Hi folks,

I opened up an old car battery charger the other day to see if I could replace the current meter which no longer works. I'm pretty sure I can, but of course now I've become curious as to how it works. Its a Chicago Electric Battery Charger 66783. There's lots of pictures of it around the internet, so I'll just show the front of it for now:



It's not antient; it has LEDs and transistors in it. I have a much older charger that has selenium rectifiers. (I've only just recently learned what those are.)

I've already learned quite a bit. It has a center tap rectifier that uses only two diodes. I didn't know that was a thing before. And switching between the different charging modes is done by a rather clever (in my humble opinion) sliding switch that changes the number of turns on the primary side of the transformer. I made up a picture of what I understand so far:



Unfortunately, when it comes to components with more that two legs, I'm pretty mystified as to how they work. I did some searching and found that someone else has drawn up a diagram of the circuit board. I've re-drawn it; hoping that it will be easier to read. I don't know if it was drawn correctly in the first place, or if I've recreated it accurately, but I spent quite a lot of time making sure I got it right.
Here is a link to the one I found originally: https://electronics.stackexchange.com/questions/268069/car-battery-charging-circuit

I think I can see a voltage divider in there, but really I don't understand how it all makes the charger behave the way it does. I'm pretty sure the SCR at the top has something to do with reverse polarity protection, and short circuit protection. I think it may also be involved in how the charger is able to tell if a battery is fully charged. Or at least is used somehow to shut off the current once the battery is full.

Anyway, I'm planning to replace the busted current meter with a digital one that also measures voltage. I plan to hook it up so that it uses the battery power to operate. That way I can see what the battery voltage is even with the charger off or when the charge cycle is complete.

While I'm in there, I'd also like to add a power switch on the AC side, as well as a fuse. On the DC side there is a thermal breaker just before the output, but nothing I can see that would protect the AC side. I've read on several occasions that the charger is supposed to be unplugged whenever connecting or disconnecting a battery. (Apparently it's to protect the charger's delicate innards.) I'm fine with unplugging it when I put it away, but it's really annoying to have to unplug it every time I want to change batteries or do a load test or something.

As far as I know, these changes shouldn't affect the charger. Unless anyone sees a reason that I shouldn't do it?

 

The transformer uses a center tap, so that the ground stays at Zero whilst the ends alternate each positive half of the AC waveform put through one diode per half cycle. very common in most old battery chargers. Yes you can put a digital ammeter/ voltmeter on , it will need its own supply .

 

I tried recreating the schematic in LTspice. I got the center tap rectifier to work, but once I added all the transistors, resistors, and SCRs I get an error message when I try to run:



I gather it has something to do with the first SCR. The nodes it mentioned are all attached to the SCR. I guess the SCR needs some kind of reference info? Also, I don't know what xu1 means. Also, if the first SCR needs something extra, why is the program not upset about the second SCR? Does it just stop at the first error it sees? Here is a picture of the whole schematic:



This isn't critical or anything. I'm just really curious. I understand there is a sweep function I might use to see how the circuit reacts to different battery voltages. That sounds cool. Also, I don't know how to add switches or breakers yet, so they're missing. I get the same error message regardless of whether the SCR is bypassed or not.

 

I tried recreating the schematic in LTspice.

Post the .asc file

 

I don't think there is one. Here is the attribute window I get when I right-click it:



I have nearly 4,000 ASC files in my computer but none are named anything remotely close to SCR. Or thyristor. I've read that they are the same thing.

 

I meant the .asc file of your schematic.

 

Oh.

For some reason I thought they were just for individual components. LTspice has been quite the learning curve so far.

 

The current meter does not work ? Or does not show current? Or there is no current ? Figure that out first. !!! Did you try with a decent battery instead ???

Did you read the charger instructions ? Most chargers have sensing circuitry to enable charging only when the battery is discharged with still some voltage but not dead. A battery dumped to a back yard last year will not be accepted by the charger, will not recover, will not charge, will show no Amperes, and is too much as paper weight or door stop.
It is the main reason of battery charger returns to stores by people that think a plastic heavy box with some weeds around can be resurrected by a shiny new modern just bought charger. In response to that, the battery chargers industry adopted an enabling circuitry to protect the charger from abuse.
Come back with findings.

 

Yes, I did read the instructions. The rest of the charger works fine. I put a clamp meter on the output and it showed current. The battery I have is pretty much fully charged, so the green "finished" LED lights up pretty quick. The SCR bypass switch works as expected.

I pulled out the circuit board today so I could identify the various components. Turns out a lot of the symbols available in LTspice have no spice model to go with them. I finally found a model that I think is close enough for the SCR. The existing SCR is an S6055K, and the closest I could find is an S6055W. I think the letters are to identify differences in the lead shape, but the internal characteristics are the same.

All the pnp transistors were listed in the program, but the npn transistors were not. I was unable to find a spice model for 2N5551, so I used 2N5550. I hope that's close enough. The component I thought was a second SCR turned out to be TL431A, which I think is a voltage reference. I couldn't find any thing close to it on the internet or in LTspice so I picked LT1009 because it looked similar. I have no idea if that part is close or not. At least it's a voltage reference. Both are listed as "adjustable". I think that means they behave differently depending on the resistor that's connected to it. That must be what that bank of parallel resistors is for.

I at least have it so the simulator will run without errors now. I was hoping to see the green LED turn on and off depending on the battery voltage. So far that's not the case. I'm either doing the DC sweep wrong, or I don't have close enough components, or I've drawn the circuit wrong... Fortunately, the traces are all on one layer of the circuit board. I'll try looking at those to see if I've got all the wires in the right place.

Attached is the updated schematic file.

 

I would crack open the non-working amperimeter and fix it if stuck. Once I found a hyperfine portion of a dB meter winding wire tangled in the movable mechanism as factory flaw and worked fine after removed. Your 'high' current meter should be more sturdy to survive surgery. Use magnifier and good light.

 

I was hoping to see the green LED turn on and off depending on the battery voltage

Your LEDs are upside-down so will never light.
The LEDs light when they are forward biased, not reverse biased.

 

Hmm... The second LED is supposed to be reversed because it lights up to indicate reverse polarity if the battery is hooked up wrong. The first LED is either off all the time, or on all the time depending on which way it's oriented. It's only supposed to come on when the battery reaches a certain voltage.

Since the circuit board is out, I decided to pull all the other parts out and clean stuff up a little better. The rivets got replaced with nuts and bolts. The Volt/Amp selector switch got taken apart and cleaned up. The transformer got cleaned up and painted. The rectifier was really loose on its plastic insulator tabs. That got fixed. I installed the AC power switch... on the wrong side.



I was out in my shed today looking for future projects and found this guy:



I don't know if it works or not, but It may be next.

 

Took a break for a while to work on other projects. Today I used the heat gun to remove the old adhesive from the front sticker.




The sticker was more like a thin sheet of plastic, so it worked pretty well. I used spray adhesive to stick it back on. Unfortunately, it shrunk from the heat so the switches don't line up with the print anymore. I also fit the new Volt/Amp meter. With a little filing, it snapped right into place in the old meter housing. I used a bit of hot glue to ensure it won't pop back out.




The AC side of things is about done. The new switch and fuse are wired up, and some of the old wires got some shrink wrap. I didn't like how many of the original connectors were exposed and very close to other conductors.


I noticed the plastic strain relief on the AC cable has worn through the outer jacket of the cable. I plan to fix that later. It's very close to being done though.

 

Finished all the wiring today. Despite my best efforts, it's still not broken. In fact, the Volt/Amp meter works perfectly, and seems accurate. I wired it so it will run off of whatever battery is connected even if the charger itself is off or unplugged. I even got the wire direction through the CT sensor right the first time.

I still don't understand how the electronics do what they do. Especially that the voltage goes up and the ripples disappear when a battery is connected. All without a single capacitor. But with the thyristor on, and having it set on 2A, it looks like it could be a battery maintainer and not just a charger. And now I understand the Charge/Start switch, this unit might also function as a quick-n-dirty 12V power supply. Probably better to only do that with a battery attached though.

Anyway, just some cosmetic stuff for the cover and it will be done.

 

I cleaned up the cover and gave it some new paint. I also replaced the battery clips.




Since the fuse and power switch are in the back, I added some labels on the top to remind me where they are. A bit of black paint covered the old lines on the front cover, and I used a gold pen to re-draw the lines in their new location (since I accidentally shrank the sticker.)




I still don't know why it works, but I guess I'll give up on that for now. The important thing is that it works, and is a bit safer and more convenient than it was. And it looks a lot better too. I'm pretty happy with it. Though I wish I hadn't messed up the front so bad. But hey, it's a battery charger, not the space shuttle, LOL.

 

Hi folks,

I opened up an old car battery charger the other day to see if I could replace the current meter which no longer works. I'm pretty sure I can, but of course now I've become curious as to how it works. Its a Chicago Electric Battery Charger 66783. There's lots of pictures of it around the internet, so I'll just show the front of it for now:
View attachment 315547


It's not antient; it has LEDs and transistors in it. I have a much older charger that has selenium rectifiers. (I've only just recently learned what those are.)

I've already learned quite a bit. It has a center tap rectifier that uses only two diodes. I didn't know that was a thing before. And switching between the different charging modes is done by a rather clever (in my humble opinion) sliding switch that changes the number of turns on the primary side of the transformer. I made up a picture of what I understand so far:
View attachment 315546


Unfortunately, when it comes to components with more that two legs, I'm pretty mystified as to how they work. I did some searching and found that someone else has drawn up a diagram of the circuit board. I've re-drawn it; hoping that it will be easier to read. I don't know if it was drawn correctly in the first place, or if I've recreated it accurately, but I spent quite a lot of time making sure I got it right.
View attachment 315548Here is a link to the one I found originally: https://electronics.stackexchange.com/questions/268069/car-battery-charging-circuit

I think I can see a voltage divider in there, but really I don't understand how it all makes the charger behave the way it does. I'm pretty sure the SCR at the top has something to do with reverse polarity protection, and short circuit protection. I think it may also be involved in how the charger is able to tell if a battery is fully charged. Or at least is used somehow to shut off the current once the battery is full.

Anyway, I'm planning to replace the busted current meter with a digital one that also measures voltage. I plan to hook it up so that it uses the battery power to operate. That way I can see what the battery voltage is even with the charger off or when the charge cycle is complete.

While I'm in there, I'd also like to add a power switch on the AC side, as well as a fuse. On the DC side there is a thermal breaker just before the output, but nothing I can see that would protect the AC side. I've read on several occasions that the charger is supposed to be unplugged whenever connecting or disconnecting a battery. (Apparently it's to protect the charger's delicate innards.) I'm fine with unplugging it when I put it away, but it's really annoying to have to unplug it every time I want to change batteries or do a load test or something.

As far as I know, these changes shouldn't affect the charger. Unless anyone sees a reason that I shouldn't do it?

The ammeter is far more useful on a battery charger because it tells what is happening. A voltmeter will only tell you how dead the battery is before you start. A power switch for the incoming power is a good idea, along with a power-on indicator lite.

 

I thought about just installing an Amp meter, but when I tried one out I found myself immediately going for a volt meter to see the before and after settling. So I figured it made sense just to get a Volt/Amp meter since it's the same cost and physical size.

I had a chance to use it a couple days ago. I have an old motorcycle that doesn't like to start after months of storage. I had just fixed an electrical issue that kept me from riding. The starter battery was full, but needed some help. I set the charger to start and 55A. The bike only drew 30A and was able to turn over quickly for several seconds and then came to life. The starter battery is able to start it fine now that it's run for a bit. And the charger did a great job.

With the Volt meter I could have monitored the voltage drop while the starter motor was running, but the bike started before I thought to check. Someday I'll get a better starter motor for that bike. The stock one was rather poorly designed and needs more power that it should to turn the engine over. It got redesigned a few years later. I'm just stubbornly hanging on to the crappy starter because I don't want to spend the money, LOL.

 

No Kick Starter??? That was always the recourse for a bike where the battery was only just enough to power the spark ignition system once the generator took over. (Before alternators became common on motorcycles.)
Two slow spins to get gas into the engine then one quick stomp to start it.

 

What I've got is a 2014 Ural M70. It has a kickstart lever, but for this year and model, very few bikes actually start with the kick start. Mine is one of the ones where the kickstart is pretty much decorative.

 

OK, Now it is clear. I recall seeing a 120 pound kid trying to kickstart an older HD'74 bike. That was entertaining. And not successful.
OK, now I understand. Not all starter motors are the same, and I recall seeing at least one that the generator was used as the starter. A rather low compression engine, poor performance but easy starting.
It might be that in addition to whatever starter motor issues there are, that there may also be an excess voltage drop in some of the wiring. I have come across that a couple of times.. A voltmeter connected between the motor terminal and the battery terminal will show that problem. It might even be on the "ground return" side of the motor connection. It is not hared to check and it might reveal a problem that was not obvious.