power supply backup

Discussion in 'The Projects Forum' started by seesy123, Jun 25, 2010.

  1. seesy123

    Thread Starter Member

    Mar 14, 2008
    anyone can check for me isit this circuit can work..because the output of voltage regulator LM7805 is 14.1V in picture.how come the output is higher than 5V?how it can charge the battery? for this circuit isit it keeping charging the battery even battery is fully charged.so the battery life is shorter right and will it explode if keeping charge.can any idea to modify it uncharge when it's fully charge in battery.

    thanks for help
  2. k7elp60

    Distinguished Member

    Nov 4, 2008
    The reason the output of the 7805 is 14.1 volts is the 9.1V zener diode from the center pin of the 7805 to ground. The battery will not overcharge because diode D1 drops the voltage to the standby charge voltage of 13.5 to 13.8V. A 12V battery will not over charge at this voltage.
  3. seesy123

    Thread Starter Member

    Mar 14, 2008
    thanks for your reply first.

    oh..mean if i want to get higher output with voltage regulator so i just need change the zener diode value right. but the voltage regulator will not spoil? but output pin of regulator is just 5V..is it possible to support higher than 5V in output pin.what is the range voltage if it can support more than 5V.

    Mean the battery will not charge anymore even it's fully charge?but not as long as the voltage is higher(13.5-13.8V) than the battery voltage(12V) then it will charge the battery?because when the battery is fully charger the maximum voltage is 12V but the supply voltage(13.5-13.8V) is higher than it,so it will keep going charge right?
  4. retched

    AAC Fanatic!

    Dec 5, 2009

    This link will explain the inner working of different batteries.

    Lead-acid batteries use what is called a float charge.

    The float charge is usually around 13.8v for a 12v Lead-Acid battery.

    In order to charge a battery, you must apply more voltage than the battery has.

    So, your charger will keep the battery topped-off without overcharging.

    If the battery is used, it will re-charge the battery and try to keep it full.

    A full 12v Lead-Acid battery usually shows around 13.8v.

    Read the above link for more information.
  5. SgtWookie


    Jul 17, 2007
    Using a linear regulator for charging a battery isn't such a great idea; the power dissipation in the regulator will be very high if the battery is significantly discharged. This means that the regulator will get very hot.

    It is difficult to provide enough of a heat sink for a TO-220 package to keep it from overheating when under a heavy load.

    Modern chargers use switching supplies to regulate charging current during the "bulk charge" phase, which quickly brings the battery up to a certain voltage; typically somewhere around 14v. After that voltage has been reached, the charger changes to "absorption mode", where the voltage across the battery terminals is held constant until the current drops below a certain level; and then comes the "float charge" phase which can be continued indefinitely.

    The actual specifics about the charging requirements needs to be obtained from the data sheet for the particular battery in question. Attempting to use a "generic" charger for all batteries will likely result in either lower performance than could be obtained, or shorter service life.

    The bottom line is, you must follow the manufacturer's specific recommendations for the battery in question by reading the datasheet for the battery. If you do not, you will kill the battery before it's time. If you really goof up, you can wind up having an explosion and/or fire, and property damage/physical harm.