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Is this the right way to charge a battery?

Discussion in 'Electronic Basics' started by [email protected], Aug 1, 2005.

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  1. Guest

    My brother got a Vectron ultralite made by Air Hogs. the battery is a
    6v 100mah nimh, the cherger used is a 13v 600ma transformer. This
    voltage and power rating sound way to high for a baattery so small.
    Will this transformer kill the battery? would a lower power
    transformer, like 7v 200ma be a good investment? Thanks in advance!
     
  2. Redink

    Redink Guest

    If you could possible give me the model number and the name of the
    charger you are useing i will be glad to research on it and tell you
    what is good to charge and not
     
  3. Redink

    Redink Guest

  4. Tom Biasi

    Tom Biasi Guest

    You don't need a transformer you need a charger.
    Tom
     
  5. Guest

    It was the kind of wall outlet transformer used to charge any battery.
     
  6. kell

    kell Guest


    You are right. The transformer is too hot for the battery.
    The crude solution, which is used in cheap chargers, is a series
    resistor.
    First: does this thing put out dc or ac? Obviously you need dc. If
    it's ac, you need to rectify it or get a wall wart that does put out
    dc.
    Then you need to limit the current with a resistor. You should go for
    a low charging current to avoid damage to the battery. That's because
    you will have no way of knowing exactly when the battery has finished
    charging, so you need a charging rate low enough that you don't have to
    worry about how long you leave the battery charging. Nicad and nimh
    batteries don't like overcharging. Find out the slow charging rate for
    the size nimh you have (google is your friend). If they suggest C/20
    for example, you need 5 ma (wow sounds small). Anyway whatever it is,
    you can then figure out the resistor value. It's just 7 volts divided
    by the charging current (7 volts across the resistor because you have a
    13 volt source going into a 6 volt battery). (Of course the actual
    voltage from your power source varies with the current, so it might not
    be exactly 13 volts.) Then the power the resistor dissipates is going
    to be current squared times resistance. Mske sure your resistor has a
    higher power rating by at least a factor of three.
     
  7. ChadMan

    ChadMan Guest

    I wanted to ask a similare question. I have a bunch of 7.2 volt 600 MAH bat
    packs. I want to make a charger for them.

    In this fellows case wouldn't it be better to use a Zener regulator so that
    it wouldn't overcharge?

    Does anyone know of a schematic for these 7.2 AA packs? I'm thinking that I
    could use something like this:


    Batt (+) ___
    o-------o----|___|-----(+)
    | Charge
    | x-former
    '-----z<---o---(-)
    Batt (-) |
    o------------------'
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
     
  8. ehsjr

    ehsjr Guest

    That will blow up the zener.

    Here's a simple circuit for slow charging a 600 mah
    7.2 volt pack:

    -------
    +12 ------Vin| 7805 |Vout------+
    ------- |
    | [R] 91 ohms 1 watt
    | |
    +--------------+-----> To Batt +

    Gnd-----------------------------------> To Batt -

    The 7805 will need to dissipate about .3 watts. The TO-220
    package does not need a heat sink at that power, but I'd
    use a small sink anyway. The circuit will limit the charge
    rate to ~ 55 mA, roughly C/10, which is desireable for
    a 14 hour charger. Note that Gnd from the 12 volt supply
    connects *only* to the negative side of the battery.

    Ed
     
  9. JazzMan

    JazzMan Guest

    Speaking of battery charging circuits, I'd like to build
    a nice NiMH charger for single cell AAs rated at around
    1700-1800mAh. These are the Energizer cells that came with
    a 4-cell charger, but I'd rather charge them with something
    that actually monitored cell temperature and actively
    altered charging current as needed to get them charged as
    fully and reliably as possible.

    I'm presuming there are ready-made ICs out there that
    can handle this? I'm not very good at design, but I
    can build like a bad dog.

    I've got a couple dozen cells here now at various
    states of discharge, and don't want to use the supplied
    charger since it seems that after only a dozen or so
    charge cycles the NiMH batteries quit holding a charge.
    I'm guessing this is the cae because the charger doesn't
    shut down after full charge is reached?

    If someone can point me to a few decent schematics I
    can do all the board layout, etching, etc myself.

    Thanks!

    JazzMan
    --
    **********************************************************
    Please reply to jsavage"at"airmail.net.
    Curse those darned bulk e-mailers!
    **********************************************************
    "Rats and roaches live by competition under the laws of
    supply and demand. It is the privilege of human beings to
    live under the laws of justice and mercy." - Wendell Berry
    **********************************************************
     
  10. Bob Monsen

    Bob Monsen Guest

    Here is a fast charge IC for NiMH/NiCD from maxim:

    http://www.maxim-ic.com/getds.cfm?qv_pk=1666&ln=en

    There are some example circuits in the datasheet.

    --
    Regards,
    Bob Monsen

    If a little knowledge is dangerous, where is the man who has
    so much as to be out of danger?
    Thomas Henry Huxley, 1877
     
  11. Guest

    Yes, the power cube gives out dc power. the toy has a light on the
    hovercopter itself that tells you when it's done chargeing, which with
    there increadibly high current source, is about ten minutes!!!
     
  12. |
    | Yes, the power cube gives out dc power. the toy has a light on the
    | hovercopter itself that tells you when it's done chargeing, which with
    | there increadibly high current source, is about ten minutes!!!
    |

    It's typical that the equipment (hovercopter) has an on-board electronic
    charging circuit which regulates the charging current and voltage, and the
    external DC power source is rated a margin higher than the controlled
    charging voltage and current.
    Thus the capacity that you note in your power source is just a little
    over-kill, and of no concern.
     
  13. | ChadMan wrote:
    | > | >
    | > I wanted to ask a similare question. I have a bunch of 7.2 volt 600 MAH
    bat
    | > packs. I want to make a charger for them.
    | >
    | > In this fellows case wouldn't it be better to use a Zener regulator so
    that
    | > it wouldn't overcharge?
    | >
    | > Does anyone know of a schematic for these 7.2 AA packs? I'm thinking
    that I
    | > could use something like this:
    | >
    | >
    | > Batt (+) ___
    | > o-------o----|___|-----(+)
    | > | Charge
    | > | x-former
    | > '-----z<---o---(-)
    | > Batt (-) |
    | > o------------------'
    | > (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
    | >
    | >
    |
    | That will blow up the zener.
    |
    | Here's a simple circuit for slow charging a 600 mah
    | 7.2 volt pack:
    |
    | -------
    | +12 ------Vin| 7805 |Vout------+
    | ------- |
    | | [R] 91 ohms 1 watt
    | | |
    | +--------------+-----> To Batt +
    |
    | Gnd-----------------------------------> To Batt -
    |
    | The 7805 will need to dissipate about .3 watts. The TO-220
    | package does not need a heat sink at that power, but I'd
    | use a small sink anyway. The circuit will limit the charge
    | rate to ~ 55 mA, roughly C/10, which is desireable for
    | a 14 hour charger. Note that Gnd from the 12 volt supply
    | connects *only* to the negative side of the battery.
    |
    | Ed

    Your circuit offers only current control using the 7805 Voltage regulator,
    which will certainly overcharge the batteries if their voltage is not
    carefully monitored.

    I'd follow the advice of another poster who references design specific IC's
    such as those from Maxim.
     
  14. Kitchen Man

    Kitchen Man Guest

    Which makes it a battery charger, not simply a transformer.
     
  15. ehsjr

    ehsjr Guest

    Rubbish. Where did you ever see a 14 hour charger
    with *any* voltage monitoring, let alone "careful
    voltage monitoring"? 14 hour chargers have been in use
    for years with *no* voltage monitoring, and no problem,
    unless the batteries are abused. Just put the batteries in
    when they are low, and take them out after 14 hours have
    elapsed. If you leave them in far in excess of 14 hours,
    or charge them when only a small percentage of the charge
    has been used, you're abusing them
    This is even worse than the other statement. You are
    recommending that he use 13 parts instead of 2, and understand
    a 17 page datasheet in order to determine parts values. Think
    about it - the poster doesn't even know that his proposed "zener
    regulator" will do nothing but blow up the zener. Do you really
    think he can follow the datasheet? Did you even look at it?
    What do you think his chances are of determining the proper
    charge rates, determining the correct values for R1 and Rsense,
    and building and testing the circuit? Can he determine what
    kind of heatsink, if any, to use for Q1?

    Ed
    P.S. I would have responded sooner, but I was travelling.
     
  16. Bob Monsen

    Bob Monsen Guest

    Well, it is fairly easy to build a simple CCCV charger. Why burn up your
    batteries by overcharging them if and when you get distracted, and leave
    them charging for a week?

    Using a TL431 and three resistors with your proposed circuit will limit
    the voltage to some chosen value. Just use the TL431 to yank on the adj
    pin of the regulator when the voltage gets to the limit point. Use a 1k
    resistor between batt+ and adj. You can build in a nice "Done" indicator
    by putting an LED between the TL431 and the adj pin. Set the final
    voltage to 8.4V, and you have it made (unless one of your cells is bad,
    in which case you'll destroy them all).

    Also, using an LM317 makes more sense, because the required resistor
    will dissipate less energy, and the required input voltage will be
    lower. Your charger will only allow charging up to 7V given 12V input.
    Using the 317, which is the same as a 7805 except that the drop is only
    1.2V, means you need a 22 ohm resistor for a similar current.

    --
    Regards,
    Bob Monsen

    If a little knowledge is dangerous, where is the man who has
    so much as to be out of danger?
    Thomas Henry Huxley, 1877
     
  17. ehsjr

    ehsjr Guest

    Good point about the LM317 and the input voltage. The
    7805 circuit won't go over 7 v with a regulated 12v in -
    he would have to use an unregulated 12 v supply that
    droops to 12 v under its rated load with the 7805 circuit.

    While is is fairly simple to use a TL431, it is not as
    simple as the proposed circuit: two components. It doesn't
    get much simpler than that. Remember, this poster doesn't
    know enough electronics yet to use a zener properly, so
    keeping it as simple as possible is the goal.

    Ed
     
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