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Old 10-11-2008, 16:27   #1
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Are my calculations right to add a fridge?

Before our next trip to MX I'm thinking about one of the Engel portable fridges. Last time down we saw a few in service and the owners raved about them. One guy had two, one used as a freezer and one as a fridge.
According to the specs the power consumption varies between 0.5 and 2.5 amps. I don't know why there is such a range. Here is a link to the one I'm interested in.Engel Australia - Portable Fridge-Freezers=

My current system is this, one group24 starting and one group27 house.
My assumptions,
Usage:
Engel running 2amp 12 hours = 24 amp/hours (based on users statement of run times)
various cabin lights 2amp 4 hours = 8amp/hours
VHF and CD a guess at 4 amp/hours
fans another guess at 6 amp/hours
various pumps 4 amp/hours
Total daily demand 42 amp/hours
Our boat is small, 27 feet, we have very few electrical items, the fridge is a huge splurge for us.

105 amp/hours from the G27 house battery.

So this puts me at about a 40% discharge rate, the current battery is a new deep cycle.

We also have a Kyocera 85W solar panel, using these assumptions
5 amp/hours 8 hours of sun 40 amp minus a fudge factor of 25% equals 30 amp/hours.

Universal M12 with a 55amp (I think) alternator.

Here is the question, is the one Group 27 big enough to run everything with the solar and say 30 minutes of engine run every morning?
Or should I get another battery in parralell for the house bank? I have the room to add one if I relocate the engine battery, easily done.

Thanks, Bill
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Old 11-11-2008, 05:31   #2
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Originally Posted by bmiller
... According to the specs the power consumption varies between 0.5 and 2.5 amps. I don't know why there is such a range...
The current draw is inversely proportional to the applied voltage - hence, if operating on 12.5V, the current is 2.5 Amps, then operating at 24V, the current should be 1.25A, and at 240V it should only be about 0.13 Amp. They must be using a lot of current to convert 240VAC to DC.
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Old 11-11-2008, 08:30   #3
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Dear Bill
The figures look reasonable, but there are often things people forget (anchor light, phone charger GPS etc). If you are listing everything you use in your energy budget I think you should just be OK. You will be running your battery down by a bit more than ideal (You wont start with 105 AHrs because it is very hard and takes a long time to charge a battery to 100%), but remember adding adittional batteries does't give you any more energy. You still have to replace what you use.
If you have room for any more solar pannels this would make more difference.
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Old 11-11-2008, 12:56   #4
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Bill,

A few comments, the general rule of thumb for daily output of modern solar panels in 12 volt systems is the rated capacity in watts * 0.3 = total daily Amp-hours. You'll be looking at about 25 Amp-Hours out of your solar system, a bit more if you can accurately aim them at the sun.

You'll be looking at a lot more than 30 minutes of engine time to really top off the batteries. The charge acceptance rate is pretty low when they approach full. I think you'd be a lot happier if you added another battery and cycled between about 60 and 85% charge, with an occasional equalizing charge. If there was any more money left after the battery, more solar capacity would be good too...

Good luck!
Bill
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Old 11-11-2008, 16:56   #5
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I have room to add a group24 if I move the starting battery. That's what I was thinking all along, you've helped me finalize it. Now I may consider replacing the original alternator.
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Old 11-11-2008, 19:09   #6
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Quote:
Now I may consider replacing the original alternator.
That is probably a poor investment. With a smart regulator on a good alternator the acceptance on a battery drops as you get closer to being full. This reduces the output from the alternator. Your new big alternator runs at full power a few minutes longer but then shuts down to what you have now. It also assumes your engine is up for the higher load.

In that sense you can't charge a battery faster unless the acceptance is higher. That comes from lots of drained batteries. That is more true with a boat with say 400 amp hours using twice the power you use. They would see the benefit to some extent as far as hours to run the engine. Conversion of gasoline to electricity stored in a battery is a very inefficient process. You want to do as little of that as you can - period.

In your case since you have such a small bank you will never see the need to use more of an alternator than you have. It goes right along with adding more batteries. After 2 days of no charging two batteries are just as dead as one. What goes out has to go in. If the numbers work with one battery they do not work better with more. Perhaps worse because a 1/2 discharged battery can absorb more energy from the alternator than 2 - 1/4 discharged batteries at least until it becomes 1/4 discharged. The rate of charge is how long you run the engine and burn fuel.

You can add all the batteries or alternators and still be in the same situation. The more efficient solution is to master the balance of in and out. A battery monitor will do that better than a new alternator and cost less.

Use the smallest battery bank that works. In a perfect case you would take the battery down and back from 50% over a regular period of time. It's the consumption that traps you into a bigger bank. Making it bigger does not solve the charging issue at all.
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Old 12-11-2008, 03:16   #7
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I considered adding and independent charging system for our refrigerator and came to the conclusion that the cost of solar panels, smart charge controllers, battery monitors and additional batteries just wasn't worth the hassle of having a tiny little "cold spot" on the boat. We plug into the grid wherever we're tied up and have no problems with our refrigerator. If we anchor out, we just go without refrigeration. The more complex the boat is, the more it costs to run, the more stuff there is to break, and the more you have to worry about things that are just not that important to the "sailing" experience. I'm amazed at how little conversation there is about actual "sailing" and how much forum space is taken up discussing all of the stuff one can bolt on to a boat hull.
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