How to charge and discharge a battery simultaneously?

Discussion in 'The Projects Forum' started by Hariharan Ayyasamy, Mar 9, 2014.

  1. Hariharan Ayyasamy

    Thread Starter New Member

    Mar 7, 2014
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    i am working on a project that runs on solar energy. i need it to operate 24*7. so used a battery to charge parllely. sometimes charging and discharging of battery need to happen simultaneously is there any possibilities????
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Current can only flow in one direction. You cannot charge and discharge simultaneously.
    You can have an alternating current (AC) but what is the reason for doing this?
     
  3. wayneh

    Expert

    Sep 9, 2010
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    3,031
    MrChips correctly notes that current can only flow in or out of the battery, never both.

    It is possible to power a load AND charge the battery at the same time, if that is what you are asking. To do this, your solar panel must provide more power than the load requires, so that a bit of excess is available for charging.

    The simplest arrangement is to place a blocking diode between the solar panel and the battery. The load is powered directly by the battery. This simple arrangement is good for small projects, but not high power systems.

    What do you really want to do?
     
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  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If you're talking about having both a solar cell and a battery hooked up to the same device so you don't have to manually switch between the two, then that can be as simple as adding a diode and a resistor if all parts have the appropriate ratings. Road signs, garden lights, security systems - lots of examples of this. Here is one way to go, assuming this is a typical low voltage, low current device.

    Pick a big enough battery to run the device all night, remembering that the battery voltage will drop during the night. So your device probably will need some kind of voltage regulation of its own.

    Next pick the solar cell so that its output voltage is slightly greater than the battery and it's output current can run the device *and* charge up the battery during the day. Remember, the number of hours during the day that the cell will actually be charging the battery is fewer than the number of hours the battery will be doing all the work at night. In other words, you probably will need a much larger solar cell array than you thought.

    Connect the device directly to the battery, and connect the solar cell to the battery through the diode and resistor. The resistor limits the charging current to a safe value for both the cell and the battery, and the battery acts as a regulator for the solar cell output voltage.

    This is the most simple case, not very efficient, etc. But that's the basic plan.

    ak
     
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  5. k7elp60

    Senior Member

    Nov 4, 2008
    478
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    An automobile electrical system is typical of this. When the engine is not running and accessories are turned on the power is coming from the battery. As soon as the engine is on, the accessories are still working but
    the system alternator is recharging the battery.
    I have solar panels and have some radio equipment and some lighting that is connected to the batteries. On cloudy days the battery is being charged, but the load exceeds the charge current. So the battery is being charged
    and discharged at the same time. I do not use an isolation diode, but the charge controller is between the solar panels and the battery & load.
    At night the battery supplies all the power and on sunny days the battery is fully charged. My solar panels provide about 16A at about 20V maximum.
    My batteries 225AH batteries so I can run the load over a week without the
    batteries being charged.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    No, the battery is not charging and discharging at the same time.
    It can do one or the other but not both.

    When the charging system (solar panel or alternator) is below the voltage of the battery, the battery is going to supply the needed current. It can supplement the charge coming from the charging system. The battery is not being charged. It is being discharged.

    When the voltage of the charging system exceeds the battery voltage, the battery is being charged. The battery supplies no current to the load.
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    k7elp60 - 20A? How many sq ft is the solar array. And if it's not secret, what is the application for radios and lights that take 225 A-h for backup?

    Just curious.

    ak
     
  8. k7elp60

    Senior Member

    Nov 4, 2008
    478
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    I have about 20 square feet of solar panels. The load on the batteries is not a secret. It is amateur radio and some lighting. The reason for the large capacity of batteries is if bad weather prevents the panels from charging the batteries I have over a week of normal use. All batteries have a AH rating and a normal discharge. Lead acid has the discharge rate of 20 hours, so I could get 11.25 amps for 20 hours. My lights are used about 6 hours a day and use about 3A. The amateur radio gear uses about 10A and is only used a about 2 hours a week. I use golf cart batteries 2ea 6V in series. I have had the solar panels about 15 years and I am on my 3 set of batteries. The batteries have been lasting about 6 years.
     
  9. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    Mr. Chips is correct, the batteries do not charge and discharge at the same
    time.
     
  10. ahmed115

    Member

    Dec 21, 2013
    32
    0
    If u want the battery charge at limit volt u can use ic 555
    http://forum.allaboutcircuits.com/showthread.php?t=95360
     
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