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Li Ion replacement

Discussion in 'Electronic Repair' started by D Yuniskis, Jul 29, 2009.

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  1. D Yuniskis

    D Yuniskis Guest

    Hi,

    I'm replacing the battery in a GPS unit. Looks to
    be an "A" cell (no, not AA or AAA), 3.7V 1800mAHr.
    Of course, the battery has pigtails soldered to it
    so I'll need something with either pigtails or solder
    tabs as its replacement (I doubt I could add any
    sort of battery holder in this small space!).

    I went grep-ing Digikey and all I found were "lithium"
    batteries but I suspect these are intended for use in
    "nonvolatile (data) backup" (i.e., low current rates)
    and not actually used to *power* something. (I think
    the battery is intended to last about 8 hours so I'm
    guessing the load to be about 250mA?).

    Can someone clarify what I need to go looking for?
    And/or a pointer to a source for same (qty 1).

    Thanks!
     
  2. spamme0

    spamme0 Guest

    google 18650.
     
  3. D Yuniskis

    D Yuniskis Guest

    Thanks! Seems to be the right size and capacity;
    but I need something with *pigtails*:

    "Of course, the battery has pigtails soldered to it
    so I'll need something with either pigtails or solder
    tabs as its replacement (I doubt I could add any
    sort of battery holder in this small space!)."

    I haven't a clue as to how I could otherwise make the
    connection to the battery (without risking explosion/damage).
     
  4. spamme0

    spamme0 Guest

    buy cells with tabs.
    do not solder to lithium cells!!!!! unless you have very good fire
    insurance, medical insurance and a death wish.
    Your wife will be very angry six months later when the GPS
    sets her car on fire. So, if the battery doesn't hurt you, she will.

    Just in case you missed it....DO NOT SOLDER DIRECTLY TO ANY BATTERY,
    ESPECIALLY LITHIUMS. People will tell you they do it.
    Are you feeling lucky?

    You need a battery tab welder if you try to use non-tabbed cells.

    Virtually every lithium ion laptop battery pack
    ever made contains tabbed 18650 cells. Buy an old battery pack
    and take it apart...carefully.
    Sometimes, you can talk the radio shack guy to give you one from his
    recycle bin. Or ask the IT guy at work.

    Cut the tabs. You can (carefully) solder to the remaining part of the
    tabs if they're 1/4 inch or more without overheating the cell.


    Your gps (probably) takes significantly less peak current
    than a laptop, so a weak laptop pack that's not completely dead
    may provide useful cells for your application.
     
  5. Especially once you know what happens when you mix
    And we all know such batteries are so poorly sealed that water pours into
    them the moment they're dunked in water...
     
  6. D Yuniskis

    D Yuniskis Guest

    Ummm.... Find a picture of a pig. Note the tail. There's no
    semblance to a battery tab.[/QUOTE]

    Note the following: ;-)
    Thanks. When I saw these were commonly used in laptop battery
    packs, I just fished a couple of laptops out of the trash and
    cannabilized the battery packs until I found cells with "decent"
    open circuit voltages. Made sure I cut the tabs that connected
    them to their neighbors so I would have a fair bit to work
    with. Soldered some pigtails on and was able to replace the dead
    battery in the GPS unit easily!

    Rescued GPS unit: $0
    Rescued Li Ion cell: $0
    Working GPS unit: priceless

    I've never had luck trying to solder to these cans
    (partly out of fear of letting the can get too hot!).

    I've a friend who will make me a little welder but I
    would have to fetch it from him (out of state) -- its
    just not worth the effort for something I do so rarely!

    Thanks, all, for the pointers!
     
  7. D Yuniskis

    D Yuniskis Guest

    Ummm.... Find a picture of a pig. Note the tail. There's no
    semblance to a battery tab.[/QUOTE]

    Note the following: ;-)
    Thanks. When I saw these were commonly used in laptop battery
    packs, I just fished a couple of laptops out of the trash and
    cannabilized the battery packs until I found cells with "decent"
    open circuit voltages. Made sure I cut the tabs that connected
    them to their neighbors so I would have a fair bit to work
    with. Soldered some pigtails on and was able to replace the dead
    battery in the GPS unit easily!

    Rescued GPS unit: $0
    Rescued Li Ion cell: $0
    Working GPS unit: priceless

    I've never had luck trying to solder to these cans
    (partly out of fear of letting the can get too hot!).

    I've a friend who will make me a little welder but I
    would have to fetch it from him (out of state) -- its
    just not worth the effort for something I do so rarely!

    Thanks, all, for the pointers!
     
  8. spamme0

    spamme0 Guest

    snip
    I'd like to hear more about the welder design.

    I messed around with low-voltage transformers, but never
    got consistent welds. Was VERY sensitive to contact
    resistance. About half the welds didn't stick.
    The other half blew thru the tab.

    Gave all that up when a CD welder
    showed up on ebay for $15. It'll put 7000Amps into a milliohm.
    Get MUCH more consistent tab welds.

    If you have a chance to get an affordable tab welder,
    you'll find other uses for it. And you'll have a lot of new
    friends in need of tab welding ;-)

    mike
     
  9. spamme0

    spamme0 Guest

    I flat-out disagree with that statement.
    Sure, you can play semantics with the words and claim it means anything
    you want...but current/time control doesn't solve the repeatability
    problem in the face of process variation.

    If your resistance is zero, all the current in the world won't make a weld.
    If your resistance is high, low current will blast holes thru the medium.

    What you need is local temperature rise to fuse the metal together.

    A current/time controlled weld is VERY sensitive to resistance.
    If you have repeatable resistance and machine enforced pressure, you can
    set the current/time
    for the particular setup and make good welds...until something changes.

    If you raise the minimum resistance, the variable portion becomes a smaller
    percentage and welds get more repeatable. .005" sheet brass that you get
    at the hobby store makes tabs that are much easier to weld.
    I never used it in a battery pack because I didn't want that added
    resistance and I was too lazy to look up the corrosion effects due to
    the dissimilar metals.

    A CD welder delivers energy and is much more forgiving of process
    variations, ie pressure and resistance changes in a hand-operated
    homebrew setup making welds on old used batteries.

    A REAL battery tab welder hits it twice. The first shot stabilizes the
    contact area and measures the resistance. The second shot uses results
    from the first measurement to optimize the weld energy.
     
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