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How to build a Li-Ion charger myself (windmill, generator, solar cells)??

Discussion in 'Electronic Basics' started by [email protected], Oct 28, 2005.

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  1. Guest

    I have a crazy idea:

    1. On my self suppored bicycle trips around the world I want to use a
    digital camera.
    2. I have faced reality and accepted that most digital cameras use
    Li-Ion batteries, which makes them very small and handy.
    3. Currently I'm interested in a Sony DSC-T7 which uses a Li-Ion
    battery. The battery specs can be found here:
    4. I don't want to carry a regular charger but I want to use either a
    regular bike generator (the ones powering for front and rear light on a
    bicycle), solar cells or a home-built mini wind mill which can be
    mounted on the bicycle and produce electricity during the night.

    So the question is:
    Is it possible to build my own Li-ion charger if I have a varying power
    source such as a wind mill? I know that Li-ion batteries are difficult
    to charge and that they may be damaged if treated wrongly. I have seen
    in some other threads that companies such as Texas Instruments are
    offering special ICs. What about the small motor/generator for such a
    mini wind mill application - does anybody have recommendations? Are
    there some published circuits on the internet which I could use?

    Regards, Lucas Jensen
  2. Funfly3

    Funfly3 Guest

  3. mike

    mike Guest

    A couple of cautions.
    Most people who design simple chargers on the back of an envelope don't
    pay attention to the little details. They build one and it works for
    their limited application. Then someone else tries to use the design
    in a completely different application and has difficulties.

    With a bike generator, you're gonna have voltage going all over the
    place. That doesn't have to be a problem as long as you design for it.
    You're gonna want spike protection on the input so the regulator doesn't
    short and apply max current/voltage and blow up your battery.

    Worry about transient conditions. The power on transient may not be a
    big deal, unless you're sitting on the edge and the transient condition
    happens on every pedal stroke. Your average charge condition may not be
    what you think. If you use an IC, it may be resetting itself a lot.
    Or may get stuck in a condition that it can't recover from.
    Ics have safety timers. If it keeps getting reset, that feature
    is ineffective.

    IF your generator can't supply enough power at the current instant,
    the charge current will drop. When the power comes back up, so will the
    current and the battery voltage. This will confuse the hell out of the
    charge termination algorithm.
    Again, not problems unless you don't design for 'em and evaluate under
    all possible conditions.

    Current limit with voltage limit is probably what you want, but torture
    test it under widely varying input conditions with a scope looking at
    the outputs.

    Wanted, Serial cable for Dell Axim X5 PDA.
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  4. Jasen Betts

    Jasen Betts Guest

    ["Followup-To:" header set to sci.electronics.basics.]
    if you can get an off-the-shelf charger that uses an external low volltage
    DC source that may simplify your task. then all you need is a windmill that
    can make 12V (etc...)
  5. Dave D

    Dave D Guest

    But some don't. It would be far easier for you if you find one which can use
    AA alkalines and AA rechargeables.
    The Li-Ion based cameras I've seen have the charging circuit built into the
    camera itself, and the mains adaptor is just a regulated supply. If you are
    prepared to charge the battery in the camera, although it may be less
    convenient, it would make things a lot simpler.

    Here's how I might do it-

    You need to find out the voltage of the mains adaptor, let's say it's 6V,
    but double check your camera before plugging anything homebrew into it!.
    It's essential that the camera sees a constant and well regulated 6V
    throughout the charge cycle, don't power it from the dynamo on your bike or
    a windmill, at least not directly. You'll at best find the battery never
    charges properly and at worst you'll destroy your camera.
    What I suggest is a car type arrangement. Use your bike's dynamo (these are
    usually around 6V AC IIRC so you'll need to rectify to DC ) to charge either
    a 6V lead-acid gel battery or 5 NiMH cells, using appropriate charging

    Build a voltage limiter to keep the battery output voltage below, say, 6.1V.
    A crowbar circuit to prevent catastrophies is recommended. Feed the DC from
    the battery into the adaptor input on your camera. An inline fuse would be a
    wise move.

    The battery should ensure that the voltage stays fairly constant, and if you
    choose large enough capacity cells, it should keep charging the Li-Ion
    battery when you aren't pedalling, and may even be able to completely charge
    the camera overnight while you sleep. IOW, the bike battery charges during
    the day, then it charges your camera overnight.

    You could even wire your lights into it so they stay on when you stop
    pedalling! You'd need to increase the capacity for this.

    So basically I'd aim to charge the camera with a battery, which in turn
    would be topped up by the bike's dynamo. This is exactly what happens when a
    mobile phone is charged via a cigar lighter socket on a car.

    This is just a very rough idea, it would need to be well thought out before
    implementing. I believe that if it's done properly, it would work out rather

  6. Kryten

    Kryten Guest

    Charging Li-Ion cells is non-trivial - they can overcharge and die

    Possible - yes, simple - no.

    You'd need a gadget to get a decent steady supply from the dynamo/windmill,
    and then you need a charger smart enough to manage Li-Ion cells.
  7. Guest

    Hi Dave,

    I've been searching the internet for suitable AA battery cameras, but
    they are all too bulky. I understand that you want to stabilize the
    voltage my means of a battery pack in between. But this is not a
    working solution, because the entire system has to be really
    lightweight. I know of some chips called something like 7805. They are
    designed to give a constant output voltage as long as the input voltage
    stays above a certain value. The 7805 would give a 5 V output. Frankly,
    I hate the bike generator solution the most because it gives more
    friction which I personally hate. I would rather stay with the mini
    wind mill or the solar cell. As for the solar cells, they readily
    produce DC current at a defined voltage. As I remember a solar cell
    will always produce the same voltage if exposed to the same light
    soruce (for instance the sun) and only the intensity will determine the
    current. This means that you should have a fairly stable voltage but
    alternating current (due to clouds etc).

  8. Jim Thompson

    Jim Thompson Guest

    LiIon cells are "managed" both in charging AND discharging. It's not
    a trivial task to build your own. I would suggest purchasing a
    commercial unit.

    ...Jim Thompson
  9. James Sweet

    James Sweet Guest

    Sure it can be done, have a look at, they make a whole
    range of Li-Ion charger IC's, and you can even get free samples to play
    with. As for the generator, you could regulate the output with a small
    high efficiency switchmode regulator, National Semiconductor makes a
    line of those which are very easy to work with, and again samples are
  10. Guest

    Jim Thompson schreef:

    Don't worry. Your Li-Ion battery is NOT managed.
    Adhere to Funfly3.
    Charging your battery is like charging a lead-acid battery: Constant
    voltage with current limiting. Correction for temeprature might be
    required. That is where the third connection is for. As far as I know
    there is no standard for a temperature sensing element in Li-Ion
    battery packs, so you have to find out!

    Take care, but don't be afraid for explosions etc. Li-Ion battery packs
    are internally protected by means of
    (1) discharge valve. prevents pressure build-up inside the cell
    (2) thermal fuse
    (3) semiconductor fuse
    So, if you do something wrong, the battery pack will be defective, but
    will not explode.

    Reason for "managing" Li-Ion battery packs (the ones with 4 or 5
    connections) is
    a) For "fuel gauging". As the discharge characteristic of a Li-Ion
    battery is almost flat, it is almost impossible to determine the
    state-of-charge from the battery voltage
    b) For faster charging. Using appropriate algorithm can charge
    batteries faster
    c) Universal battery chargers. The battery charger can obtain battery
    pack data from the battery pack.
    The "managed"battery packs have a serial interface (normally the Smart
    Battery Bus) for communication between battery and camera or charger.
  11. You might find this site vaguely interesting.

    Cheers, J/.
  12. Funfly3

    Funfly3 Guest

    chargers do not need any algorithms as when each cell gets to 4.2 volts its
    charged and the current will have dropped to zero, there are plenty of 12v
    chargers for the model industry and all these just limit the current and
    voltage even a school boy could design one, ask on one of the radio control
    groups as we are big users of Li-Ion and Li-Po batteries from 3.6 volts to
    40+ volts and current ratings from MAh to tens of amp hours I have a $10
    charger that will charge 1 to 3 cells at various current
  13. Rich Grise

    Rich Grise Guest

    I'd try to arrange it so that the windmill turns the same bike generator -
    just gear it suitably. I'm also thinking some kind of vertical windmill,
    maybe even with a telescoping shaft, but then you'd need guy wires
    or something...

    Have Fun!
  14. PeteS

    PeteS Guest

    Sandra wrote:

    <<Charging your battery is like charging a lead-acid battery: Constant
    voltage with current limiting.>>

    Bzzt. You missed an important point.

    A Li+ battery *can* be constant voltage charged (if you can live with
    the loss of life - figure 60% vs. constant current / constant voltage),
    but what you forgot is the voltage limiting. Never charge a Li+ to more
    than 4.2V per cell unless you want to see interesting light and nasal
    effects (I know this to be a fact - I have a charger where the voltage
    management failed).


  15. Funfly3

    Funfly3 Guest

    I think you missed the point A Li-Ion is constant voltage charged but "with"
    current limiting ,at the beginning of the charge the current is the limiting
    factor at the end voltage is the limiting factor?
  16. Kryten

    Kryten Guest

    To save long on-line discussions, got to:
    for all you need to know about batteries.

    How often do you expect to use your camera, and will you need a flash. Once
    you fill a card, you will have to store or send it somewhere: will you be
    carrying a PC or PDA around too? If so, that is going to need power as well.
    Theft is a risk, if you are travelling outside the first world - carrying
    something worth say $600 is the same sort of temptation to a desperately
    poor person as ten to a hundred times that amount is to someone in an
    affluent country.

    It is a bit strange that in 1990, I simply mailed my films home.

    In 2005, the memory cards are too expensive to mail home and buy new ones.
    So you need an expensive gadget to do this for you.

    Maybe one day cameras will be able to send photos somewhere safe via the
    cellphone network. I know some phones can do this but they are generally
    rather low resolution compared with proper cameras, but I was thinking more
    about a simple phone tacked onto a camera than a simple camera tacked on a
    mobile phone.

    However I can see the appeal of what you are doing.
    There must be a market for a power supply for the technovagrant :)
  17. budgie

    budgie Guest

    BZZZZZTT! Wrong. As anyone who has worked on the design of CC/CV chargers
    would know, when transition from CC to CV occurs, the cell is about 60% charged.
    Also, the current does not "drop to zero" at transition - rather it starts an
    almost exponential decay from the CC value. Typically this will take 2 hours or
    more to drop to 10% of the CC value.
  18. Guest

    any one can tell why the comp. lock isn't accurate.
  19. Chris Jones

    Chris Jones Guest

    The above idea about using a second battery to power the camera seems likely
    to work. If you want to charge the battery outside the camera then the
    following information might be of use:

    It looks like your battery is a single cell.

    Some information on charging lithium batteries is here:

    Most lithium packs have built-in MOSFETs and protection circuitry to
    disconnect the battery from the terminals if any cell is charged over some
    very accurate threshold which (according to Panasonic) is 4.30V +/- 0.05V.
    The safety circuit will probably also disconnect if you try to discharge
    the cell to too low a voltage, i.e. it disconnects at 2.3V +/-0.1V, but you
    shouldn't discharge it below 3V. Don't rely on the safety circuit to
    control your charging, it is meant as a backup, and I wouldn't be surprised
    if once it disconnects, it's time to buy a new battery pack.

    If you are charging single cells, then the important things are:

    Never exceed 4.2 Volts/cell, regardless of current or anything else. For
    some cells this should even be 4.1 Volts, depending on the cell chemistry.
    This needs to be an accurate regulator (1% accuracy will do).

    Never exceed the current limit specified for the cell (which is usually
    about 0.7C, i.e. the current you would have to draw to flatten the cell in
    one hour). For your battery it would be about 0.45 Amps

    Never flatten the battery below about 3V or so, nor allow it to
    self-discharge below about 2.5V, the battery will undergo permanent damage
    and some people say it becomes dangerous. Basically don't store them
    completely flat for long periods. I have seen it suggested that optimum
    life results from storing with 40% charge in the battery.

    If the battery happens to have a voltage below 2.9V, the charger is supposed
    to reduce the charging current to 0.1C (the amount of current that would
    flatten a charged battery in 10 hours). This would be 68mA for your
    battery. If you can manually verify that you haven't flattened it this far
    then that ought to suffice.

    So you need to limit the current to 0.45Amps and you need to limit the
    voltage very accurately to 4.1 or 4.2V. If the voltage has reached 4.2V
    and the current dies down to 0.1C (68mA for you) then the battery is full
    and you should disconnect it for best life, though I don't think it is
    dangerous to continue charging it at 4.2 Volts.

    There are 'intelligent' charging chips available from several manufacturers
    that will manage the whole charging cycle, but these chips would probably
    expect a steady and constant supply voltage from start to finish which you
    will not have unless you use the second battery route when you might as
    well use the camera's internal charging circuit.

    Since you require opportunistic charging, I would suggest rectifying the
    generator output and filtering it with a big electrolytic capacitor, and
    limiting the peak voltage with a zener diode of perhaps 8.2V, 5 Watt or
    whatever power rating you find necessary. This can then be applied to a
    non-intelligent charging IC such as the ADP3820.

  20. Bob Monsen

    Bob Monsen Guest

    You should use a set of solar cells. A windmill would be pretty annoying,
    and probably powered by your legs rather than the sun.

    If you only want to charge a single cell (4.2V) you can use a 6V solar
    cell pad. Limiting the current to 0.5C won't be an issue with solar cells,
    assuming you don't go nuts and buy too many. The cells are generally
    speced for a particular current. The voltage limit will be the main issue.
    Use a regulator, like an LM317 (see the datasheet at for
    an appropriate circuit). You can get these at radio shack, where the
    packages have schematics on the back. A small 1 hour timer would be a good

    Look here and elsewhere for information on solar cells:

    Bob Monsen

    It is mathematics that offers the exact mathematical sciences a certain
    measure of security which, without mathematics, they could not obtain.
    - Albert Einstein
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