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Unusual Battery Charging System

Discussion in 'Electronic Basics' started by Geoff, Jul 19, 2004.

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  1. Geoff

    Geoff Guest

    I'm going to be going into rural Nepal for 8 months starting this
    winter. I would like to be able to take pictures for this time.
    However, there will be no electricity. I've got a nikon coolpix 4500,
    which uses a Li-ion rechargeable battery (#EN-EL1) which is 7.4V /
    680mA. The proprietary charger (#MH-53) takes AC at 100-240V and
    outputs DC 8.4V / 600mA. The MH-53 supplies only a slightly greater
    power than the EN-EL1 (5.04W vs. 5.032W)

    Anyway, I'd like to design a design a charger that acts as a
    substitute for the EN-EL1. I plan on doing this with a solar panel,
    which will probably end up being 7-10W, outputting, ideally at the .6A
    that the charger would. However, I suspect that the panel will end up
    being quite a variable source, and, as I don't want to damage the
    battery, I'll need to stabilize it to outputting at 8.4V/.6A.

    Now, I've never done this sort of thing before, but I've taken an E&M
    class recently and would be confidant putting anything that's
    relatively simple together. Having done some research already I think
    that if the panel put off a constant flow at .6A, then I could use
    something like the LM338 in conjunction with a few resistors (and
    perhaps a potentiometer, in case I'd like to re-use the system for
    something else) to regulate the voltage to the 8.4V. However, this
    will probably not be the case. http://www.national.com/ds/LM/LM138.pdf
    details a 'adjustable current regulator' (on page 12) that looks to be
    something, but I don't know. (help?)

    Ultimately, is there a self-contained object that I can build that,
    having a variable source put into it, outputs a pre-set current at a
    pre-set voltage? Or is there no way to get around not having a
    constant current to start with?

    Thanks for your help.
     
  2. Bob Pease of National Semi did something similar, and its documented
    here

    http://www.elecdesign.com/Articles/ArticleID/4251/4251.html

    His experience might help with your design.

    Regards,
    Bob Monsen
     
  3. If you come up with a solar panel that puts out at least 10 volts at
    ..6 amp, you can make a regulator with an LM317 that lets through only
    8.4 volts. The charger is in no danger if the regulator can provide
    a bit more than .6 amps. It will consume only the current it needs.
    Extra voltage is what will damage it.
    Don't forget a small heat sink for the regulator.

    You should add a capacitor on the input and output of the regulator as
    specified on the data sheet to make sure it is stable.
    http://cache.national.com/ds/LM/LM117.pdf
     
  4. Marlowe

    Marlowe Guest

    I'm into R/C model airplanes and there has been a lot of news about Li-Poly
    batteries exploding and causing fires and other mayhem. Most of the
    problems seem to be caused by crash damage or failures while charging.
    Anyway, I have been using Li-Ion cells for some time in my electric powered
    planes with no problems, but I take great care in charging them. I use only
    a charger designed for Li-Ion/Li-Poly batteries that are selectable to the
    battery capacity and number of cells. I NEVER leave them charging
    unattended or for longer than it takes to recharge the cells (no overnight
    charging as we do for our Ni-Cad batteries).

    I realize that Li-Ion batteries are used all over the place in many
    applications like the ubiquitous cell phones, but do a Goggle search on this
    topic and you will see many horror stories. I once charged a two cell
    Li-Ion pack with a Ni-Cad/Ni-MiH designed charger and burned them out (no
    explosion) even though I monitored the current and voltage. Go figure.
     
  5. Marlowe

    Marlowe Guest

    More on the safety issue:

    Emergency Safety Alert: Lithium Battery Fires
    (Added 4/23/04)

    Lithium batteries are becoming very popular for powering the control and
    power systems in our models. This is true because of their very high energy
    density (amp-hrs/wt. ratio) compared to Nickel Cadmium (Ni-Cds) or other
    batteries. With high energy comes increased risk in their use.

    The principal risk is fire which can result from improper charging, crash
    damage, or shorting the batteries. All vendors of these batteries warn their
    customers of this danger and recommend extreme caution in their use.

    In spite of this many fires have occurred as a result of the use of Lithium
    Polymer (Li-Poly) batteries, resulting in loss of models, automobiles, and
    other property. Homes and garages and workshops have also burned.

    A lithium battery fire is very hot (several thousand degrees) and is an
    excellent initiator for ancillary (resulting) fires. Fire occurs due to
    contact between lithium and oxygen in the air. It does not need any other
    source of ignition or fuel to start, and burns almost explosively.

    These batteries must be used in a manner that precludes ancillary fire. The
    following is recommended:

    1. Store and charge in a fireproof container, never in your model.

    2. Charge in a protected area devoid of combustibles. Always stand watch
    over the charging process. Never leave the charging process unattended.

    3. In the event of damage from crashes, etc., carefully remove to a safe
    place for at least a half hour to observe. Physically damaged cells could
    erupt into flame. After sufficient time to ensure safety, damaged cells
    should be discarded in accordance with the instructions which came with the
    batteries. Never attempt to charge a cell with physical damage regardless of
    how slight.

    4. Always use chargers designed for the specific purpose, preferably having
    a fixed setting for your particular pack. Many fires occur in using
    selectable/adjustable chargers improperly set. Never attempt to charge
    lithium cells with a charger that is not specifically designed for charging
    lithium cells. Never use chargers designed for Ni-Cd batteries.

    5. Use charging systems that monitor and control the charge state of each
    cell in the pack. Unbalanced cells can lead to disaster if it permits
    overcharge of a single cell in the pack. If the batteries show any sign of
    swelling, discontinue charging and remove them to a safe placeóoutsideóas
    they could erupt into flames.

    6. Most important: NEVER PLUG IN A BATTERY AND LEAVE IT TO CHARGE UNATTENDED
    OVERNIGHT. Serious fires have resulted from this practice.

    7. Do not attempt to make your own battery packs from individual cells.

    These batteries cannot be handled and charged casually such as has been the
    practice for years with other types of batteries. The consequence of this
    practice can be very serious and result in major property damage and/ or
    personal harm.
     
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