Battery Isolator

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Battery Isolator

Postby Frank » Mon Nov 14, 2005 8:38 pm

Ok, I am going with a Dorm. type refig. and have bought a 1200 watt inverter to power it. The plan is run off the inverter/car alt. untill reaching the camp site. At that time switch over to shore power. My figures come out to about 35 amps from the tow vehicle alt. when the frig is running. Do I really need an isolator? Will be disconecting the tow vehicle upon arrival and the tear also has it's own battery.

Frank
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Postby madjack » Mon Nov 14, 2005 9:24 pm

I'll second what PB said and no you don't have to have an isolator unless you want it...just remember to disconnect the lead to the car when ya stop for more than a few.......
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Postby GeorgeTelford » Tue Nov 15, 2005 4:02 am

Hi Frank,

When the Fridge kicks in it will drag around 50 Amps for under a second and then settle to around 10-12 amps. The compressor only runs for about 5 mins out of every 20 (I tested mine in warm weather too)

The average consumption is under 3 Amps Average over an hour and this was on a Full size mains fridge with small freezer section. I have pictures somewhere of the battery managment system during a 48hr Test.

Regarding the invertor where is this to be situated?

Heres the fridge set up in the bus that I was building

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and here's the battery bank

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with that battery bank I could wild camp for a week with the mains fridge running!
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George is that your bus?

Postby Guy » Wed Nov 16, 2005 1:49 pm

Dear George,

Is that the bus you are working on? Got any pics of the outside to see what you've done so far? It looks like it doing fine.
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Guy
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Postby Frank » Wed Nov 16, 2005 2:22 pm

George,

Don't think I can fit that battery bank into the weekender! :lol: Yesterday came up with the idea of putting the battery and the isolator in front of the A/C unit. Will help greatly on the tongue weight and also put the two of them close together so as not to need as much #2 welding cable. At the same time it creates a problem of getting the hot exhaust air out from the A/C, as now the battery is in the way. Going to talk to Woody about how he put a fan in there to pull, his A/C is fully enclosed also. Could put a small vent so as some fresh air would come in and cross the battery to keep exposive gas out of there.
I also made up a auto transfer switch for the frig.out of Alan Bradley control parts, it's only good for 7 amps,it is a breaker and the frig draws 3, has a 30-60 second timer on it to let the frig pressure leak down before coming back on. Will not be using the inverter for anything else except maybe TV on a Wet weekend. Just got one battery in my battery bank. :cry:
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Postby GeorgeTelford » Wed Nov 16, 2005 4:19 pm

Hi Guy

That bus is no more, the engine blew and the local commercial vehicle repairer took way to long to start, since then I bought another bus, but now I have split up with the wife, so a big family size bus is too much now. I only posted the pics to show that I had experience of running a mains fridge via batteries.

Frank

How do you know what the fridge draws? My fridge above only averaged 2.9 Amps, but on start up of the compressor draws 50+ Amps and while running uses 10-12 Amps.

So the average is the same BUT 50+ amps and a 5 min burst of 10-12 amps, is you transfer switch going to cope?

George
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Postby Frank » Wed Nov 16, 2005 5:25 pm

George,
going by the name plate. Breakers are kinda like a time delay fuse, they will hold more just for a second or two than their rating. Have not hooked up the transfer switch to the frig. just tried it out on the work bench to see it work, will do that tomorrow. The time delay relay is the weak point, could not find one with more amps. They were bought for control circuits. The transfer switch could be made out of two sets of contactors.
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Postby GeorgeTelford » Wed Nov 16, 2005 5:58 pm

Hi Frank

Assuming your fridge stays in line with my fridge, you want to be sure all the parts will be happy with ; 50 Amps for 1 second, 12 Amps for 5 mins at a time. That average current on the plate will never actually occur (ie it will never be running at 3 amps other than by "average")

your breaker will need to be 10 Amps (if you are lucky a 10 Amp breaker will cope with the 50 Amp 1 second draw, do test it under warm conditions though) or to be on safe side a 15 Amp breaker.

I have no idea of the specs on Alan bradley bits (any links?)

Also what make and model fridge are you using?

Is this transfer switch to remove Invertor and go over to mains?
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Postby WoodSmith » Wed Nov 16, 2005 9:15 pm

Harbor Freight has a Battery cut off switch on sale right now for 2.99. It has 100 amp at 12 v. lot no 92688.
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Postby Gerdo » Thu Nov 24, 2005 9:47 pm

How do you know how large of an isolator to install? Do you go on battery amperage or alternator size?
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Postby GeorgeTelford » Fri Nov 25, 2005 4:45 am

Hi Gerdo

Battery amperage is meaningless as far as switch size is concerned all batteries could supply a massive ampage for a brief period.

Alternator amparage drops pretty rapidly after initial "charging" although battery to battery equalisation can run for a while longer.

An Isolation switch of 100 Amps min is probably about right.
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