Help with Maths (fridge consumption)

[robertager1962]

I wonder if some kind soul can help me with a simple maths problem that I seem completely unable to work out.

We are liveaboards currently in Spain and I am just thinking about changing my batteries and am trying to calculate my required capacity for my fridge.

It draws 3 amps and runs for 1 minute in every 5 (1 minute on and 4 minutes off). Ambient temperature 20 degrees.

I just want to calculate its consumption for every 24 hours. Currently I make this 288 amps.

I am sure that I am making a mistake as we only have capacity of 270 amps in the batteries and we never go below 50%.

I have solar panels that produce a maximum of 8.0 amps in the winter so my figures just don't seem to add up.

Can anyone put me out of my misery please.

 

[rogerthebodger]

3 amps x 24 hours x 1min on / 4mins off =

18 amp hours

 

[noelex]

14.4 AHrs per day

 

[AngusMcDoon]

You need to understand the difference between Amps and Amp-hours to make that kind of calculation. You don't have 270 Amps of capacity, you have 270 Amp-hours (Ah) of capacity.

How to Calculate Amps, Watts, Amp Hours, Volts & Hours for your Deep Cycle Batteries - KT Cables

 

[RJJ]

If it was running continuously, you'd be doing 3A *24 hours = 72 AmpHours.

20% of 72 = 18 Ah = 18 Ah.

If your solar panels give you max 8A, then the question is - for how many hours a day do you get 8A, in winter? If you get 3 hours of 8A output (obviously it'll vary according to the weather) then you have 24Ah per day. Which if the fridge is the only load is fine; but presumably you have some other items?

3A at 20% sounds a bit low to me, but if that's right then you are doing fine with 270Ah batteries.

 

[Slowboat35]

As previously stated the correct figure is 14.4 ...You can do that calculation on your head!

 

[reeac]

1 minute on then 4 minutes off means that it's running for 1/5 of the time, not 1/4, so it's 3 times 24 times 1/5 ie. 14.4 hours.

 

[GHA]

It draws 3 amps and runs for 1 minute in every 5 (1 minute on and 4 minutes off). Ambient temperature 20 degrees.

Sorry to make things worse....... but it's likely to be up towards 3 minutes every 5 when the weather is much hotter. Mine is running 15% of the time at the moment in round about 20degC, but will be up over 50% when the temp is more like 30, or on all the time after a load of warm beer is put in....

Though still manage with 300w solar getting back to really fully charged a few days a week in the summer.

 

[differentroads]

Sorry to make things worse....... but it's likely to be up towards 3 minutes every 5 when the weather is much hotter. Mine is running 15% of the time at the moment in round about 20degC, but will be up over 50% when the temp is more like 30, or on all the time after a load of warm beer is put in....

Though still manage with 300w solar getting back to really fully charged a few days a week in the summer.

Good point. We're in the Med too and the fridge runs nearly 50% for much of the day in summer. Our solar panels (200W) produce more power for longer, which helps to compensate, of course.

I have a 450AH battery bank to help increase the time to 50% discharge. The longest we've been without mains or engine charge is 10 days and we were nearly at 50% discharge from starting fully charged. Our 200W of solar won't ever get us above 80% charged - the downside of a bigger battery bank. And in winter it gives us five or six days before I need to charge from the mains, even when tilted towards the sun. I wish I had space for another 100W.

 

[Mistroma]

Good point. We're in the Med too and the fridge runs nearly 50% for much of the day in summer. Our solar panels (200W) produce more power for longer, which helps to compensate, of course.

I have a 450AH battery bank to help increase the time to 50% discharge. The longest we've been without mains or engine charge is 10 days and we were nearly at 50% discharge from starting fully charged. Our 200W of solar won't ever get us above 80% charged - the downside of a bigger battery bank. And in winter it gives us five or six days before I need to charge from the mains, even when tilted towards the sun. I wish I had space for another 100W.

Same here. It looks as if you are just heading towards the Mediterranean, just wait till it gets to 40C (fortunately not very often).

You will find the compressor running a lot more than 20% of the time and 50% or even 60% will be more usual in peak summer. I don't know what compressor you have but some have variable running speeds. My Danfoss BD35 can run between 2,000 - 3,500 rpm and tends to use around 5A in summer with cycle time in ~40%. That means it is using about 48Ah per day. (5A x 24h x 40%)

I think it gets up to around 70Ah per day in really hot weather. It would be worth finding out which compressor and control unit you have fitted.

Tip: We made up insulated covers with reflective foil and foam inside. They fit the work surface on top of the fridge and made a big difference to power use in mid-summer. The worktop was often almost too hot to touch at times if sun was shining through the hatch. The covers keep it cool to the touch and cost almost nothing to make.

 

[robertager1962]

Thanks so much everyone. It should have been straightforward, but I don't know where my brain was this morning. Just as well it's not me doing my son's maths lessons.
We have two 100watt panels but spent the last two summers getting here, so the longest we have anchored is for 1 week. Next summer we plan on spending most of our time on the hook for this reason I am calculating our needs. We do have a petrol generator in emergencies but I don't really want to use this, if I can avoid it. I may look at increasing my battery capacity but this would be pointless if I can't replace the charge on an daily basis.
Anyway just heading to the back of my classroom with my dunce's hat on.
"

 

[GHA]

We do have a petrol generator in emergencies but I don't really want to use this, if I can avoid it.

Might have to for a while in the morning once a week or so, otherwise your batteries will never get fully charged and die early.. Takes a looong time to get to really full even without the regulator going to float.

 

[TonyMS]

We have a similar problem - 300W solar panels, but only 2 are ever pointing at the sun - and a fridge that uses 4A when its on, and is on about 1 min in 3 when it's hot, less when it's colder.

My solution is to run the fridge off the load connection on the solar panel controller, and to connect everything else directly to the battery. And the load is set to come on at around 13.2V, and cut off at 12.8V. That way, the fridge is only working when the panels are working, and can't kill the battery. In practice, the fridge gets very cold in the middle of the day, and retains enough coolth to preserve things overnight. Crucially, the beer is at optimum temperature while the bar is open.

 

[Mistroma]

GHA is quite right. Use your generator early in the day to get batteries reasonably charged and let solar do its best for several hours. You should probably plan to add more solar, 300W might be enough for quite a bit of the summer. Just a guess as it depends on your lifestyle and current charging systems. Rigid panels aren't very expensive.

It is always a balancing act as you already know. Huge battery capacity allows you to spend days at anchor before getting to 50% SOC. Then you pay for it trying desperately to put it all back. Or having a large capacity means SOC is still 95% in the morning and then you end up putting in 15% the next day in order to get back to 100% (just guessing).

I imagine you will find a sweet spot with capacity, usage, generation working well together. The latest Hi-Tech batteries simplify things by having good charge retention, faster recharging possible and most importantly, high charging efficiency. Welcome to the merry-go-round.

We charge laptops, tablets and phones via peak solar during the day. It all saves a little as they recharge to 100% more efficiently than our Trojan T105s.

 

[robertager1962]

Same here. It looks as if you are just heading towards the Mediterranean, just wait till it gets to 40C (fortunately not very often).

You will find the compressor running a lot more than 20% of the time and 50% or even 60% will be more usual in peak summer. I don't know what compressor you have but some have variable running speeds. My Danfoss BD35 can run between 2,000 - 3,500 rpm and tends to use around 5A in summer with cycle time in ~40%. That means it is using about 48Ah per day. (5A x 24h x 40%)

I think it gets up to around 70Ah per day in really hot weather. It would be worth finding out which compressor and control unit you have fitted.

Tip: We made up insulated covers with reflective foil and foam inside. They fit the work surface on top of the fridge and made a big difference to power use in mid-summer. The worktop was often almost too hot to touch at times if sun was shining through the hatch. The covers keep it cool to the touch and cost almost nothing to make.

Very good idea. We have Southerly with big windows and this reason I have made up insulated reversible covers. In the winter we have them dark side out and it stops the condensation and in the summer the silver bubble wrap (saved from when insulating my house before we left) faces outwards. This has helped a lot, although so far we have only reached 38° C in the Guadiana.

 

[vas]

out of curiosity, when you guys say fridge run 1min out of 4 or whatever, that's the average you calculate over a period of time? Wonder how you make this up as it's different in the summer even throughout the day (starts 15m an hour, but then midday could be 50-50, to drop to 1 in 4 at night again) You should really have an average for when sun is up and solar worknig vs the night with just consumptions...

The good thing with solar is that they are there when you need them most...
Was checking my VRM data a minute ago and with I guess 16-17C in the cabin and no sun today, fridge was cycling at 15-18min every 2hours!

 

[GHA]

My solution is to run the fridge off the load connection on the solar panel controller, .....

Not there yet but hopefully soon I'll get round to changing the fridge thermostat to a microcontroller and PCB, then program in dynamic control of the temperature based on duty cycle, also measures current/voltage and has control of the speed so see what works best.
5 boards cost $20, Or was it £..

 

[Mistroma]

We had a Southerly for about 24 years and our current boat has equally large windows. I made up narrow covers for side-decks and large flat windows forward of the mast. They run from the grab handles to the lower safety wire and keep things a lot cooler in the saloon. A bit of a nuisance at anchor but just enough room to walk forward. Our sprayhood and awning are blue, really bad idea. I have a spare white cover over the awning and top of sprayhood. Metallised covers over sprayhood windows (centre window rolls up for ventilation).

Covers roll up and don't take long to deploy at anchor and we usually spend quite a few days (or even weeks) in the same place.

I hope you are enjoying the Guadiana, we spent 5-6 weeks there a few years ago and had a great time.

 

[Baggywrinkle]

As already stated 1 min ON to 4 mins OFF is a duty cycle of 20% (1/5)

3A over 24 hrs is 72 Ah ... times the duty cycle of 20% = 14.4 Ah which is about 0,173 kWh - (14.4 Ah x 12 V / 1000)

0,173 kWh seems ridiculously low for a fridge in the med IMO ... mine uses approx 0,450-0,550 kWh

If you want to measure exactly how much it uses ... get one of these ...

ARCELI 150A Watt Meter Power Analyzer High Precision RC with Digital LCD Screen for voltage (V) current (A) Power (W) Charge(Ah) and Energy (Wh) Measurement: Amazon.co.uk: Sports & Outdoors

... and connect it in line with the fridge and measure over a couple of days.

 

[GHA]

I hope you are enjoying the Guadiana,

Pain to get charged up there in the winter.. sun takes a while to clear the hills in the morning and often fog in the mornings anyway. Though well up the river there will be ice on the deck in the morning so fridge doesn't need to work all that hard