monitor battery under charge

Discussion in 'General Electronics Chat' started by Circuits101, Mar 15, 2010.

  1. Circuits101

    Thread Starter New Member

    Feb 24, 2010
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    I have been designing a battery charger for my EE senior design project. The charger itself works great in multisim (Have yet to build it) but now we are trying to implement a 5 level LED monitoring circuit to alert the user of the charge left in the battery. The circuit works create when connected to the battery but when the battery is being charged the monitor tells us that the battery is full when we know it is about 50%. How could I have the monitor ignore the charging input and get the current state of the battery.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    To assist you with diagnosis of your problem we are going to need you to provide the schematic of the relevant portions of the circuit.

    hgmjr
     
  3. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Your monitor is only designed to monitor battery voltage levels from the nominal full charge level (load on, charger off) down to the depleted level.

    When you charge a battery, the voltage goes well above "full" very soon in the charging process yet the charging is not complete until the battery reaches its fully charged level (charger on, load off).

    e.g. A two cell lithium battery has a nominal voltage of 3.7V when charged, but 4.1V at the end of the charge cycle, when the charger is on.

    You need a different circuit to monitor charging process as the voltage slowly goes from 3.7V to 4.1V.
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    " A two cell Li battery " Do you mean 2 cells in parallel, ifixit? ' thought single cell about 4V?
     
  5. Circuits101

    Thread Starter New Member

    Feb 24, 2010
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    I attached our charging circuit with the level indicator. We are charging and monitoring the levels of a 12v lead acid battery. What could we do differently with the indicator to get it to display the battery's current status regardless of being charged or not
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    Nobody said what type of battery. Lead-acid? Lithium? Ni-MH? Ni-Cad?

    A Lithium cell voltage reaches 4.2V when it is only 70% fully charged. You must not let the voltage to go higher than 4.2V. Its current will slowly reduce until it is low when the cell is fully charged and then the charger must turn off.
     
  7. ifixit

    Distinguished Member

    Nov 20, 2008
    639
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    Bernard,
    I was thinking of Lithium AA cells, not lithium-ion cells at the time, they are 1.5V each, but they are not rechargable. So my post should say 1 cell of 3.7V nominal.

    Thanks for that correction.
    Ifixit
     
  8. Circuits101

    Thread Starter New Member

    Feb 24, 2010
    11
    0
    i'm working with lead acid batteries not lithium-ion. Does anyone know how I could adjust my circuit. Also my charger is a multi stage charger with stages constant current, constant voltage and standby voltage mode
     
    Last edited: Mar 16, 2010
  9. Audioguru

    New Member

    Dec 20, 2007
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    A lead-acid battery shows a higher voltage while it is being charged.
     
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