How many hrs can I run my fridge?

[carrswood]

my electrical knowledge is limited - could you help me with some calculations.

When I have some money saved I intend to replace my 12v fan cooled fridge box with a more efficient external compressor type (isotherm, waeco etc).

In the meantime could you help me with my current fridge:
12V supply
Consumption = 46W. Therefore am I right she's using 3.8 ah ?

NB: this fan cooled fridge has no thermostat - so its always cooling and constantly drawing 3.8 ah.

But, the good news is an overnight test and thermometer inside showed a temp of 2.2 degrees (more than cold enough).

My question is, if I was to connect this fridge to a dedicated 120ah battery for the fridge only - how many hrs use could I run it for?
My understanding is a good quality battery should not reach a state of charge less than 50%.

My simple maths would say I would have 60ah of usable battery life to utilise - ie 15.8 hrs running. Now I know it doesn't necessarily work like that!. For one the current is perhaps weaker in the latter stages and the more you de-charge the battery then the longer it will take to re-charge the battery with your alternator.

Could someone help me here with the True facts I should be aware of?

 

[earlybird]

I think that your calculations aren't far out, and the battery shouldn't "weaken" too much if in good condition but two points:-
2.2 deg seems very cold for a fridge, 6 or 7 would be OK, your unit is doing a lot of work, hence draining the battery.
Putting 60 aH back into the battery from the alternator will take a lot of engine running.
Ed. missed the aH units error, but numbers were near enough

 

[whipper_snapper]

In the meantime could you help me with my current fridge:
12V supply
Consumption = 46W. Therefore am I right she's using 3.8 ah ?

NO! It uses 3.8 Amps at 12V

Amp hours are completely different things. Amps is the flow rate of the electricity, amp hours is a measure of the capacity of a battery.

My question is, if I was to connect this fridge to a dedicated 120ah battery for the fridge only - how many hrs use could I run it for?
My understanding is a good quality battery should not reach a state of charge less than 50%.

3.8Amps for 1 hour takes 3.8 ah from the battery.
So yes, in principle, you can run the fridge for 15.8 hours to take a 120amp hour battery to 50% charge.

 

[johnalison]

I suspect that by the time you've got a big enough battery to run your Peltier-effect box you'll have used up more money and space than by getting a compressor box. A lot depends on the degree of insulation. I have something over 200 Amp/hr on the service side and leave the (built-in) fridge running continuously. Even without a solar panel I have been off-charge for up to a week with an acceptable voltage-drop.

 

[CaptainBob]

We've got an external compressor type. It's pretty heavy on the batteries, though I've no idea of its rating/current consumption. I'd be dubious about calculations anyway as the batteries degrade, ambient temperature affects things, what you have in the fridge affects things.

I'm pretty religious with our routine.

We have put quite a few large cool blocks in the fridge, some inside the element, some held up against it. After it has been running for some time, these freeze and hence have a smoothing effect on the temperature fluctuation graph.

When the engine is running (which usually is not for very long), if we're without shore power at our next destination, the fridge is run at maximum.

As soon as the engine is not running, we switch the fridge down to a very low setting (ie runs warm), but due to the cool blocks it holds temperature for quite some time.

If we're overnighting I switch to one battery only to ensure we can start the engine if need be.

Exception to this is when the wind is up, in which case I leave both batteries on so long as the wind gen is keeping them much about about 12.8V when the fridge is on. This has happened a lot so far this year!

So far I don't recall anything being spoiled. And all this is done with two 110Ah leisure batteries. On occasion we've done 3 days straight without power or engine, and this includes fairly extensive use of laptops running off the same system.

Having the Marlec charge controller with individual battery voltage readouts is extremely handy!

 

[carrswood]

thanks
earlybird - at 2.2 degrees I could almost afford to have the fridge running
1/3rd of the time (by some time clock arrangement or something like that)

I'd still love to know how much a certain 'rated' alternator (say 80ah) will put back into a 120ah battery over 1hr daily?

 

[bobgoode]

Your 120 AHr battery is unlikely to be fully charged by a standard alternator...you would need some sophisticated controller to get anywhere near fully charged. Probably best to work on 100 to 110 AHr capacity which if you discharge to 50% will give a useable 50 / 55 AHr. At a continuous 4 amps thats 12 to 14 hours use.

 

[William_H]

Fridges

I'd still love to know how much a certain 'rated' alternator (say 80ah) will put back into a 120ah battery over 1hr daily?[/QUOTE]

To answer Carswood question. This depends on the exact voltage the alternator is regulated to and the degree of discharge of the battery. It is unlikely that the 80amp rating of the alternator will be a limiting factor in the recharge. A bit like havinga car capable of 100miles per hour in reality it does not determine how long it takes you to get home from work in the evening. Speed limits trafic etc.

The only way to tell how many Amp hours have been rechraged into a battery in one hour is one of the battery monitor devices. This will record and totalise the current in very short time slots. An ampmeter and you regularly monitoring can also give you an idea.

Roughly speaking with a 50% discharged 120AH battery you might see 20 amps going in for about 5 minutes then this will taper to perhaps 5 amps to near the end of the hour.
The recharge rate can be kept higher for longer with a higher regulated voltage ie14.5 instead of the usual 14v. This can automatically be done with a smart or stepped charge controller.

Re Fridges. I think for short voyages a big block of ice in a well insulated box is as good as carrying extra batteries. The peltier (fan cooled) fridges are to my mind useless. I can only suggest that if you got down to 2.2 degrees that it must have been cold outside. The compressor fridges are far better using much less total current on a thermostat.
Another approach might be to have powerful compressor fridge that can pull the temperature down quickly while the engine is running or on dockside power. Then rely on insulation to keep it cold until the next engine run. Strore cold in stead of electricity. Or a combination of both. Certainly a fridge system on a small boat is unlikely to be a turn on and forget like it is at home. It will need constant husbanding of power resources. good lcuk olewill

 

[john_morris_uk]

I'd still love to know how much a certain 'rated' alternator (say 80ah) will put back into a 120ah battery over 1hr daily?

To answer Carswood question. This depends on the exact voltage the alternator is regulated to and the degree of discharge of the battery. It is unlikely that the 80amp rating of the alternator will be a limiting factor in the recharge. A bit like havinga car capable of 100miles per hour in reality it does not determine how long it takes you to get home from work in the evening. Speed limits trafic etc.

The only way to tell how many Amp hours have been rechraged into a battery in one hour is one of the battery monitor devices. This will record and totalise the current in very short time slots. An ampmeter and you regularly monitoring can also give you an idea.

Roughly speaking with a 50% discharged 120AH battery you might see 20 amps going in for about 5 minutes then this will taper to perhaps 5 amps to near the end of the hour.
The recharge rate can be kept higher for longer with a higher regulated voltage ie14.5 instead of the usual 14v. This can automatically be done with a smart or stepped charge controller.

Re Fridges. I think for short voyages a big block of ice in a well insulated box is as good as carrying extra batteries. The peltier (fan cooled) fridges are to my mind useless. I can only suggest that if you got down to 2.2 degrees that it must have been cold outside. The compressor fridges are far better using much less total current on a thermostat.
Another approach might be to have powerful compressor fridge that can pull the temperature down quickly while the engine is running or on dockside power. Then rely on insulation to keep it cold until the next engine run. Strore cold in stead of electricity. Or a combination of both. Certainly a fridge system on a small boat is unlikely to be a turn on and forget like it is at home. It will need constant husbanding of power resources. good lcuk olewill

Will, I was about to reply in exactly the same sort of terms! You might be interested to know that some of the modern fridge installations available in the UK automatically detect whether 'extra power' is available from the engine charging/wind generator/solar panels and switch to super cooling mode to freeze down a holding plate in the cool box.

I agree 100% with your comments on batteries and charging. As we have agreed in the past, an 80 amp alternator very rarely actually gives out 80 amps into a battery bank - even if the batteries were capable of accepting such a high charging current.

 

[vyv_cox]

I have a 55 Amp alternator charging three 110 Ah batteries through a Sterling alternator controller, monitored by a digital ammeter/voltmeter. I run a water cooled refrigerator 24 hours per day, with 125 watts of solar panels, in Greece. The refrigerator consumes about 3.5 amps. First thing in the morning the batteries are at their lowest, about 12.3 volts at present but reducing as solar power becomes less in the year. When we start engine at this time the alternator delivers the full 55 amps, indicated by the ammeter. Batteries are fully charged, as shown by the 'float' mode on the Sterling, in about one hour, when current is usually about 5 amps.

These batteries are now 6 years old, so the treatment they are getting doesn't seem to bother them.

 

[Chris_Robb]

Reply to Captn Bob
Quote
Exception to this is when the wind is up, in which case I leave both batteries on so long as the wind gen is keeping them much about about 12.8V when the fridge is on. This has happened a lot so far this year!

Bob - you may be caught out if the wind drops with 2 flat batteries. Why bother to have the engine battery being charged by the wind gen. Other than starting - thats all the current it needs and it should be recharged again by the time you have motored out of the harbour.

The main problem with Westerly Fridges is that they were ultra mean with the insulation and made them too big as well. The tops (other than the lid) usually had no insulation. If there is room, try to add more insulation to them round the outside it there is room. I took the drastic action of ripping it all out and starting again.

 

[CaptainBob]

Reply to Captn Bob
Quote
Exception to this is when the wind is up, in which case I leave both batteries on so long as the wind gen is keeping them much about about 12.8V when the fridge is on. This has happened a lot so far this year!

Bob - you may be caught out if the wind drops with 2 flat batteries. Why bother to have the engine battery being charged by the wind gen. Other than starting - thats all the current it needs and it should be recharged again by the time you have motored out of the harbour.

I keep a close eye on it. Reason I do it this way is that the current draw from each battery when using the two batteries is obviously halved - which means neither gets very low at all in this situation - which means better longevity for my batteries. Also, I don't have a dedicated engine start battery. I prefer to use both at once if neither is too low (again, half the current for each battery). Plus I alternate which battery is used as domestic when in a single battery (no wind) situation. Again this will maximise the lifespan of both batteries as each will have had fewer charge/discharge cycles.

The main problem with Westerly Fridges is that they were ultra mean with the insulation and made them too big as well. The tops (other than the lid) usually had no insulation. If there is room, try to add more insulation to them round the outside it there is room. I took the drastic action of ripping it all out and starting again.

Our lid is about 2.5 inches thick. The fridge motor comes on (at the moment) about once every 30 minutes - and for about 3 minutes each time. This is with the fridge set at about 7 degrees C. Insulation around the body of the fridge is minimal - but it's enclosed in a cupboard with a big air gap around it - so I think we're pretty well sorted.

Only real issue I have though is that my Eberspacher main pipe runs through the same cupboard, under the fridge, so despite putting some reflective insulation above it - when we run it, the fridge kicks in a lot more often... but we tend to use the Vapalux for heat when cruising.

 

[carrswood]

I can only suggest that if you got down to 2.2 degrees that it must have been cold outside.

Hi William H, would you believe it the fridge was left overnight in my kitchen at home where the temp was 22 degrees
This makes sense though - the instructions say the unit will cool to 20 degrees below ambient. I know its not efficient with no thermostat but thats pretty impressive cooling from a standalone air cooled box

 

[William_H]

Hi William H, would you believe it the fridge was left overnight in my kitchen at home where the temp was 22 degrees
This makes sense though - the instructions say the unit will cool to 20 degrees below ambient. I know its not efficient with no thermostat but thats pretty impressive cooling from a standalone air cooled box

Yeah like I said pretty cold! No actually it is pretty cold here at the moment at 16 degrees max but I don't start singing till it gets over 35 degrees.
Funny I have a little job to do today to help a friend on his 32ft sail boat. He wants the fridge disconnected. It was accidentally switched on recently and flattened the batteries. He reckons he will stick with ice. olewill

 

[Refueler]

3.8A is pretty low actually by my experience for these cool-boxes. My large Rubbermaid job is nearer 7A. Another common Supermarket job I used of a friend was near 5A steady.

For mine I have fitted a standard car headlamp relay that cuts in when engine starts to power the box. Being still plugged in via the cigarette lighter socket / plug (heavy duty of course !) I can still connect direct to battery or in car if needed. It's easy to wire one in by using the Alternator exciter / light circuit.

Personally if I was changing from a cool-box, having had 2 people use them on board my boat - I'd go for a Waeco fridge-freeze box similar to the cool-box but with regulator, lower power consumption etc. etc ... plus a battery cut-off if voltage drops too low. Both froze my beer !! Can't be better proof than that ..

 

[pappaecho]

I dumped my Peltier effect coolbox for the same reason. Even though it had a thermostat when run on the mains, on 12 volts it had none and thus over cooled as you have found.
I replaced with a Waeco compressor unit, which has a proper thermostat, and which only is on about 1/3rd the time, and so whilst it used 60 watts when running it is for far less time.
Maybe one solution would be to fit a 12 volt thermostat if you can find one

 

[carrswood]

For a 35 L Halfords fridge its quite impressive really. I just need to find a way of stopping it running 100% of the time! A time switch or something - then perhaps we would achieve 5 degrees instead of 2.2 ! And use far less battery