scaling battery charger. simple as decreasing the resistance?

Discussion in 'The Projects Forum' started by Dentsu, Jul 31, 2011.

  1. Dentsu

    Thread Starter Member

    Jul 7, 2011
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    hi, here is a schematic of a li ion battery charger and it is currently configure to charge 1 battery between 4.08v to 4.26v at 100mA. would this same setup be able to charge 3x3.7v 880mAh battery in series? i was thinking of just changing some of the value to equal the amount i need. anyone can advise me of it is a good idea or what is the best way to acomplish this? also will it still work if i use a cheaper regulator like a LM317 or equivalent? :confused:

    [​IMG]
     
  2. iONic

    AAC Fanatic!

    Nov 16, 2007
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    The circuit has no means to terminate charging, which is not a good thing with LiIon batteries. Also you will need a higher input voltage, probably 15V - 20V. Charging them in series always leaves you open to overcharging, especially if one battery is defective, and with LiIon this overcharging can and will destroy the remaining batteries and even possibly fire.

    You would do much better with a dedicated LiIon battery charger IC such as the following:

    LM3420
     
    Last edited: Aug 1, 2011
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  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    To decrease the output voltage to 3.7V you could lower the 2M resistor to 1.67M (or you increase the 806k.) The output voltage will adjust to have 1.23V at the feedback pin. However, this circuit doesn't have a current limit. The chip itself seems to have a current limit (I had a short look at the datasheet) but I don't think it's supposed to work always under these conditions.
    So depending on the depletion of your battery the output current could be limited by the IC's protection circuitry, which is not desirable. I think it'd be better to have something that limits the output current, but have not yet figured out the best way.

    EDIT: oops, I didn't read the "3.7V cells in series"... I go with IONIC then...
     
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  4. Dentsu

    Thread Starter Member

    Jul 7, 2011
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    the battery i will be using is a protected 16340 cr123A so it should take care of the overcharging problem. for teminating charging i think ill use a led indicator and have the user manually remove it so wondering where i can place the led? would a 12v input like this work? are there any other risk i should know about?

    Product Description:

    Li-ion Lithium
    Capacity: 880mAh
    Voltage: 3.6V
    No memory effect
    Short Circuit and Over-Current Protection
    Voltage of full charge & empty condition is 4.2v & 2.75v respectively
     
  5. iONic

    AAC Fanatic!

    Nov 16, 2007
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    When three LiIon batteries are in series their fully charged voltage will be about 12.6V. The you have to consider the dropout voltage of the regulator itself which could run you another few volts. That brings you to about 15V, but I'd hate to run the device on the fringes of inadequacy and that why I was pushing 20V. 18V might be O.K.
     
  6. Dentsu

    Thread Starter Member

    Jul 7, 2011
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    alright you have convince me to go with the 20v source. what about the placement of the led indicator when no current is being drawn? maybe i can make use of the "Shut Down" feature or what is the correct name for the charging cap terminator thingy your refering to? i will go ahead and re-assigning the value.
     
  7. iONic

    AAC Fanatic!

    Nov 16, 2007
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    The "Shutdown" feature on the IC you linked to will have nothing to do with the IC I mentioned. The device will not sense that the battery is overcharged and terminate the charging.
     
  8. Dentsu

    Thread Starter Member

    Jul 7, 2011
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    i hope i am on the right track with this math and would probably stick a led between the 270k and 50k resistor.

    [​IMG]
     
  9. iONic

    AAC Fanatic!

    Nov 16, 2007
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    With a 12V Source you will not get 12.6V out..
     
  10. Dentsu

    Thread Starter Member

    Jul 7, 2011
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    i guess it should work if the input source is around 20V? can i get a second option?
     
  11. iONic

    AAC Fanatic!

    Nov 16, 2007
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    A second option or a second opinion?
    A second option is a source of 14V - 15V. If the 12V source is a New 12V Lead Acid battery you may have enough to charge @12.6V for a very short time before it drops below the full battery level, leaving the batteries undercharged.
     
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  12. Dentsu

    Thread Starter Member

    Jul 7, 2011
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    for future reference if i decided to take this on and wanted to add in the load and a pair of LDR led from my other project all i have to do is connect the end of my new LED circuit to the + end of this battery like what you see below? also what is your thought on this lm317 charger? it has 12.6 ouput and led indicator but from 7 years ago.
     
  13. wayneh

    Expert

    Sep 9, 2010
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    You're ammeter in the first diagram is connected in parallel to your chain of diodes. It needs to be in series to be meaningful.
     
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