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Help Building a battery pack

Discussion in 'Electronic Basics' started by optimizer, Dec 5, 2005.

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  1. optimizer

    optimizer Guest

    Hi
    I built a 7.2 V battery pack using two rechargeable 3.6V dc batteries
    They eventually died and I purchased some different new 3.6
    replacements. Unfortunately, when these are charged they pump out
    total of 8+ volts and shut down my device
    I'm an electronics dummy other than the basics. I thought I was goin
    to be able to buy a 0 to 32 volt regulator from the Shack and jus
    turn the switch (or adjust it) on it to 7.2 Volts, but upon seein
    one in the store, there is no switch/adjuster, just a block and
    metal pins

    Basically, I want something that I can just attach to the battery pac
    output wire that will ensure that 1/ no more than 7.2 V DC get
    output, 2/ it allows voltage less than 7.2 Volts (it doesn't stop th
    circuit if the battery is getting low), 3/ Is safe - I can just attac
    it into the wire and no real electrics shock protection (it's no
    going to build up a huge charge or anything), "just cover wit
    electrical tape" type of project

    I suppose whatever is suggested may cause heat, need heat to b
    released. Let me know/advice if that might be an issue

    It's just for a small video camera - so doesn't use much juice

    Thanks for doing the forum. Hope I get some good help, and hopefull
    use it for some other projects
     
  2. Guest

    Some time ago, a friend asked me to rig up something for the same
    purpose. In my part of the world, the original 7.2V pack is difficult
    to get, quite expensive and doesn't last very long. OTOH, 6V
    general-purpose sealed rechargeable batteries are cheap and easily
    available. So I took two of them and used an LM317 IC to regulate the
    resultant 12V down to 7.2V.

    The camera now receives a constant 7.2V supply even when the 12V
    battery voltage level varies from full charge to full discharge. He's a
    semi-pro and doesn't mind the extra weight and bulk.

    The linear regulator generates considerable heat when the camera is
    running, so I used a fairly hefty finned heatsink from my junk box. A
    switched-mode regulator would be more efficient but more complex.
     
  3. Bob Monsen

    Bob Monsen Guest

    On Mon, 05 Dec 2005 03:34:55 -0600, optimizer wrote:
    <his lust for an adjustable supply exposed>

    Try www.mpja.com: http://tinyurl.com/733hn

    I have one, and it is great. It would be nicer if it had kelvin sense
    leads, and the adjust knobs were better quality (or had a digital input,
    like some of the pricier ones) but it works fine for me, and didn't cost a
    fortune.

    In particular, for your battery job, you can limit both voltage and
    current. I'm guessing your batteries are liion, so you probably want to
    limit the voltage appropriately.

    Another idea would be to look on EBay; bench supplies come up all the
    time.

    Another idea, you can get a nice charger at frys for $10.

    --
    Regards,
    Bob Monsen

    Man, as a social animal, can no more escape government than the
    individual can escape bondage to his bowels.
     
  4. ehsjr

    ehsjr Guest

    You're talking about an LM317 regulator chip. Here's how you can
    use it:
    http://cache.national.com/ds/LM/LM117.pdf
    Look on the first page for "Typical Applications" - there
    is a diagram there of the LM317 with two additional parts -
    a 240 ohm resistor and a 5K potentiometer. The capacitors
    on the diagram are not always needed - see the notes.

    Note that the input voltage has to be at least 3 volts higher
    than the output voltage. If you want to keep the maximum
    possible voltage to ~7.2, yet still be able to adjust it lower,
    use this circuit: (one added 1500 ohm resistor)

    -----
    +12 ---in|LM317|out---+------> + Vout
    ----- |
    Adj [R1] 240
    | |
    +----+---------+
    | | |
    | / |
    | \R2 |
    +--->/5K [R3] 1500
    \ |
    / |
    | |
    Gnd --------+---------+-----> Gnd

    Install the LM317 on a heat sink.

    Ed
     
  5. optimizer

    optimizer Guest

    Hi
    Well, that's the regulator I was looking at Radio Shack/Tand
    Adjustable-Voltage Regulator LM317T (see link below). I was expectin
    to see a dial on it when I went to the store, but it didn't look lik
    it had a dial or switch on it at all, from what I could see shakin
    it in it's container. The radio shack people knew nothing..
    So how on earth do I get it to reduce voltage to whatever outpu
    voltage I want to set it to

    I've looked at electrical books/diagrams, and I've seen one that use
    another component in the layout - but the diagram was an electronic
    diagram with electronics coding etc, so is confusing to a
    electronics dummy that just wants in-out lines-components. Thank

    http://www.radioshack.com/product/i...kwCatId=2032058&kw=regulator&parentPage=searc
     
  6. optimizer

    optimizer Guest

    Unfortunately I just need to lower it around 1 volt
    I had some NiCD batteries, and got NiMH's to replace. The NiCD used t
    work fine. The NiMH's seem to charge to a total of around 8 volts o
    more even though they are two 3.6 volts batteries
    Anyway, if the input needs to be 3 volts higher, I'm missing 2 volts
    Thank
     
  7. ehsjr

    ehsjr Guest

    I think you may be able to get this to work with just 1 cheap
    part.

    If all you need to do is lower it (your battery pack) about
    1 volt, and you want to do that as simply as possible, put
    a 1N4001 diode in series with the pack and the device, with
    the banded end of theconnected to the negative side of the
    battery. That will reduce the voltage about .7 volts.
    *However*, the pack will soon reduce to the nominal 7.2 volts,
    and the diode will further reduce that to about 6.5. If your
    device is "happy" with that, you can use this simple approach.
    It does not precisely meet your stated requirements (#1 and #2,
    as it will allow about 7.4 volts output, not 7.2 as you
    stated, and it might stop the device from working by reducing
    the voltage to the device to 6.5, even when the battery is not
    low.) But it is the simplest you can get, and it stands a good
    chance of working for you.

    Here's what is going on that is causing your 7.2 volt pack to
    provide 8+ volts: a 7.2 volt pack consists of 6 cells whose
    nominal voltage is 1.2 volts per cell (6 * 1.2 = 7.2). When you
    charge the batteries, they get charged to a hair over 1.4 volts
    per cell (as they should be), and the pack voltage is over 8
    immediately after the pack is taken off the charger. It soon
    reduces to 7.2 when used.

    Finally - if the single diode approach won't work, use the
    LM317 circuit already posted. The input voltage will need
    to be over 10 volts - which you can get by using 3 3.6 volt
    packs, or a 12 volt DC wall wart supply.

    Ed
     
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