"Fixing" crap Harbor Freight battery charger

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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
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Re: "Fixing" crap Harbor Freight battery charger
On 08/05/2013 08:05 AM, Roger Blake wrote:
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Old chargers used selenium stacks, which tapered naturally.

Re: "Fixing" crap Harbor Freight battery charger
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Re: "Fixing" crap Harbor Freight battery charger
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nice.


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.

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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?

Re: "Fixing" crap Harbor Freight battery charger
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12 volt lead-acid car batteries, I should have mentioned that if I didn't!

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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...

--  
  Roger Blake
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Re: "Fixing" crap Harbor Freight battery charger
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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).



Re: "Fixing" crap Harbor Freight battery charger
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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.

--  
  Roger Blake
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Re: "Fixing" crap Harbor Freight battery charger
On 08/07/2013 10:07 AM, Cydrome Leader wrote:
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Physicists tell me that SCRs will work as transistors.

Re: "Fixing" crap Harbor Freight battery charger
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that's nice.

Re: "Fixing" crap Harbor Freight battery charger
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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.


Re: "Fixing" crap Harbor Freight battery charger
On Fri, 9 Aug 2013, Cydrome Leader wrote:

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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


Re: "Fixing" crap Harbor Freight battery charger

Roger Blake wrote:
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   Why would anyone use phony 1% resistors when they are dirt cheap
these days?  they are pennies each in leaded, and less for surface mount
in manufacturing quantities. A lot of OEMs stopped stocking 5% SMD
parts, because the 1% were cheaper. We did that at Microdyne, starting
in the late '90s.

   Cheaped out on rectifiers?  They likely paid more for that CT
transformer that they would save on two 30 cent diodes. The fact that
there are no ICs is significant in what way?

--  
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.

Re: "Fixing" crap Harbor Freight battery charger
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The Chinese are infamous for cheating on specifications. Whether these
are actually phony or not would obviously require measuring them with an
accurate meter.

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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.

--  
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Re: "Fixing" crap Harbor Freight battery charger
On Mon, 5 Aug 2013 15:05:05 +0000 (UTC), Roger Blake

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Incidentally:
<http://802.11junk.com/jeffl/pics/drivel/slides/dead-battery-chargers.html
All of them died float charging standby and backup batteries on
mountain top radio sites.  Mostly, the electronics on the PCB's (with
the numbers ground off) were what failed.  I managed to fix one or two
by guessing the regulator IC that was used, but they again failed in
short order.  The chargers were replaced by various Statpower/Xantrex
chargers, which have been working quite nicely for the last 7 years.

--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: "Fixing" crap Harbor Freight battery charger
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were these hunk-o-junk units plugged into those hardwired site wide surge  
surpessors?



Re: "Fixing" crap Harbor Freight battery charger
On Fri, 9 Aug 2013 21:00:44 +0000 (UTC), Cydrome Leader

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Sorta.  All the sites were on some form of backup or emergency power,
such as an autostart generator.  The switching transitions are
anything but graceful, with no provisions for zero voltage switching.
The resultant glitches, spikes, surges, and over-voltage hiccups could
easily have blown up the chargers.  However, all the sites also have
big MOV surge protectors and LC line filters just after the circuit
breakers, which does a fine job of protecting against glitches and
such.  There was plenty of other equipment in the buildings, including
commodity networking hardware, scanners, computahs, weather station,
etc all of which are somewhat sensitive to glitches.  Nothing failed
except the battery chargers.


--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: "Fixing" crap Harbor Freight battery charger
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It's pretty sad lead acide battery chargers were the most sensitive  
devices out of that mix of hardware.

Any interesting lightining strike stories?

Re: "Fixing" crap Harbor Freight battery charger
On Mon, 12 Aug 2013 16:37:05 +0000 (UTC), Cydrome Leader

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I still don't know exactly what killed all the chargers.  I would
expect such high power devices to be built rather rugged, but
apparently, that's not the case.

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Nope.  I live on the left coast, where we don't have much lightning.
I've had some nearby hits blow up various antennas, radios, and
assorted boxes, but nothing spectacular.  We've eliminated most of
those type of problems with careful grounding, lighting arrestors, and
ritual sacrifice to the lightning god.  Lighting is one problem that I
don't have to deal with (much).

The only interesting horror story I've heard was from another service
company, where copper thieves axed the big ground wires from the tower
legs to the ground rods.  When lightning hit the tower, it went into
the building instead of directly into the ground.  I didn't see the
carnage, but was told it was huge.  I'm considering adding a ground
wire integrity test to the SCADA alarm panel.

--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: "Fixing" crap Harbor Freight battery charger
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Very odd. Some sort of fatal design flaw with the regulation parts is what  
it sounded like.

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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?




Re: "Fixing" crap Harbor Freight battery charger
Cydrome Leader formulerede spørgsmålet:

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Measure the resistance between the tower and a separate ground  
connection.

Leif

--  
Husk kørelys bagpå, hvis din bilfabrikant har taget den idiotiske  
beslutning at undlade det.



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