Very simple battery-guard circuit needed

Discussion in 'The Projects Forum' started by Jacob J, Apr 28, 2011.

  1. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
    159
    0
    Hello all

    How can I make a very simple battery safeguard, that cuts the power off when the voltage gets below, let say 11 volts. I guess this is how the commercial units work?

    I have thought of a comparator of some kind. But how it should be put together is a bit hard for me as ive never worked with comparators before.

    As simple a circuit as possible is prefered, but suggestions that have some nice features are very welcome.

    /Jacob
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    That's it?
    What's the battery starting voltage supposed to be?
    What is the expected current range?
    What battery chemistry are you dealing with?
     
  3. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
    159
    0
    The battery is a ordenary boat/car battery. 12 V, maybe a bit more is on it atm (1 month since I measured the volts on it, but thats what its supposted to be).

    The current is a bit more difficult. I have different boat devices, like depth sounder, VHF radio, boat lights and so on. The fuse after the battery is 10 amps, but it shouldnt get much higher than 3 amps total.

    The chemistry of the battery I dont know anything about. If its important I can take a look at it, when I get home to the boat.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, marine batteries are different than car batteries; they are designed to be discharged more deeply than car batteries are. They have thicker plates, and fewer of them. This means that they cannot provide as much current at one time as a car battery can, but the plates won't fall apart as quickly as a car battery will when deeply discharged.

    Car batteries are designed to output several hundred amperes for a short period of time (about 30 seconds) and then must be recharged immediately. If they are ever deeply discharged, they will fail rapidly.

    Both auto and marine batteries are 100% charged when they measure 12.7v-12.8v at room temperature (25°C, 77°F). If the charge is allowed to decrease below around 12.5v, plate sulfation begins, which will eventually reduce the ability of the battery to accept or release a charge. Heavily sulfated batteries seem "dead".

    You should maintain your battery on a "float charger" at 13.4v-13.6v when not in use.

    If you allow the battery to become discharged below ~12v, it's life will be shortened considerably. If you discharge it to 11.4v, it is considered 100% discharged, and it will have a short service life.

    Rather than a flat cut-off, you should consider using a voltmeter to monitor the charge state of the battery, and deciding yourself how low you wish to allow the battery to discharge, knowing that the deeper you discharge it, the shorter its' life will be.

    You should get the manufacturer's name and model number of the battery, and find its' datasheet using a search engine like Google. Then you should follow the charging and discharging recommendations specified by the manufacturer.
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    It might not be such a good idea to have an automatic low voltage cut-off on a boat battery. Whatever benefits it might bring in terms of battery longevity, would you really want your running lights VHF radio, etc to be cut off suddenly?

    Imagine if this happened just at the moment you needed to make a "Mayday" call!
     
    Lightfire likes this.
  6. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    Look at Cirkits Solar Power Center for an example and description of how it works. Though it is a solar charging circurt it could do just what you want. It has both a over charge and over discharge protection using comparators.
     
  7. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
    159
    0
    Thanks for the advice on the batteries. I didnt know that if it falls below 12V, it then would be dead (or close to). The battery safeguard should then cut the power off at 12,5volts to be good enough.

    Regarding to cutting off the power for all the safety devices like VHF radio and navigation lights, I will only use the battery safeguard for my cooling box I have onboard.

    The main problem with this cooling box is, that it doesnt have a thermostat and thats what I want to build. But a thermostat isnt enough. I need this safeguard to cut the power when there isnt enough for this kind of luxus.
     
  8. Lightfire

    Well-Known Member

    Oct 5, 2010
    690
    21
    Thumbs up!
     
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