Technical (a bit) battery/fridge question

Loggo

Loggo

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I have a 'fridge that plays heck with my batteries espcially on hot days (natch'). It has a consumption of 3.8w.
I have found a solar panel that claims an output of 8w. I know that this is a maximum but there seems a good margin, and 'fridge current requirement should rise with solar output.
This just leaves the battery to cope with the overnight running when no-one is opening the door to remove beers etc. and, hopefully, the compressor will be running less.

Am I clutching at straws ?
 
yeah, but watts and amps are so misleading at times. :)

No solar panel of 8w is going to run a fridge.
 
Well guys - all my frailties regarding anything of a practical nature are now well exposed - I went the arts route in school !

The manual went to some length to explain how to hang the door from the right or left hand side but does not go into details like power consumption (to be fair it ignores it in a very wide variety of European languages)

I got the 3.8 whatevers from a verbal enquiry but obviously bloomered it up.

The 'fridge is a Engel SRBD and makes short work of an 85AH battery. Current (ha !) practice is only to run it when the engine is running but that is far from satisfactory. Seems a shame to get a generator just to keep a few esentials cool though.
 
I have ample solar power and a new 250AH service batts and I always switch my fridge off overnight. If every thing melts you need more insulation. By their nature front opening fridges are hideously inefficient. Think waterfalls of cold air! 85Ah is not enough. And 3.8amps is quite a thirsty fridge, my ten yr old Frigoboat Paris 35 draws 2.8amps. It's very rusty but just resurrected by completely redoing power supply.
 
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Loggo, we are having a bit of a stir here in order to help.

You have a fridge that uses about 4 amps - every hour that's 4 Amp/Hours extracted from the battery.

The battery is small for such work. Even the nominal 85 AHr capacity will in practice be about 70 AHr, and it will die at about 45 AHr, when the voltage will drop below the level needed to run the fridge.

So the usable capacity of your battery is about 25 AHr (70 - 45), and the fridge is extracting 4 every hour. That's a life of about 6 hrs.

You need a bigger battery, and some means of ensuring that it is fully charged - that usually means an intelligent charger. A solar panel of about 60 watts would keep the battery topped up, but no more.

NB. Figures are approximate to keep the maths attractive, but should not be too far out. Honestly. :)
 
Thanks Sarabande. I wish I could get six hours but the battery is five years old and well past the first flush. Perhaps apair of 110s will get me through the night (providing I don't open the door)
 
My fridge consumes around the same as yours, interestingly maximum when it first comes on and reducing by about 0.5 A until it cuts off. I have 125 Watts of solar panel and three domestic 110 Ah batteries. This power runs the fridge (and other consumers) easily at this time of year 24 hours per day but is only just enough towards the end of September. This in Greek sunshine.
 
vyv_cox said:
My fridge consumes around the same as yours, interestingly maximum when it first comes on and reducing by about 0.5 A until it cuts off. I have 125 Watts of solar panel and three domestic 110 Ah batteries. This power runs the fridge (and other consumers) easily at this time of year 24 hours per day but is only just enough towards the end of September. This in Greek sunshine.
Click to expand...

We're 24volt and the fridges are water cooled so it's plus pump.

In the UK climate I was informed a fridge as rule of thumb will run 8 out of 24hrs and up to 12hrs in hotter climes.

There is so many other factors other than temperature though, healthy and adequate batteries, insulation, front or top loader, how full the frdges are, times of entry........

Unfortunately we have a front loader and this only ever gets used for cans and bottes and is only used on shore power or engine. Otherwise all else goes into the top loader.

Without doubt I would say the fridge is the worst power soak for us all and must be managed to get the best use of AH's we can.

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First things first.

85 ampere hours is very small for a service battery. The whole 85 is never available, even with a new battery when fully charged, so I would fit as big a battery as you can.
About the fridge.
I have an Isotherm which draws about three amps when the compressor is running, which is for about third of the time, so reckon it as equivalent to 1 amp over 24 hours, which is 24 ampere hours. It is supported by an 80 watt solar panel, which generates just over 3 amps maximum in sunny Spain.
And when the sun is low, or the panel is in shadow, it will be less, not to mention at night. Say 2 amps average for 12 daylight hours, or 24 ampere hours
Note that even the maximum figure does not equal 80watts. 3 amps times (say) 14 volts (charging voltage) equals 52 watts.
The discrepancy is caused by the fact that the manufacturers all quote a wattage gained by multiplying the on load max current by the off load max voltage, (which is about 19 volts), which is an impossible situation There is technical term for this sort of claim; it is called 'lying'
Anyway, this size of panel will supply enough to run the fridge, with a bit, but not much, spare in the summer when daylight lasts longer. I hope this puts things into perspective.
I would recommend Nigel Calder's Boat Maintenance book as having the best 'beginners guide' to understanding batteries on boats that I have read.
 
I think I have the same fridge. I have a Rutland 913 and a 60w solar panel too. I am soon adding another 60w panel as it can't cope with the demand.
I'm not too pleased with the 913 it doesn't give out much current at all compared to the panel
 
grumpygit said:
We're 24volt and the fridges are water cooled so it's plus pump.

In the UK climate I was informed a fridge as rule of thumb will run 8 out of 24hrs and up to 12hrs in hotter climes.
Click to expand...

My fridge is also water cooled. The pump used to be a fairly large Shurflo one which I thought would be a big consumer. I replaced it with a PAR Max1 and simultaneously fitted a volt/amp meter. I was amazed to find how little either pump consumes, less than 0.5 A running at almost zero pressure, simply pumping water through the exchanger and overboard.

Ours only varies very slightly from 33% on, 66% off, even in the middle of summer in Greece.
 
vyv_cox said:
My fridge is also water cooled. The pump used to be a fairly large Shurflo one which I thought would be a big consumer. I replaced it with a PAR Max1 and simultaneously fitted a volt/amp meter. I was amazed to find how little either pump consumes, less than 0.5 A running at almost zero pressure, simply pumping water through the exchanger and overboard.

Ours only varies very slightly from 33% on, 66% off, even in the middle of summer in Greece.
Click to expand...
Ah! my air-cooled one runs (at 38Cambient) 20% of the time - takes 4.5amps on start-up and 2.2 amps running.

Always said air-cooling was more efficient ;-)
 
charles_reed said:
Ah! my air-cooled one runs (at 38Cambient) 20% of the time - takes 4.5amps on start-up and 2.2 amps running.

Always said air-cooling was more efficient ;-)
Click to expand...

Hi Charles

I can't argue with your comment about air cooling except that the water temperature will be more constant than ambient air temp.

As I said in my earlier post, there are so many other factors and you may be more efficient in one or more of those fields to get as low as 20%,..... which can only be good for you.

____________________________________________________________________
 
charles_reed said:
Ah! my air-cooled one runs (at 38Cambient) 20% of the time - takes 4.5amps on start-up and 2.2 amps running.

Always said air-cooling was more efficient ;-)
Click to expand...

Charles.
What type of fridge? Mine is a Waeco 60 litre front opener, walls and door are just not thick enough to have very efficient insulation.
On the Sadler the airflow over the air cooled heat exchanger was very poor and it ran almost constantly, even in Holland. We changed to the current arrangement to get over that drawback. 20% on is pretty impressive though.
Vyv
 
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