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PostPosted: Sep 13th, '06, 20:43 
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notice that its the same power for double the air flow? :)

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PostPosted: Sep 13th, '06, 20:54 
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I was about to buy a new air pump, but mine consumes only 10W for 30L/hr the new nes there are almost double the power consumption :(

And power is at a premium when you're talking about a backup :(

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PostPosted: Sep 15th, '06, 20:17 
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The little marine 12V DC sump pump I use for doing my water changes draws 2.5amps to pump 38 litres a minute at 1 meter head.

Can somebody with more electrical knowledge than me tell me how long that would last being run by a car battery if I was to set it up as my fail-over pump?

Not wanting to sound too stupid, what is the relationship between amps and watts. Are amps used when DC?


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PostPosted: Sep 15th, '06, 20:28 
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VB, Id think it depends on the size of the battery, what is its amp-hour rating? Yes, amps are used in DC. Since you stated your 12v dc pump draws 2.5 amps you already answered that question yourself. :wink:

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PostPosted: Sep 15th, '06, 20:43 
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Thanks GF - I should have known that because I have been talking to Murray about stuff to do with his switches and he had said that a normal car battery is about 70 amp hours. Therefore I guess I would expect the pump to run for about 28 hours - or 1 day to be conservative. Sounds good - I could definitelly run my current system on this, providing aeration from the gravity return from the grow-bed. Therefore plants and fish would be okay. I'll get all the stuff I need and run a test. My only problem would be that to get my flood and drain cycles a use a timer on a 30 mins on 30 mins off cycle. I wonder if I could run some sort of timer on the battery system. Normally the timers rely on the power to run them - suppose I would need a special 12v dc one unless I could do a mod on the basic 240v ones. Any ideas anyone?


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PostPosted: Sep 15th, '06, 21:00 
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VB, the amp hours of the battery would have a coralation(sp?) to how long it would last but I dont think its a direct coralation, not a 1:1 ratio. Then of course you would have system losses through the wiring as well. I'd have to do some reading up to be sure.
For sure the more you add for a load on the battery the faster the drain (in reference to a 12v timer).
I don't believe its good to fully drain your car battery (lessons thier life dont it?)
As a quick guess I'd think 12 hours would be the max, looking forward to your tests.

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PostPosted: Sep 16th, '06, 10:44 
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VB, Amp hour rating are usually quoted at the 20hr rate (might be 10hr)

I'll explain using the 10hr rate (for easy maths)

70 amp hour battery would last 10 hours drawing 7 amps from it.

But if you draw 70 amps from it it definatly wont last 1 hour as GF stated its not a linear graph.

The more i think abut it the more i'm sure its quoted at the 20 hr rate, so that means you can draw 3.5 amps for 20 hours.

Hope this makes sense.

ALSO be very aware that a normal car battery it NOT the ideal choice for stuff like this, as its not meant to be cycled. The life expectance in this sort of application would be very low. If you cmpletely flatten a normal car battery you reduce it capacity anywhere upto 50% (ever noticed that once you get a flat battery you always have troubles with it again, unil you buy a new one?

Deep cycle batterys are the one you want they are used in Recreational stuff, like boats and camping stuff (much more expensice though) or you can buy SLA's (the sealed lead acid batteries used in alarms and stuff) but you won't find one with even a 35 A/H rating.

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PostPosted: Sep 30th, '06, 08:40 
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I'm going to bite the bullet and get my backup power supply going. Would hate to see my system grind to a halt now it all seems to be going well - would set me back months.

I am going to buy a 75 amp hour deep cycle battery (about $140), a failover switch from Murray (which I have orderred) and a charger that will turn itself off completely when battery is charged and switch back on to top it up (about $100 for one that will take a day or 2 to charge the battery fully). Haven't decided whether I will run a waterfall usign my dc sump pump on power failure or whether I will buy a DC air pump. The DC air pump would set me back about $80 plus postage - so may hold off on that one till I need more stuff from the aquarium supplier.


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PostPosted: Sep 30th, '06, 08:45 
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Sounds good, remember pics when you set it all up. Water falls are so soothing.

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PostPosted: Sep 30th, '06, 08:48 
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I mean water fall literally - simply water falling, nothing flash.


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PostPosted: Sep 30th, '06, 08:53 
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Ohh poo stick a rock under it before it enters the tank and ya have it. :blob:

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PostPosted: Sep 30th, '06, 08:59 
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I'll see what I can do :P :D


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Oh - forgot to mention that the battery will have dual purposes. I will also use it to run an electric motor that I intend to get for a canoe that I have (you know - take the kids fishing on the local lake). Helps to justify the purchase of the battery and charger - both to myself and the misses :wink:


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PostPosted: Sep 30th, '06, 10:26 
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Hmm. I'm electrically challenged. I want backup though.

Let me see if I've got this. I need a failover switch, a deep cycle battery, (I'm guessing a charger for the battery) and a dc pump.


When my power fails the switch kicks in turning on the DC pump run by my deep cycle battery.

Once I find the appliance I want to run (pump or air) I tell you guys the specs and you can tell me what battery size I'll be looking for to give me 12 hour back-up.

Living in auckland we get power before anyone else in the country if there is major problems :)

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PostPosted: Sep 30th, '06, 10:32 
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Good one VB, always go for the double use.

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