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Repairing a 12v battery charger.... What the heck is this?

Discussion in 'Repairing Electronics' started by WranglerSK, Aug 15, 2011.

  1. WranglerSK

    WranglerSK New Member

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    Hey everyone,
    Quite excited to be on the site, and hope to gain ++ information from everyone here.

    Yesterday I started tinkering on an old 12v battery charger i picked up in a pile of stuff from an auction. As soon as I opened it up, I noticed corroded connections and melted wires. I started replacing them with copper (rather than the aluminum it had) and upped it to 18g. my next piece as I was looking at it, has a steel plate with connections located. These are where the wires connect to. The bottom pair are intact with a small black ?resistor? and the top two are not there with some black arc noted on the plate. I pulled one of the little items out, but as much as I google it, I can't find out what it is, and where to get some. I stuck an ohm meter to it, but doesn't show any resistance.... Any tips? I'll post some more photos of the unit when I get a second. I'll try to get a pic of this little thing asap.
    Thanks for your help in advance.
     
  2. Sceadwian

    Sceadwian Banned

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    Text descriptions are virtually useless for identification, decent pictures are the only way you'll get even a clue.
     
  3. WranglerSK

    WranglerSK New Member

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    Alright, I just decided to go out and get some photos taken right now.

    Here's the unit

    Here's the setup inside. Note the new wiring placed in there.


    Where you can see the clips attached to the new wiring, that is where this little black piece goes - I'll draw an arrow on it.

    Here's some photos of the little black piece. There is writing on the side of it: It has the Motorola symbol - then 1504T1E27

    Hope this helps. - I apologize it's blurry - old school Blackberry....
     

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  4. dave

    Dave New Member

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  5. Sceadwian

    Sceadwian Banned

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    There's four arrows there, so four black pieces? They're diodes for a full bridge rectifier for the output of the main transformer maybe, not a capacitor in site though, this isn't exactly state of the art in charger design =)
     
    Last edited: Aug 15, 2011
  6. WranglerSK

    WranglerSK New Member

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    Thanks for the info. How do I determine what size of diode they are? And yes, 4 of them. 2 still there, although the one I popped out has a crack down the side, so I would probably get 4 new ones...if I can figure it out. Thanks again
     
  7. Sceadwian

    Sceadwian Banned

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    Well they're rectifier diodes so they need to be rated for the full current they're intended to draw, which in this case is 15amps for charging, and it has to have a 100amp peak power handling ability, probably with a rated voltage around 20 volts. From what little I can recall they're called disc diodes, and the thing on the outside is probably not part of the diode package but an insulator. You'll want to replace all four no question, the form factor of the replacement diodes is kind of important because those types of diodes are very closely physically linked to the chassis so they basically use the housing as a heat sink. If you use a radial diode or TO220 type package heat sinking becomes important.
     
    Last edited: Aug 15, 2011
  8. debe

    debe Active Member

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    Probably cheaper to use a $5 100V 35A bridge rectifier on a heat sink. These are what ive used as replacement in small battery chargers.
     

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  9. debe

    debe Active Member

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    A free source is old car altenators with press fit diodes, theres 6 of them. 3 Pos & 3Neg polarity. They will do the job easily.
     

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  10. Sceadwian

    Sceadwian Banned

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    Best clean em up better than those pics =\
     
  11. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    You don't need capacitors in a battery charger, in fact it's probably better not - as the raw DC pulses help to 'agitate' the lead acid battery.

    Obviuously it's crude, but that's all you need for a manual battery charger, and new ones today would be similar (if a little less antiquated).
     
  12. WranglerSK

    WranglerSK New Member

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    I'm going to look for the 100v 35A bridge rectifier today when I'm in the city. Hopefully something comes up. I assume that I can basically override that whole steel plate and just connect the 4 wires to this?

    Thanks for all of your help. If I can't track this down, then I'll maybe tear apart an old alternator!
    Thanks again.
     
  13. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I suspect you haven't considered that the original parts 'may' be important.

    A battery charger basically consists of three items:

    1) A transformer.

    2) A rectifier.

    3) A current limiter.

    It's the third which is 'vague' - it could be an actual physical resistor, it could be a specially would and designed transformer, it could be the special rectifier (metal plate), it could be the wiring between them (one reason for thin aluminium wiring - which only seems to be used in the USA?). Or it could be a combination of all of them.

    Replacing the wiring with thicker copper, and the rectifier with a modern silicon one, 'could' remove any current limiting features.
     
  14. Sceadwian

    Sceadwian Banned

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    Nigel the statements you said about a charger not needing a capacitor and the definition of the three basic requirements for a battery charger only apply to flooded lead acid batteries, that's a pretty important qualification which is why I mention it, if you try to charge anything other than a flooded lead acid battery using this better have a fire extinguisher nearby. Even sealed lead acids (Absorbed glass mat) will have a slightly different float charge which is critical to pack life.

    Nigel, you assumed in post #10 that it was a lead acid charger, obviously it is, but it wasn't mentioned at any point previously in this thread, remember the average user that comes to these forums has very little to no experience with any of these things which is why it's important to fully define everything from the start, hopefully the original poster knows that this even if he did replace the diodes could never be used to charge anything other than a flooded lead acid battery, and that's if he's lucky =) Though when attached to a big capacitor, or even a battery it'd make one heck of a 12V power source.


    The transformer itself is the current limiting device in these types of chargers, they just tap different coils depending on switch(s) position, they're inherently current limited, which would explain why the only active component of this circuit appears to be the bridge rectifier from mains.
     
    Last edited: Aug 16, 2011
  15. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Obviously it's a lead acid charger, and presumably it says so on the casing?

    The transformer is the current limiting in SOME chargers (using specially designed transformers), and most don't have taps anyway - the old metal rectifier will limit current, as will thin aluminium wires. It's also pretty common for old chargers to have a limiting resistor as well, presumably before some clever guy invented the self limiting ones.

    Most chargers which had switchable charging currents used resistors, and switched different values in.

    You wouldn't believe the massive antique charger we still have at work :p
     

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