only monitoring of battery voltage and charge current can indicate how it is all going.
good luck olewill

 

[macd]

As well as the usual confusion about A and Ah (not to mention the OP's abandonment of A altogether), we've had the old saw about a fridge being more efficient when full. This is only true of front-loading fridges (or bottom-loading, if you can find one): the heavier cold air falls out, ambient takes its place and then has to be cooled. It is very much less true of top-loading fridges, 'cos most of the cold air stays put. That's why most boats have 'em. If you want to be hyper-efficient, don't let people with fat arms rummage in your fridge

William H's suggestions about 'storing cold' does, however, make sense, particularly in hot climates...if you're prepared to routinely mess around with the thermostat/on-off switch and anticipate using the engine (or some other power source) on a regular basis.

To the OP's question: for a fridge on a boat that size, I'd say 6-8A draw is a tad high. Probably more like 5A, possibly less. Given reasonably good insulation, in UK summer (ha!) conditions, the compressor should be working no more than half the time and ideally somewhat less. So the mean drain on the batteries would be around 2.5A, or 60Ah over 24 hours. That's as much as I'd want to drain a 170Ah battery bank on a regular basis, especially as it takes no account of other drains on the batteries. As someone wrote, without knowing more about the OP's charging regime, it's impossible to say how long it would take the engine to replenish those 60Ah.

And agreed, the OP would be much better able to assess all his total power budget, fridge included, with a battery monitor, or at least an ammeter. Not sure what he means by "full power" and "low power".

 

[vyv_cox]

We run a standard, front opening Waeco 60 litre fridge, with a water cooled Danfoss compressor instead of the standard air cooled version, in Greece. We turn it on in April/May and off again when we leave the boat, usually in October. At the start of its cooling cycle it takes about 4 amps, reducing to about 3 amps after 6 - 8 minutes. It then switches off on the thermostat for about 12 minutes, dependent upon ambient conditions.

We originally had 2 x 110 Ah batteries. These would run the fridge reasonably well but their voltage would be down to about 12.0V first thing in the morning, after finishing fully charged from solar panels the previous evening. We then increased the battery bank to 3 x 110 Ah, with which the battery voltage is rarely less than 12.3 - 12.4V first thing.

With a Sterling alternator regulator it almost never takes more than one hour to recharge either of these battery banks. Initial current draw is anything up to 55 amps, usually declining within about 10 minutes to half that, although occasionally the higher figure will take somewhat longer. We usually see float voltages in around 50 minutes.

 

[vyv_cox]

As well as the usual confusion about A and Ah (not to mention the OP's abandonment of A altogether), we've had the old saw about a fridge being more efficient when full. This is only true of front-loading fridges (or bottom-loading, if you can find one): the heavier cold air falls out, ambient takes its place and then has to be cooled. It is very much less true of top-loading fridges, 'cos most of the cold air stays put. That's why most boats have 'em. If you want to be hyper-efficient, don't let people with fat arms rummage in your fridge

This is an interesting one that I tried to get YM interested in testing but they refused for 'political' reasons (another writer is the 'fridge expert'). The specific heat of air is low, around 1.0 kJ/kgK, whereas of water it is 4.2 kJ/kgK but of course a kilogram of air occupies 1000 times the volume of a kilogram of water. This suggests to me that it will take little energy to reduce the temperature of the ambient air by 20 - 30 degrees C, even if the fridge is empty. I would have expected the energy consumed in replacing a cold beer can with a warm one would be far greater than, say, leaving the door open for 30 seconds..

 

[nigel1]

One way to reduce power consumption to the fridge is to keep it tidy and organised, so you are not rummaging around trying to locate something.
The other thing I do is to keep zip lock bags stuffed with that polystyrene packing stuff, and use them to fill the voids (dont overfill the fridge or circulation will be lost). When the fridge door is opened, the cold stays in, and less cold air to fall out.

 

[Rossynant]

I would have expected the energy consumed in replacing a cold beer can with a warm one would be far greater than, say, leaving the door open for 30 seconds..

Another way to reduce power consumption - old trick of freezing beer (in cans) at home or at store to stuff the fridge with may still work to advantage; this was a good way for traditional ice-box. Not sure if cans aren't weaker nowadays, but easy to try.
Naturally freezing a lot of water (let's call it such...) inside fridge, while plugged in marina or on engine, can keep it cold for a day or two without any power consumption - until ice melts or get's used

 

[sailorman]

The old trick of freezing beer (in cans) at home or at store to stuff the fridge with may still work to advantage; this was a good way for traditional ice-box. Not sure if cans aren't weaker nowadays, but easy to try.
Naturally freezing a lot of water (let's call it such...) inside fridge, while plugged in marina or on engine, can keep it cold for a day or two without any power consumption - until ice melts or get's used

we always double up on certain home cooking & freeze the spare all ready to take sailing with us. it saves cooking time o/b & is a good start for the fridge.
we also freeze milk & do the same
last summer rolleyes we took 4 home frozen meals on holiday, mainly if late in hbr we had a ready meal. one meal was still frozen after 3 weeks in what we call "the football pitch"

 

[vyv_cox]

In some countries, Greece, France and Italy in my experience, you can buy bottled water that has been deep-frozen. Cost is usually little more than buying it as normal. Helps a lot with the fridge temperature but we find the freezing process throws out solids/salts and changes the taste so we don't usually drink the water once it has thawed out. OK for cooking though.