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how should i charge this battery?

Discussion in 'Electronic Basics' started by [email protected], Jul 6, 2005.

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  1. Guest

    I have very little experience in electronics and only know the basic
    math equations and funtion of all the basic components from resistors
    to transistors. i have just started reading my first book on beam
    electronics and need to know how to charge a battery pack. I took two
    cordless phone batteries(one 3.6v 400mah the other 3.6v 600mah) and
    wired them in paralel to get a 3.6 1000mah battery pack (is that
    right?). What is the perfect voltage and amperage for charging this
    pack? If I wired the pack in series, then how many mah would I have?
    How would i charge this new pack?

    Thanks in advance!!
  2. Tom Biasi

    Tom Biasi Guest

    You have NiCads. Nominal 1.2 volts per cell. A rule of thumb (nice and safe)
    is to charge at 1/10 the mAH rating for 10 Hrs.
    At that rate the NiCad temperature usually does not need to be monitored. In
    practice you really won't get 1000 mAH out of the parallel pack but you will
    get more than 400 mAH.
    The voltage could be about 1.4 volts per cell. When charging in series it is
    really desired that the battery packs be all the same type and rating.
    Series by definition passes the same current through all cells.
    If you ask me I would charge the packs separately. I would use 4.2 volts
    per pack and limit the maximum current to about 40 mA. Do you have a way to
    control the current? A discharged battery will draw more current than a
    partially charged battery but keep the maximum to about 40 mA and charge
    over night. This is not the optimum charging for the batteries but will do
    the job.
  3. Bob Monsen

    Bob Monsen Guest

    Wiring them in parallel isn't a very good solution, unless you use
    diodes to prevent one from running current through the other.
    Unfortunately, that will waste a bit of the output voltage.

    A better solution would be a larger pack, since it would probably be
    lighter too. You can get 1700mAH 1.2V nimh batteries for a buck each,
    and wire 4 together.

    look for

    HR-AAU overstock w/TABS

    It'll be lighter, and you won't have to worry about cross-charging.

    Your homemade 1000mAH pack needs about 100mA for 14 hours. Take care not
    to overcharge it, or you'll kill the nicads. You can tell when you are
    overcharging it, because the cells heat up quickly.

    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
  4. ehsjr

    ehsjr Guest

    You need to charge them separately. It's simple & cheap.
    Get a 12V DC wallwart, 2 LM317 voltage regulators and 2
    resistors. Wire it up like this:

    +12V ----+-----in| LM317 |out------+---Batt1--+
    | --------- | |
    | | | |
    | +---/\/\/\-----+ |
    | R1 |
    | |
    | |
    | --------- |
    +-----in| LM317 |out------+---Batt2--+
    --------- | |
    | | |
    +---/\/\/\-----+ |
    R2 |

    Batt1 is the 400 mAh pack, batt2 is the 600 mAh pack.
    R1 is a 33 ohm resistor. R2 is a 24 ohm resistor.

    Be *sure* to connect batt1 to the 33 ohm resistor
    and batt2 to the 24 ohm resistor. Note that the
    batteries are *not* in parallel or series. The
    negative side of both batteries goes to the negative
    from the wallwart. The positive side of each battery
    goes to its own LM317/resistor combination.
    Charge for 14 hours.

    Do not use these batteries in series. Doing so could
    damage the 400 mAh pack.

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