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Old 03-10-2012, 02:27   #1
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Seperat Battery for fridge

I'm just thinking, since fridge is in the mostly cases the biggest consumer og amps, would it give any sense to have seperat battery for fridge? If for instance I added a 100 w solar to the system, would it be faster/easier to charge one 100 amph bat than lets say 2x100 amph. This is the same logic behind bat for bowtruster, some have seperate some don't! Are there any benifits by doing the same with fridge?

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Old 03-10-2012, 02:33   #2
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Re: Seperat Battery for fridge

No it's more efficient with a larger battery.

A separate battery is used in some cases for a bow thruster because the very high current and long distance from he battery requires very large battery cables which are expensive and difficult to install.

The fridge has a much lower power draw and is normally close to the house batteries.

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Old 03-10-2012, 08:01   #3
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Re: Seperat Battery for fridge

However I would not rely on a 100 watt solar collector to be able to provide enough power to run a frig, irrespective of battery size. Do the math, frig drawas 10 amps and runs 30% of the time = 10 * 0.3 * 24 = daily requirement of 75 amp hours.
100 watt panel puts out 72 watts average for about 8 hours = 50 amp hours per day leaving you 25 amp hours short. Put in your own numbers and see where you stand.
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Old 04-10-2012, 09:29   #4
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Re: Seperat Battery for fridge

Separate batteries for different loads is a bad idea. You want the largerest house bank you can get to run everything.


(By Nigel Calder - I DIDNíT write this!!!)

The popular arrangement of having two house banks alternated in use needs scrutiny before I go any further.

LIFE CYCLES: As we have seen, the life expectancy of a battery in cycling service is directly related to the depth to which it is discharged at each cycle - the greater the depth of discharge, the shorter the batteryís life.

This relationship between depth of discharge and battery life is NOT linear. As the depth of discharge increases, a batteryís life expectancy is disproportionately shortened. A given battery may cycle through 10% of its capacity 2,000 times, 50% of its capacity 300 times and 100% of its capacity around 100 times.

Letís say, for arguments sake, that a boat has two 200-ah battery banks, alternated from day to day, with a daily load of 80 Ah. Each bank will be discharged by 40% (80 Ah of one of the two 200 Ah banks) of its capacity before being recharged. The batteries will fail after 380 cycles, which is 760 days (since each is used every other day). If the two banks had been wired in parallel, to make a single 400 Ah battery bank, this bank would have been discharged by 20% of capacity every day, with a life expectancy of 800 days, a 5% increase in life expectancy using exactly the same batteries!

But now letís double the capacity of the batteries, so that the boat has either two 400 Ah banks, or a single 800 Ah bank, but with the same 80 Ah daily load. The two separate banks will be cycling through 20% of capacity every other day, resulting in a total life expectancy of 1,600 days. Doubling the size of the battery banks in relation to the load has produced a 210% increase in life expectancy. The single 800 Ah bank will be cycling through 10% of capacity every day, resulting in a life expectancy of 2,000 days - a 25% increase in life expectancy over the two (400 Ah) banks, and a 250% increase in life expectancy over the single 400 Ah battery bank!

There are two immediate conclusions to be drawn from these figures:

1. For a given total battery capacity, wiring the (house) batteries into a single high capacity bank, rather than having them divided into two alternating banks, will result in a longer overall life expectancy for the batteries.

2. All other things being equal, any increase in the overall capacity of a battery bank will produce a disproportionate increase in its life expectancy (through reducing the depth of discharge at each cycle).

Stu Jackson
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Cowichan Bay, BC, (Maple Bay Marina) SR/FK, M25, Rocna 10 (22#) (NZ model)
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Old 04-10-2012, 10:31   #5
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Re: Seperat Battery for fridge

Do you now have a fridge? If so have you seen how much it draws and about how long it runs?

We use a 2.3 cu. ft. portable in one boat and in hot weather with cabin temps in the 80's it uses 30-40 amp/hrs a day. We had 180 watts of solar on that boat and that provided over 90% of our electrical needs and we also run a computer a lot.

We made and installed a 4 cu. ft. box with a small freezer section in the Endeavour and in 80 deg. cabin temps it seems to be using less than 20 amp/hrs a day. So almost twice the size of the portable but 1/2 the amp usage per day. I hope to put an hour meter on it when we go back to get some hard figures. We would usually add about 4 drinks a day to each fridge that had to be warmed up and then also other items as needed.

The key is...

...lots of insulation. The portable doesn't have it and the bigger box does. The other key is being realistic on how large of a box you need. The 2.3 was fine for a 2 month trip and we felt that we didn't really need that much larger of a box on the Endeavour so kept the interior at a realistic size which allowed for more insulation. The freezer section is large enough to keep a couple weeks worth of...

...meat frozen. We usually resupply every 2-3 weeks somewhere.

In conclusion if you don't have a box now try and make or get as efficient one as you can. Run one house bank like the guys above mentioned. Be realistic on your needs,


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