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"Fixing" crap Harbor Freight battery charger

Discussion in 'Electronic Repair' started by Roger Blake, Aug 5, 2013.

  1. Roger Blake

    Roger Blake Guest

    Given the less-than-steller reviews and HF's reputation for electronics,
    I should not have bought this thing, a 12 Volt 6 or 2 amp (switch
    selectable) charger for car batteries. The problem is that instead of
    providing the tapering charge and switch to trickle it is supposedly
    designed for, this "charger" discharges and kills batteries instead. (In
    contrast, I have a 4 amp Schauer charger that is about 40 years old
    which still works just fine.)

    Instead of throwing out the HF unit, I was thinking that it has a perfectly
    usable case, transformer, ammeter, and wires with alligator clips. Just the
    crapulent Chinese electronics are bad so I'd like to trash that stuff
    and turn this into an "old school" type charger like my Schauer.

    The transformer secondary is center tapped and reads about 28 volts AC
    across the ends, 14 volts on each leg from the center. The original
    electronics are on a small board with 7 transistors, 2 SCRs, a couple
    of diodes, and numerous resistors. (That's a lot of stuff just to
    kill batteries!) Searching online I'm finding a bewildering array of
    home-brew battery charger circuits, everything from simply using a bridge
    rectifier on the transformer secondary to more complex circuits to taper
    the charge and either switch off or go to trickle when the battery is
    fully charged.

    The behaviour of my old Schauer is to gradually reduces current until
    the battery is fully charged, where it will stay at a low level and not
    hurt the battery if left on overnight or even for a few days. I have
    not opened it up yet to see what's inside, but being an early 1970s unit
    I'd be surprised if there is much aside from a transformer and rectifier.

    Anyone have a simple circuit handy that works well for this? I've seen
    a few plans online that just say to use a bridge rectifier of suitable
    capacity, but I want to be reasonably sure that this thing won't kill
    any more batteries.
     
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    Kalyan Ram Guest

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  3. It might be easier to fix the thing, unless you want a project.

    I can tell from here that it's probably using some sort of half-bridge
    setup. Apparently two extra diodes cost more than a smaller more efficient
    transformer. Heck, it's unlikely to have the correct sized transformer to
    start with, they just removed two diodes to save whatever it costs to
    keep the school kids/prisoners fed.

    It might be good to mod the thing to crowbar if it overshoots 13.8 or 14.1
    or whatever you want your batteries charged to. There should be some sort
    of rectifier to prevent the battery from discharging back into the
    charger, check on that.

    I can't say they're good or bad, but I've made NiMh chargers that were
    just constant current sources that maxed out at the float voltage. The
    charging current tapers off as the thing starts to choke itself out, but
    it does work. The only reason I even had the constant current section was
    to prevent the transformer from burning out if you threw a heavily
    discharged cell into it. Old NiMh batteries really didnt like fast
    charging either.
    How large are the batteries you're charging? A brute force charger may be
    ok for smaller batteries, but if you're trying to charge some 2 ton off
    the grid battery locker, you're going to have to smell that transformer
    burn up.

    What parts are on that board you have now? Can you post a photo?
     
  4. Roger Blake

    Roger Blake Guest

    12 volt lead-acid car batteries, I should have mentioned that if I didn't!
    It does look like they cheaped out on a couple of rectifiers, hence
    the 28V center-tapped transformer. (The transformer looks surprisingly
    hefty, though.) But then again there are 2 SCRs on heatsinks and it's
    surprising how many discreet transistors this thing has, enough to make
    a decent transistor radio, but who knows how many are real. A bunch
    of what are probably phony 1% resistors completes the components. No
    ICs. No capacitors. The center tap of the transformer feeds into the
    center section of the board through a circuit breaker. Each side of the
    secondary then feeds into its own symmetrical section of the board.

    I tried taking a picture but my camera does lousy closeups, will have
    to borrow one...
     
  5. Are these "SCRs" really SCRs or just large power transistors? Did the
    thing sing or make horrible PWM sounds when it worked?

    I have seen real SCRs in telecom rectifiers (giant battery chargers/power
    supplies).
     
  6. Roger Blake

    Roger Blake Guest

    The board is silkscreened "SCR1" and "SCR2" in those locations - the
    components themselves are black plastic about 3/8" square and 1/8" thick,
    labeled "BT151 25J TRANSUN", metal tab at top for heat sink attachment,
    3 leads on the bottom.
     
  7. Roger Blake

    Roger Blake Guest

    The Chinese are infamous for cheating on specifications. Whether these
    are actually phony or not would obviously require measuring them with an
    accurate meter.
    Some of the choices in the design do seem odd given that lowest possible
    manufacturing cost was probably the primary constraint.

    As far as the lack of ICs I was just describing the contents of the
    board, though it does seem unusual these days to see a a circuit based on
    discreet transistors.
     
  8. that's nice.
     
  9. were these hunk-o-junk units plugged into those hardwired site wide surge
    surpessors?
     
  10. didn't mean to be a jerk with the last reply.

    so I did about 3 seconds of searching and here's how that charger probably
    works.

    something is measuring the voltage of rectified DC and turning the SCRs on
    and off when the input voltage is just right for charging the battery.

    It's basically 120Hz PWM. They may even be using the negative or postive
    voltage from the other leg of the center tapped transformer to shut off
    the SCR on the opposing side. Cheap and simple. Sadly, I don't recall if
    you just short the gate of a SCR to something to shut it off, or if it
    needs a reverse voltage of some sort to really make "sure".

    Failure modes as imagined by me are

    1) SCR shorting and outputting full voltage into your now dead battery.
    Seems not too likely- SCRs are pretty damn tough.

    2) the gate circuitry getting botched up and the SCR NOT being shut off
    before the voltage hits the overvoltaged and killed battery zone. SCRs
    latch so if that's the case, your battery is basically being murdered 120
    times as second because the gate shutoff circuity is failing 120 times a
    second.

    The of course, maybe the thing is a GTO, but the idea is the same either
    way.
     
  11. You saw that sort of thing in the early days of SCRs. There seemed to be
    much more use of SCRs (or at least much more application for them) when
    they came out, and then that faded. Not sure if there were problems, or
    other things came along or what.

    Michael
     
  12. It's pretty sad lead acide battery chargers were the most sensitive
    devices out of that mix of hardware.

    Any interesting lightining strike stories?
     
  13. Very odd. Some sort of fatal design flaw with the regulation parts is what
    it sounded like.
    I just gave this a few minutes of though and drew a blank. How do you
    directly test to make sure nobody stole your ground?

    The only thing I could come up with was run a thin wire along your ground
    system in a loop and assume scrappers will steal that too. If that guard
    wire of whatever you'd call it opens or goes missing you've triggered an
    alarm?
     
  14. Leif Neland

    Leif Neland Guest

    Cydrome Leader formulerede spørgsmålet:
    Measure the resistance between the tower and a separate ground
    connection.

    Leif
     
  15. That still sounds tricky as there may be multiple grounds all connected to
    your ground busbar. I just took a peek at the telecom room on this floor
    and the plate on the wall had nearly a dozen connections, of which 3
    seemed to be going back to the electrical riser for the floor. You could
    remove most of those connections and everything would still be grounded,
    enough that nobody dies from a frayed power cord, but just not able to
    take a lightning strike or some major electrical contractor screw up.

    I'd love to see the AT&T manual not on how to sweep floors, but on
    grounding everything.
     
  16. Shaun

    Shaun Guest

    "Roger Blake" wrote in message

    Given the less-than-steller reviews and HF's reputation for electronics,
    I should not have bought this thing, a 12 Volt 6 or 2 amp (switch
    selectable) charger for car batteries. The problem is that instead of
    providing the tapering charge and switch to trickle it is supposedly
    designed for, this "charger" discharges and kills batteries instead. (In
    contrast, I have a 4 amp Schauer charger that is about 40 years old
    which still works just fine.)

    Instead of throwing out the HF unit, I was thinking that it has a perfectly
    usable case, transformer, ammeter, and wires with alligator clips. Just the
    crapulent Chinese electronics are bad so I'd like to trash that stuff
    and turn this into an "old school" type charger like my Schauer.

    The transformer secondary is center tapped and reads about 28 volts AC
    across the ends, 14 volts on each leg from the center. The original
    electronics are on a small board with 7 transistors, 2 SCRs, a couple
    of diodes, and numerous resistors. (That's a lot of stuff just to
    kill batteries!) Searching online I'm finding a bewildering array of
    home-brew battery charger circuits, everything from simply using a bridge
    rectifier on the transformer secondary to more complex circuits to taper
    the charge and either switch off or go to trickle when the battery is
    fully charged.

    The behaviour of my old Schauer is to gradually reduces current until
    the battery is fully charged, where it will stay at a low level and not
    hurt the battery if left on overnight or even for a few days. I have
    not opened it up yet to see what's inside, but being an early 1970s unit
    I'd be surprised if there is much aside from a transformer and rectifier.

    Anyone have a simple circuit handy that works well for this? I've seen
    a few plans online that just say to use a bridge rectifier of suitable
    capacity, but I want to be reasonably sure that this thing won't kill
    any more batteries.

    --
    Roger Blake
    Change "invalid" to "com" for email.
    Google Groups killfiled.

    Harbour Frieght products are crap! You get what you pay for! Why don't you
    just go out a buy a new one from a reputable store. The rest of the parts
    in your dead battery charger are probably crap too.
     
  17. "Michael A. Terrell" wrote in message

    WTH is 'Harbour Frieght'? Another of your drug induced nightmares?

    Harbor Freight is a chain selling cheap tools. Some of their stuff is good,
    and/or good value for the money.
     
  18. Shaun

    Shaun Guest

    "Michael A. Terrell" wrote in message


    WTH is 'Harbour Frieght'? Another of your drug induced nightmares?


    If your so fucking stupid that you can't figure that out, you shouldn't be
    posting. "Harbour" is the Canadian way of spelling that word, and Frieght
    is just a typo, nobody is perfect, we all make mistakes - look at your
    parents; they had you ASSHOLE. What do you get out of critiquing someone
    else's spelling? Does it make you feel superior to other people, do you
    need to do that for your low self esteem. You are a pathetic excuse for a
    Human being!
     
  19. Shaun

    Shaun Guest

    "Michael A. Terrell" wrote in message


    Your the one who is brain fucked. There is no US company named
    Harbour Frieght, so go **** yourself.

    Splitting hairs proves nothing. I think the content and general idea of a
    message to be important unlike some anal retentive prick like you that looks
    for problems.
     
  20. Roger Blake

    Roger Blake Guest

    Just as a followup here, for the moment I've solved my need for a
    second automotive battery charger by picking up another 35-40
    year old charger at a flea market. It works great! There's something
    to be said for simplicity.

    I'll play around with the Harbor Freight unit when I have a chance,
    there are a lot of possible circuits out there to play around with.
     
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