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Battery Packs, Parallel connection, current load balancing?

Discussion in 'Electronic Design' started by Jon.boston, Jan 23, 2004.

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  1. Jon.boston

    Jon.boston Guest

    I'm looking at connecting three battery packs in parallel to get more
    current carying capability. If each battery has a maximum current
    draw of 4 amps is there a standard circuit to use to help with current
    sharing? I'm worried that all the current will come from the pack
    with the higher voltage.

    I'm looking at getting something like 9 Amps from the three packs in
    parallel?

    Battery packs are li-ion with saftey circuits.

    Thanks,
    Jon
     
  2. Jon.boston wrote...
    If you use three high-current Schottky diodes for current sharing, that
    should work well. For example, the TO-220-package MBR1545 or MBR1645
    diodes drop about 0.45V at 3A each. Some larger parts do slightly better.
    E.g., a 30CTQ045 has two diodes in a three-lead TO-220 package. With two
    sections in parallel, you get a lower roughly 0.4V drop at 3A per package.

    You can do even better with FETs and comparators for current-sharing.

    If one battery is more charged than the rest, it'll deliver more current
    but this will stop as it approaches the same charge state as the others.
    In the meantime its internal resistance and resistance of the current-
    sharing diodes will help equalize the currents sooner rather than later.

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  3. James Meyer

    James Meyer Guest

    You don't have to do any additional balancing for batteries in parallel
    as long as the batteries are all the same type.

    Once they're in parallel, NO battery will have a higher voltage than any
    other.

    Jim
     
  4. Mac

    Mac Guest

    Not for long. ;-)
    AFAIK you don't need to do anything special to discharge in parallel.
    There are battery groups that know more about this question. I think there
    are, or can be, issues with charging in parallel. Because you don't
    actually know what the current into each individual pack is.

    regards,
    Mac
     
  5. Jerry G.

    Jerry G. Guest

    We've assembled large gel battery systems using batteries in parallel. The
    key is to purchase matched batteries from the manufacture. This way there is
    even loading and charging of the batteries.

    --

    Greetings,

    Jerry Greenberg GLG Technologies GLG
    =========================================
    WebPage http://www.zoom-one.com
    Electronics http://www.zoom-one.com/electron.htm
    =========================================


    I'm looking at connecting three battery packs in parallel to get more
    current carying capability. If each battery has a maximum current
    draw of 4 amps is there a standard circuit to use to help with current
    sharing? I'm worried that all the current will come from the pack
    with the higher voltage.

    I'm looking at getting something like 9 Amps from the three packs in
    parallel?

    Battery packs are li-ion with saftey circuits.

    Thanks,
    Jon
     
  6. mike

    mike Guest

    LiIon cells are routinely hooked directly in parallel inside laptop
    battery packs. Seems to work ok as long as they're matched.
    So, what's your definition of "parallel"? If they're hard-wired,
    you shouldn't have any trouble if you start with matched cells.
    If you have swappable packs that can be inserted in any state of
    charge, your problem is isolation more than sharing. You have to
    add some (variable) voltage drop to the highest voltage pack so that
    it delivers only 3A at the voltage of the lowest pack. You don't have
    a lot of headroom between the 3A and 4A numbers. The most important
    spec might be the internal resistance of the cell pack (over it's
    lifetime). Every bad Lithium laptop pack I've seen has failed due to
    high internal cell resistance. If you want long battery life, you need
    to make sure your circuit will work with lots of battery internal
    resistance. Diodes may provide isolation, but may not provide enough
    resistance to keep the max A/pack below 4A (over the useful life of the
    pack).

    And you have the same problem charging. Have to keep the max charge
    current below the max rating at each cell. May be an issue for
    high rate charging.

    What are you doing to keep the temperatures down? I've dissected
    several laptop packs. Seems that there's often only one pair of dead
    cells. They're the ones nearest the hot part of the laptop. So, even
    if you start with matched packs, they may not stay that way for long.

    Where did the 4 amp max current come from? The 75% of the ABSOLUTE
    maximum number may be horrible for cell life. A pair of 18650s is
    typically used in a laptop that idles at 800mA or so. Even then, life
    ain't that great.

    Sure would be nice to start with bigger cells.

    mike


    --
    Return address is VALID.
    Bunch of stuff For Sale and Wanted at the link below.
    Toshiba & Compaq LiIon Batteries, Test Equipment
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    Tektronix Concept Books, spot welding head...
    http://www.geocities.com/SiliconValley/Monitor/4710/
     
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