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Ni-Mh Battery charger - Ohms law

Discussion in 'General Electronics' started by suraj, Oct 2, 2003.

  1. suraj

    suraj Guest

    I have Ni-Mh Batteries that say Standard Charge : 160 MA for 14 hrs.
    Now I want to make a charger for it . If I have a 6Volt Transformer
    and After Rectifying, what Resistance Should I use to obtain 160 mA @
    3V DC?

    Also what else Should I do to ensure that the Current that I give to
    the Batteries is clean and Best for the Batteries?

    Also If I fast charge the Batteries (500 mA for 5hrs) what is the
    disadvantage ?

    Many Thanks,Suraj
     
  2. default

    default Guest

    The easy answer is 19 ohms at two watts. The power line can vary
    plus/minus 10% and 19 or 20 ohms is good if cost is the consideration.
    The typical technique in cheap battery chargers it to use a
    transformer with a lot of magnetic leakage to limit current. The
    better way is to add a current limiting circuit.
    The batteries should be designed for fast charging if that is what you
    want to do. The disadvantage is usually shorter life. Left
    unattended, without automatic circuitry, the cells can overheat and
    rupture.
    Good NiMh chargers watch the voltage as the cell charges. Great
    battery chargers watch the voltage and temperature of the battery. By
    monitoring battery voltage, the charger can throttle the charge
    according to what the battery can use. NiMH batteries have a
    decrease(!) in their voltage when fully charged, good chargers watch
    for that and shut off when they see it.

    1.2 volts per cell, so you want to charge two cells? Are they a
    standard size and can they be separated? Rayovac makes a nice charger
    that does AA and AAA NiMH batteries and alkaline. Monitors and shuts
    them down as each of four cells charges. Cost in the US is ~$20 and
    worth it. Walmart has cheap Quest chargers that cost ~$20 with a set
    of four AA batteries included.

    See> http://www.powerstream.com/NiMH.htm
    http://www.sensorsmag.com/articles/0303/battery/main.shtml
    http://www.extremetech.com/article2/0,3973,1155273,00.asp

    Several IC manufacturers have chips designed expressly for charging
    batteries.
     
  3. Chang

    Chang Guest

    As your line voltage may change +/- 20% and the voltage drop on the
    resistor depends on the current I recommend you built a current
    source.

    Answer to the question : 6V * SQRT(2) = 8.5 V. Across the resistor :
    5.5 V with the current of 160 mA you need an R = 33 Ohm ( 1 W ).
    (R=U/I)
     
  4. Arachnoid

    Arachnoid Guest

    I have Ni-Mh Batteries that say Standard Charge : 160 MA for 14 hrs.
    R = V/I?

    you need to include the internal resistance of the battery before you
    do the calculation.

    NiMH cells are far more sensitive to overcharging than NiCads

    If you are planning to fast charge you should measure the voltage /
    cell and when it descends below a certain limit you should stop
    charging.

    If you are cycling NiMH you need to check that the voltage output /
    cell doesn't go below a threshold value.

    Can't remember all the numbers, but they are out there on the web
    somewhere
     
  5. You need a constant current source. The easiest way to achieve something
    close is a light bulb in series with the accu. The resistance of the
    glowing wire in the bulb increases with current, this will tend to keep
    the current constant (measure current as a function of voltage through a
    bulb to see the effect). Of course, electronic circuits can do a better
    job, see for example a book on standard OP-Amp-circuits.

    Provided that you use accus certified for that, and that you keep an eye
    on the accus temperature (reducing current as temperature increases),
    none.
     
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