"Fixing" crap Harbor Freight battery charger

Some kind of design error might be a possibility, but why would charger from various vendors all have the same problem? I wish I had spent some time doing a post mortem, but I just wanted to get rid of them and replace them with something that works.

That's exactly what I was thinking and probably the best way.

I could also measure the ground resistance to another nearby ground rod, and trigger an alarm on any abrupt change in resistance. The trick will be to design it so that it will survive a lightning strike.

I vaguely recall this being discussed in an article in Above Ground Level magazine, but don't recall the issue or method.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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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.

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  Roger Blake 
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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.

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

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

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.

I am familiar with Harbor Freight. I have bought from them for over

30 years. I used to buy drill bits by the pound, by mail. They were all seconds, due to an uneven coloration but they were quite good. A cordless drill I bought there ten years ago ran for over seven years before the 9.6 volt battery died. I replaced it with a spare 18 V pack and still use it. I had one bad LED flashlight, and one 6in1 screwdriver in all those years.

There is a local store, and I am a member of their 'Inside track Club'. In all that time I've had less than 1% defective items which is much better than my record with Craftsman. I've had their lousy screw drivers break the first time I used them to remove the screws from a computer case.

The problem that I see is that most people don't know what the hell they are buying, or how to use it. A character I know bought their lightweight cement mixer. He burnt the motor up by loading too much mix & water into the drum. They replaced it twice, and he continued to overload the mixer. He says that he's an 'Idiot Savant'. :(

I have talk to a lot of tradespeople about HF tools and they tell me they rarely break one, but they have them disappear on job sites. They will buy several sets of wrenches, and hide a spare set, just in case. :)

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

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.

--
  Roger Blake  
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  Google Groups killfiled.

The following link is a rough schematic of the 12V 6A-2A / 6V charger:

wdPGT03vcmpefG

Please note: All resistor values are measured and approximate! Theory of Operation: The input transformer has some primary side switching to change the seconda ry voltage for 12V 2a, 6a and 6V. SCR's AC1 and AC2 are triggered on alternate half cycles of the ac input to charge the battery. Triggering is done by P1 turning on via N1 turning on, N2 and P2 turning off. The voltage divider of 23K and the parallel 9.4k re sistors sets VREF as a percentage of battery voltage. The TL431 has a nomin al reference level of 2.495 volts. Therefore, V_BATTERY must be > 2.495/(4.

7/(23+4.7)) or about 14.7 volts to turn on VR which disables charging by tu rning on P2 - N2 turning off N1,P1, AC1, AC2. P3 provides hysteresis to hol d VR conducting by raising VREF when VR turns on. P4 turns on when P1 is on and V_BATTERY exceeds the rectified AC from D1 D2 via P1. P4 on turns on N 3 which clamps VREF low turning VR off and continuing to enable triggering. The sensing process repeats on each AC half cycle. AC1 and AC2 will turn o ff at the end of their positive cycle if there is no gate drive from P1. Design Notes: V_BATTERY is required to power N1,N2,P2,VR,P3,N3. Without battery voltage, N1 will not conduct and P1 will not trigger AC1 and AC2 => No battery cha rging. The reference voltage divider is fixed for a 12 Volt lead acid battery. Hen ce the note on the charger stating that 6Volt battery charging must be manu ally terminated. N2, N3 have no base emitter bypass resistors. Collector - base leakage curr ent could turn them on. There is no protection for reversing the battery leads. It May be OK as is but I have not tried it. Possible failure modes: AC1 and/or AC2. would be likely candidates. BT151's are about $0.75 each in low quantities. AC1/AC2 triggering problems require chasing through the en tire PC board to see where the signal is lost.

Overall, the HF unit is well constructed for the price. I could envision sc enarios where it would fail. YMMV ;-)

AJG