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Help needed w/battery charger circuit

Discussion in 'Electronic Repair' started by David Nebenzahl, May 8, 2006.

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  1. The problem is finding a battery charger for a cordless drill. I
    inherited a Skil 12-volt drill and battery, but no charger. The only
    hope of finding one seems to be getting a used one, which I haven't been
    able to find.

    I opened my friend's charger and drew the circuit, which I've attached a
    link to. (PDF, and a big one, sorry; somehow a little bitty 35 KB JPG
    made the file bloat up over a megabyte.)

    Can someone comment on this? I've got all the values except for the one
    resistor in series with the LED, which I can probably figure out (or
    maybe just omit altogether, as it seems only to be an indicator lamp).

    What's the function of the capacitor? (I believe it's a non-polarized
    electrolytic in the original unit.)

    Would this work if I constructed it?

    The other problem is that the battery has 3 terminals instead of just 2.
    Two are marked "+" and "-", but there's an unmarked middle terminal. But
    the charger only had connections to the two terminals. What's the 3rd
    terminal for? (Picture of bottom in the PDF.)

    Oh, yeah, the PDF is here:

    Pierre, mon ami. Jetez encore un Scientologiste
    dans le baquet d'acide.

    - from a posting in alt.religion.scientology titled
    "France recommends dissolving Scientologists"
  2. TimPerry

    TimPerry Guest

    some rechargeable batteries have a third contact which goes to a thermistor.
    this is used with a "fast charger". the battery is charged at a high
    current. when it gets warm enough it is considered to be mostly charged and
    switches the charger either to off or to trickle charge.

    some chargers (i.e. Makita) you push a button on the charger to start the
    charge. when its done it shuts down.

    with this type of contacts it should be very easy to used an appropriately
    sized variable supply and alligator clips to recharge. always observe
    correct polarity.
  3. lsmartino

    lsmartino Guest

    David Nebenzahl ha escrito:
    That charger seems horrendous to me. If for any reason you touch any of
    the exposed battery terminals of the charger, without the battery in
    place, while the charger connected to the AC, you will get instantly
    shocked by 120 VAC. I can´t believe that such a charger, without any
    interlock switches can be manufactured, and approved to be sold.

    I would design a charger using an AC transformer to reduce the 120VAC
    to 24VAC. Then I would rectify that voltage, and use a drop resistor to
    reduce the voltage to 12V. It would be interesting to know the aH
    capacity of the battery.
  4. lsmartino spake thus:
    Look, save your horror and outrage, OK? The thing is mass-produced by a
    well-known reputable company, is UL, SA and who knows what else approved
    up the wazoo, so apparently the regulators think it's safe. The whole
    thing is safely encased in plastic, so there's hardly a chance someone's
    gonna get their fingers zapped by line voltage.

    Or maybe you prefer the version of the world where everything is covered
    with soft foam rubber bumpers, with flashing lights, sirens and klaxons
    going off every time you attempt to plug something in?

    Pierre, mon ami. Jetez encore un Scientologiste
    dans le baquet d'acide.

    - from a posting in alt.religion.scientology titled
    "France recommends dissolving Scientologists"
  5. This appears similar to my black and decker, but mine has a 12v wall wart.

    - Mike
  6. TimPerry

    TimPerry Guest

    the circuit makes little sense to me as Imax through the 22K resistor would
    only be 5 mA the bulk of the current would have to go through the series RC
    circuit 2 ohm + whatever Xc is (at operating frequency). finding the
    capacitor might be a trick in itself :) the 120 V rating (of C) is
    suspicious because when charging a depleted battery nearly all the 120 V
    will be dropped across it. that's not much of a safety factor.

    it was probably done this way to partly avoid the heating you get with
    series resistors.

    lets assume for now its a 1 AH battery. to charge it in 10 hours would
    require 100 mA (or a wee bit more) all you need is to come up with a
    charger that will pump 100 mA into a 12V load that wont fail or burn up or
    set you shop on fire. there is probably 50 or do ways to do it but I'd not
    go the transformerless rout.

    a cheap way might be to get a car battery trickle charger and put flashlight
    lamp bulbs in series to limit the current or calculate the needed voltage
    drop by measuring Vno load minus 12 and sizing the appropriate resistor .

    another approach might be to use a "wall wart" supply. i'm a little hesitant
    to recommend that as a buddy of mine clams that one failed and burnt his
    house down.

    i'm in the same situation as you with an old flashlight that someone gave
    me. it's not worth finding a charger for it. i just pull the battery pac and
    tape gator clips to the contacts and hook to a bench supply with an ammeter
    in series. set it to 50 mA and let it charge for a day.
  7. lsmartino

    lsmartino Guest

    OK, but you would be unable to replicate that circuit safely, because
    you wont be building a safe enclusure for it, and probably you will be
    using alligator clips to connect the charger to the battery.

    So, using a circuit like that with alligator clips *is* a safety

    And still, is an awfull design for a charger.
  8. Dave D

    Dave D Guest

    Quite a lot more. To charge a 1Ah battery would take 100mA for 14-16 Hrs or
    140-160mA for 10 hours.

    I agree the charger is awful. I prefer a transformer and 'proper' constant
    current designs, such as those based on voltage regulator ICs arranged in
    constant current mode. The last one I knocked together for my dual band
    handie used a LM317 and a trimpot to set the current, very simple but
    effective. Unlike a series resistor this design keeps the current precise
    over the entire charging cycle, regardless of the state of charge of the

  9. Dave D

    Dave D Guest

    You asked for people's views, then you jump on someone who offers a
    perfectly valid and sensible opinion, what is your problem?
    What has that got to do with it?
    The question of whether the original is safe or not is completely
    irrelevent, the issue is whether your attempt at copying the circuit will be
    safe or not. The original had to pass stringent safety tests that a
    wall-wart powered design would not because of the potential mains shock to
    the user. Just because you copy the circuit does not in any way imply the
    safety approvals will magically transfer to your build.

    Frankly, I'd stay a million miles away from that design, it's crude and
    unnecessarily dangerous for a hobbyist to copy.
    You're being ridiculous now. I don't mean to sound rude, but I hope it is
    down to ignorance and naivity rather than bone headed arrogance. Either way
    you need to get over your attitude of self importance before you injure or
    kill yourself.

    Tip- if you need to ask about the suitability of such a simple circuit and
    how it works then your electronics knowledge is perhaps not yet good enough
    to be so arrogant and dismissive of people who are trying to help you, or to
    build mains circuits such as this one.

  10. Dave D spake thus:
    Maybe you can be excused for *assuming* that anyone who asks questions
    like these is an idiot who's likely to get zapped by line voltage the
    first time they experiment with a circuit.

    I can assure you that I'm not in that category. I am not the world's
    foremost electronics expert (hence my questions), but I certainly know
    all about protecting myself and others from shocks, fire, etc.

    So you misplaced concern for safety is totally unnecessary. If I did
    build a charger like this, it would be strictly for my own enjoyment.
    I'm not going to be selling them on eBay, or over at the flea market.

    Apparently, you've never heard of line-operated radios, either. Maybe
    you're not old enough to remember when the canonical 5-tube radio was
    operated directly from line voltage with no isolating transformer.

    Or perhaps you're one of these safety fanatics, like the ones who
    apparently are responsible for the fact that now on TV whenever there's
    the remotest chance of anything getting warm or bubbling, the
    technicians are wearing plastic goggles and protective clothing.

    Don't get me wrong; I'm all for safety. I don't like the idea of
    environmentally dangerous stuff like lead, asbestos, benzene and TCE in
    air, water and soil. It's a good thing that those big old exposed knife
    switches are a thing of the past. But even safety can be taken too far,
    you know.

    Pierre, mon ami. Jetez encore un Scientologiste
    dans le baquet d'acide.

    - from a posting in alt.religion.scientology titled
    "France recommends dissolving Scientologists"
  11. And this technique is now rightly banned. Radios in those days didn't have
    external connections for headphones and CD players etc.

    The fact is the contacts on your charger are likely to be accessible to a
    finger, etc. So it should conform to the regs for such things.

    You might find the capacitor 'dropper' technique on some self contained
    devices - perhaps a rechargeable electric razor. But not with your
  12. lsmartino

    lsmartino Guest

    Maybe you can be excused for *assuming* that anyone who asks questions

    Please excuse me, but if you have to ask about how to construct a
    circuit to charge a battery, then one can assume that your electronic
    knowledge or understanding is not the best in the world. The fact that
    you had to open a charger just to see what a charging circuit looks
    like, and after that you couldn´t apreciatte that the design you
    encountered it´s pretty crude *and* unsafe to replicate, was also
    another pointer to your lack of electronics experience.

    Just because a suggestion given by one poster didn´t suit your
    mindset, is no excuse for being rude. If you ask questions, you should
    be prepared to receive any answer, even if you dislike the answer.

    And please, don´t tell us about vacuum tubes, line operated radios and
    things like that, which are very well buried in the past because of
    their limitations.
  13. Dave D

    Dave D Guest

    I didn't assume that at all. I do however think you unfairly jumped on
    someone who voiced valid safety concerns.
    I think you're missing the point. You implied that copying the design would
    be safe because it had originally passed approvals, and you were rude to
    someone who was simply offering the opinion you requested.
    Yes, and radios where the mains cable was a resistive dropper! Very nasty.
    Not at all.

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