Need power detect circuit

Discussion in 'General Electronics Chat' started by lgoodisman, Jun 12, 2011.

  1. lgoodisman

    Thread Starter New Member

    Jun 12, 2011
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    I have a cabin and blackouts are common, especially when I'm not there and often when it's cold. That means the pipes, even with the heating tape I have around them freeze burst and ruin the floor, since the heating tape run off the main power.

    The solution of a general diesel (or similar) generator with power interruption detect switch would cost about $2,000 plus $1,000 in wiring and is not what I need.Also too expensive.

    In the event of a power outage in freezing weather, I need a small space heater to heat the pipes under the house, a small insulated area. I have wired electrical outlets there.

    I have identified a small 12Volt heater (designed for RVs) by Back Seat Heat Plus and another by ThermTech and these seem perfect; but I need a few other components. Both are 300 Watt, 25 amps, 12 V.

    1. I need to calculate for how long a 12 volt (auto) battery would run the heater and figure out how to put 3 or 4 or whatever number of batteries in parallel to give me maybe 24 hours. Anyone agree or have another idea?

    2. I need a power interruption detect switch that will turn the power to the heater on if the power goes out during the winter, ideally when it is freezing cold but I can live without the temperature sensitivity. Anyone know where I can get such a switch and how I'd put it in?

    3. I need a recharging system that would automatically charge the batteries when the power is available. Anyone know what that's called, where to get one?
    Thanks
    Len
     
  2. #12

    Expert

    Nov 30, 2010
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    The power detect can be a simple relay that is held open (not conducting power) whenever the regular voltage is available, but if you're going to run a 12V system, complete with charger, you can forget the relay. Just hook up the 12V system and hope the power comes back on before the batteries are empty.

    As for size of batteries, amphours (as listed on the battery) divided by amps (for the heater) = hours. A 100 amphour deep cycle battery will provide 12V for 4 hours to a 25 amp heater.

    The charger is a retail item. It's called a "battery charger". Very common for 12 volt batteries as most cars use 12 volts.
     
  3. KJ6EAD

    Senior Member

    Apr 30, 2011
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    363
    I designed a system using home automation components that would shut off the water supply and drain the pipes in the event of freezing conditions. It would also shut off the water supply in the event a leak was detected near any toilet, sink, dishwasher or washing machine. The controller was linked to the phone line so you could be notified of any fault and monitor/control the system remotely.

    http://www.smarthome.com/_/index.aspx
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    If you want long lasting power from batteries, get Golf Cart batteries. Many RV enthusiasts swear by replacing their 12 volt deep cycle R/V- Marine duty battery with two 6 volt Golf Cart batteries in series. They are also designed for deep discharge duty.
     
  5. lgoodisman

    Thread Starter New Member

    Jun 12, 2011
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    0
    Thanks for the info. The idea is to have the heater running off the batteries which are being charged as needed from the main power and a 12V battery will last 4 hours so four in parallel may give me about 16 hours. If the heater runs on a thermostat this sounds good. And maybe golf cart batteries in series are better.

    The last point however remains tricky for me. The "battery chargers" I have seen for autos all seem to be the kind where you hook up one battery and press on and come back later to move a next battery into place. This clearly will not work in this application. Is there a battery charger which will plug in and keep four batteries or so charged as they need recharging?
     
  6. #12

    Expert

    Nov 30, 2010
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    When you put (2 equal size) 6 volt batteries in series, you get 12 volts and the same amp-hours as one of the batteries, not both. When you put 4 (12V) batteries in parallel you get 12 volts with 4 times as many amp-hours as any one battery.

    Next, You are looking for a battery charger that will settle into "float" when the batteries are full. That part is called, "float charging" with a "float charger". It will cost more than a brute-force, "unplug it yourself" charger, but it will keep the batteries full and ready with very little attention from you.

    I think 300 watts is more than you need, but the thermostat will cycle the heaters off and on, so that won't be a problem.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    It would be a lot less expensive and far more reliable to simply drain the plumbing before you close up the cabin.

    If there is no water in the pipes to freeze, then your problem goes away completely; it requires no maintenance or ongoing costs (such as running the pipe heater tapes, recharging the batteries, etc.)
     
  8. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
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    Did you miss this one? with a few solenoid valves, you could negate the problem indefinately & not have to worry about how long your batteries will last.

    Even cheaper still, you could install manual drain valves and empty your pipes whenver you leave the cabin.
     
  9. lgoodisman

    Thread Starter New Member

    Jun 12, 2011
    3
    0
    I was not ignoring your posts on draining the pipes and appreciate them but I can't do them. We do drain the pipes to the extent we can, but the plumber ran the pipe through the crawl space and put the cut off at the far end so the cut off does not actually keep water out of the pipes. Also if I were to insert a drain, the water would drain into the crawl space which is unacceptable or I would drill through the foundation which would also be difficult and not a great solution, because the pipe enters the crawl space under the porch. It's a plumbing screw up but that's what we have.

    I really need to find a heat tape system that runs off rechargeable batteries and sets up a backup defrost system so I can have some time if the power goes out when it's cold. So far, the other options don't seem workable.

    Any help in locating that tape would be greatly, greatly, appreciated.
    Likewise, help in setting up several deep cycle (rechargeable) 12 V batteries in parallel would be appreciated. What kind of connectors do I use? What kind of wire?

    And I understand a trickle charge will recharge them when they are used.

    Thanks again.
     
  10. #12

    Expert

    Nov 30, 2010
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    That was easy! Amazon.com Search for "pipe warmer". I see 30 feet for $28 with thermostat included. Problem is, thay all want 120 volts. Not so easy :(

    For my "float" problem, I made a 78L12 charger set at the float voltage, using the chips "self protect" current limiting feature to stop it at 100ma. There are chips with higher current limits available.
     
  11. #12

    Expert

    Nov 30, 2010
    16,285
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    Further searching does not turn up any 12 volt pipe warmers. Unless someone else gets lucky, you are going to need a 120 volt inverter.

    The wire you use will have to be able to carry the amount of current the pipe warmer needs. Not much. You'll have to read the label on the pipe warmer you buy. The safe way to do it is to use common house wiring wire and make an outlet for the 120 volts. Probably 14-2-g wire.

    There are people here that warn of dire consequences when putting 12 volt batteries in parallel if they are not perfectly matched and in the same state of charge as each other.

    What have you accomplished so far?
     
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