Battery charger for Li ion cells.

Discussion in 'The Projects Forum' started by bluegreen, Jul 22, 2009.

  1. bluegreen

    Thread Starter Member

    Jul 22, 2009
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    I want to charge two Lithium ion cells using a 12V,1A adaptor.The power supply should charge the battery and also supply current to the load.
    When power supply is not available the batteries should discharge through load.The batteries should last atleast 1hour 30minutes.Can someone suggest a schematic including power path management.Thank you.

    Load requirements:7.5V,500mA
    Battery specification:3.6V,750mAH.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    Your load needs 7.5V but the two lithium batteries will have an output of 8.4V when fully charged then the voltage will drop to 6.0V when they need re-charging.

    Will your load work with such a range of supply voltage?
     
  3. Externet

    AAC Fanatic!

    Nov 29, 2005
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    A battery pack with two Li-ion cells must have its own charger controller manager chip, or each cell has to be charged independently from isolated charging supplies of at least a simple 4.100V constant voltage source.

    The hard way to make the simple 4.1000v sources is with two independent supplies connected each to one cell only.

    Or the way I do it, with 3.9V beefy zeners in parallel to each cell and connected to them only while the charging happens.
     
  4. bluegreen

    Thread Starter Member

    Jul 22, 2009
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    <Audioguru>I think it should work with a 6V supply.But how do I charge the cells simultaneously in series.What will be the maximum charging current?My load requires a minimum of 400mA current.So the power supply should be capable of providing minimum 400mA to the load and charge the cells at the same time.When supply is unavailable the load should draw from the batteries.What arrangement can I use?

    <Externet>Are you sure that the Zener method is a safe option.
     
  5. Externet

    AAC Fanatic!

    Nov 29, 2005
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  6. bluegreen

    Thread Starter Member

    Jul 22, 2009
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    Ok.Can u post a schematic.Thank You.
     
  7. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Trying a sketch:

    (+) ------------/\/\/\/\------------ZK-----S---->>-----Cell +
    DC supply
    (-) -------------------------------ZA---------->>-----Cell -

    The resistor value depends on the voltage of the supply, the desired charging current rate and the power dissipation capability of the zener.

    ZA and ZK are anode and cathode of the zener diode. I believe I use 1N5335 but writing from memory, will check later and correct if wrong.

    --->>--- are the cell charger holder contacts where you drop the cell onto.
    I use individual 5V power supplies for each cell.
    I selected the resistor for the zener to be never warmer than touchable temperature (<60°C) at worst condition = no cell in the charger.

    You would need to insert a mosfet or relay switch at "S" that turns on when the charger supply is on for charging several cells in series or permanent connection of the charger.
    Hope helps.
    Miguel
     
    Last edited: Jul 24, 2009
  8. bluegreen

    Thread Starter Member

    Jul 22, 2009
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    Thank You Externet for the schematic
     
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