"Fixing" crap Harbor Freight battery charger

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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  Roger Blake 
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Roger Blake
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Old chargers used selenium stacks, which tapered naturally.

Reply to
dave

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Reply to
Kalyan Ram

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.

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?

Reply to
Cydrome Leader

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...

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  Roger Blake 
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Roger Blake

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).

Reply to
Cydrome Leader

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.
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  Roger Blake 
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Roger Blake

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?

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Reply to
Michael A. Terrell

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.

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  Roger Blake 
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Roger Blake

Physicists tell me that SCRs will work as transistors.

Reply to
dave

Incidentally: 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.

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

that's nice.

Reply to
Cydrome Leader

were these hunk-o-junk units plugged into those hardwired site wide surge surpessors?

Reply to
Cydrome Leader

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.

Reply to
Cydrome Leader

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.

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

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

Reply to
Michael Black

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

Any interesting lightining strike stories?

Reply to
Cydrome Leader

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.

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.

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Jeff Liebermann     jeffl@cruzio.com 
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Jeff Liebermann

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?

Reply to
Cydrome Leader

Cydrome Leader formulerede spørgsmålet:

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

Leif

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Leif Neland

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