Fridge power consumption and calculating the power draw...

While I have dual batts in the Patrol, the secondary is a 105AH deep cycle, I now have a 75Ltr Combi Engel and when we go camping I don't want it connected to the car if we go out 4wheeling. So a third battery in a box I think is needed. So. The fridge maximum current draw is 4.2amps. How do I work out what the best sized battery would be to buy to run this for atleast three days. Mind you I do have a solar panel setup too...
Plus what an AGM battery? I had in mind to just buy another deep cycle. D70ZZ.

Cheers.

Mudski, If fridge uses upto 4.2a/h, say battery is 105amp hour, Divide 105 x 4.2 =25hours at max draw. If you cool down fridge 2 or 3 days before you head out, you will save heaps of power. Make sure your food is cold before you put in fridge. Keep fridge full. Only refill (DRINKS etc)of an evening before bed. All this will extend battery amp hours. Obviously if you have a 80+ solar setup, you will be more than fine for 3 days.
Steve

Yeah I wasn't sure on the calculation. As for pre cooling the fridge, that sometihng I have always done. Plus keeping it full. I even use old 2ltr juice bottles full with frozen water to fill the empty spaces. I should have bought a 60 combi but I what I paid for the 75ltr combi with transit bag, new too, was cheaper than a 60l combi with bag.

Cheers.

Doing that Mudski you should last 3 days easy without solar back up.
Steve

Many variables which will affect how much the fridge sucks out of the battery. What temp the fridge is set at, whether you have additional insulation around the fridge, whether it's in the shade or not, ambient temperature, temperature of the contents, how often the contents are or have been accessed & probably others that fail to come to mind at the moment.

Well worth spending a bit on some extra insulation - the thicker the better, all around excepting the airways to & from the compressor. Maximum thickness would be determined by what is practical to store, but suffice it to say that 6" will save a heap more battery power than half an inch.

Assuming you have paid attention to insulation & usage patterns as mentioned above, it is probably reasonable to work on a 50% duty cycle, that is 4.2amp hours per hour for 12 hours out of 24 = let's say 50ah per day (I don't actually believe a 75 litre fridge freezer will be that efficient, specially id the freezer section is being used as a freezer - probably more like 70ah or 80ah per day - lets split the difference at 75ah). in which case, for 3 days you would need batteries which will give you 3 x 75ah. Ignoring for the moment any solar input this would mean that you would need a battery capacity of around 450ah in order not to draw them down below 40 to 50% (for long battery life). Clearly this amount of battery capacity in most 4wd's is impractical as well as prohibitively expensive. This is where the solar panel(s) come in. The idea is to try to balance what is being drawn out by the fridge with what is being put in by the solar ....... but it's not quite that simple. The reason being that as long as you are only using the fridge short term (3 days) the solar only needs to put in enough to keep battery capacity above that 40 to 50% threshold where all the damage to it's longevity start to occur. So if you had say 2 x 120w solar panel, you could expect that to feed (in good solar weather) around 80ah per day into your battery, keeping up with the requirements of your fridge & your battery fully (or close to) fully charged. If you then have a day of poor solar weather your battery would provide you with the 'buffer' to get through that day, & you would hope that the weather improved the next day. Battery capacity is your buffer against poor solar weather. So over 3 days you'd probably be pretty right if you ensured you put the battery onto a smart charger as soon as you got home (or have an in car smart charger). You could have less solar & more battery, but this would be heavier & a lot more expensive, but would give a similar outcome. Or you could just decide to run your battery until it was 'flat' & make do with a single 125w solar panel & accept that you will kill your battery far more quickly, in which case you would only buy a cheap battery.

There are many ways to set these things up. The first thing to do is to test what amps the fridge draws in real life. the figure of 4.2a is meaningless without knowing how often the fridge runs. If it is 45 minutes per hour, that's more than double the daily current draw of 20 minutes per hour.

You certainly could just go & buy another battery & keep your fingers crossed & you may or may not get lucky, but I wouldn't be paying a lot for the battery, & would expect a max of 12 to 18 months use from it. Cheaper in the short term, but poor value in the longer term.

I'm sure my answer isn't what you hoped for, it's never simple & straightforward when these sort of questions are asked, invariably everyone just wants to know 'What size battery & what size solar'. Anyone who tells you there is a simple answer has either failed to understand what is involved, or is trying to make a sale!

Also don't forget any other power usage no matter how small, it adds up over a few hours - e.g. lighting. Everyone says how much more efficient todays LEDs are & how they use little power compared to halogens or even fluoros.... which is true ...........BUT.... of course it depend upon how many you use. When I started using LEDs a few years back, all my lighting needs pulled no more than 0.5 to 1ah. Today with the long awning lights with lots of LEDs in them, the light is great, but it would't be unusual for them to draw 3 to 5ah. Over the course of an evening that can easily add up to 15 to 25ah - a significant draw that needs to be reckoned together with the fridge (& the CD player?).

Hope my rambling helps a bit.

You'd be pushing it up hill on the Murray in 30 degrees opening and closing fridge for drinks over 3 days. You'll need solar back up trust me
As suggested refill at night or put one can in upside down to replace one taken. use wine bladders to fill voids{filled}.
I only use Engle for Food mainly as I have a ripper ice box which will keep ice all weekend real world. forget formulas
Pop over I'll show you

On a hot day trying to recover you may be pulling 8 amps plus on the bigger engel EG 60 ltr will pull 6 ah easy little effort
Refilling one can at a time is better than restocking with 12 in one hit. Been doing this way too long to know what works etc

Cuppa your rambling always helps mate. I would not be running anything except for the fridge on the battery. I would use the cars secondary battery for any lighting at night. I also do believe that this fridge will draw more than its says as When I do use it, even full with coldies, its does run more often than my little 32ltr Engel.
Cuppa I don't know if you remember my thread about my 180W panel setup I have and was having a few issue with it. It was using 3 x 60w 24v panels into a 12/24v reg. I still have it. But I did buy the reg you suggested in that thread. A suntrans PR2020 reg.
3 days was an average, on Easter we would go away for maybe five...
Also how would I charge a third battery via the car? Say if the third battery was dead flat, and I wanted to go out for a few hours or even just let the car run to charge the battery up. I have in the rear of my troll a dual socket setup if it helps.

Cheers.
Mark.

Taking most Easters down here rain recharge via the Cig lighter, not ideal.
IMO your solar is over kill as 80 watt panel will do what you want.
re wire your 4x4 with an Anderson outlet in the back for your 3rd battery in a battery box
leave home with it charged and top up getting to camp.
this battery box can now be used for camp lighting only, leaving you 2 x batteries to run fridges
Ask BA about my battery box and that last me 4 days and only 18 AH battery

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Originally Posted by mudski

Also how would I charge a third battery via the car? Say if the third battery was dead flat, and I wanted to go out for a few hours or even just let the car run to charge the battery up. I have in the rear of my troll a dual socket setup if it helps.
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Depends on your setup in the car. If it were me & I had the usual dual battery set-up with a voltage sensitive relay between crank & aux batteries, I would run a cable from crank battery to battery no.3 in the back of the vehicle via a dc to dc charger mounted somewhere close to battery 3. I'm not familiar with the dual socket set up you have in the back of the car, it may or may not be ok depending upon the gauge of the cable which supplies it & the current carrying capacity of the sockets. If it were me I'd just run a dedicated heavy cable ( 8 B&S probably ok) direct to the charger & then connect to the battery. With the battery inside the vehicle it would need to be an AGM - wet batteries will smell & gas when being fully charged. AGM would be far safer.

Cuppa

Battery wont be inside the car at all too btw. It will be in a trailer. The other way I was thinking of when in transport I can plug the fridge into the socket in the back of the car and run the lead into the trailer where the fridge will be and then just swap it over to the 3rd battery when on site.
That was my first thought anyway, it was the calculation and what sized battery to get...Taking this all on board.