Battery charging question

Firstly, you need 12 Volts DC, not 120VDC. Why don't you use a transformer, your bridge rectifier and filter cap and an LM7812 thus creating a 12VDC supply. Connect this to the charger.

First off, you need to stop trying to kill yourself. There are easier ways that won't surprise you as much as what you are currently doing.

You need to drop all that variac crap and to one of the following :

1) Get a spare car battery and use it as a power source, charging as needed. 2) Get one of the 13.8VDC power supplies designed to do what you are doing. A little more information about the battery charger would be needed to determine what kind of current you will need.

Jim

Not forgetting to use a power transistor to boost the current capability of the poor old LM7812.

--
I'm kinda confused here.  It sounds like you're saying that your
charger takes 12V from a car battery and then steps it up to
whatever's needed to charge other batteries. 

If that's true and you're charging the car battery with rectified AC,
you shouldn't be able to see any ripple _at all_ on the output of the
bridge, regardless of whether you have a capacitor across it or not.
Matter of fact, you don't even need the cap, since the battery will
smooth the voltage at the outout of the bridge better than any cap
could.

So, if you're measuring any ripple at all, I suspect a high-resistance
connection between the VARIAC and the bridge or between the bridge and
the battery.  Also, miswiring the bridge could do the same thing, and
remember that a VARIAC is an autotransformer, so if you've got the
output of the charger connected to mains neutral for some reason,
OUCH!!!

John, don't feel pregnant. I can't figure out what the OP is trying to accomplish either. I'm guessing that he got his hands on a variac, and is trying to use it as a 12VDC supply which would emulate a car 12V supply.

When he says "a cap", that could mean anything. Steamer, you need a big bank of big honking capacitors to make a supply like this have a smooth output.

The charger tripping out, and the output of the variac/diode not being smooth indicates that you have nowhere near enough filter capacitance.

Do you know how to read capacitance values? For what it sounds like you're trying to do, you need about 10,000 uF of capacitance - that's about the size of a Foster's beer can.

Do you have a proper multimeter? Or preferably two? (or access to a second one - I notice you got your hands on a scope.)

And, if your "charger" is designed to plug into a lighter socket, you'd probably be better off setting your supply voltage to something like

13.5-14 volts, where they typically sit while the engine is running and it's on float charge. I'd say a car battery that is showing a terminal voltage of 12V is sorely in need of a proper charge.

Hope This Helps! Rich

You don't really need "smooth DC" to charge a lead acid battery. The pulsating voltage from the rectifier is actually preferable although most people don't realize it. Battery manufacturers are probably the least well informed in the art of battery charging. And since most electrical engineers are stupid and believe anything they read are were taught, they continue to revert to ancient concepts regarding battery charging just because some moron was able to write a text book with misinformation in it. And by the way, a fully charged lead acid battery in prime condition is fully charged when it reaches 12.6 to 12.8 volts. Anything above that is "surface charge" and is worthless. Once a battery is loaded with "load tester" the voltage drops immediately to about 11.5 volts and remains there if the battery has not suffered too much from sulfation or grid corrosion. OK you dorks and PE's who don't agree, fire away!

--OK here's the problem: I've added a bridge to convert the output of my variac to DC and then I used a cap to smooth it a bit, but according to the oscilloscope the output is anything but "smooth". Anyway I'm just trying to use this to juice my battery charger, which is designed to run off of a car battery, but it turns out that the variac trick ain't working because the charger keeps tripping out; i.e. if I try charging a 12v battery it takes *days* to get it up to snuff; I talked to the guy at the local hobby shop and he said this is a symptom of insufficient power being sent to the charger. I get better results using a 15v wallwart. --What I need is to be able to charge a couple of different batteries, including motorcycle batteries and my 3 remaining 24-v packs of NiMH cells. What would be nice is a benchtop power supply that puts out 10 to 20 amps of smooth DC, but I haven't been able to find one. --Any suggestions appreciated..

--
        "Steamboat Ed" Haas         :  The IRS giveth refunds and 
        Hacking the Trailing Edge!  :  Blue Cross takes them away...
                   http://www.nmpproducts.com/intro.htm
                   ---Decks a-wash in a sea of words---

--
Hey, I'm all ears...  _Why_ is it preferable?

charged

charge"

has

And do any of your "professional" circuit designs really work?

--- In an effort try to see where you're coming from, I just went back and took a look at your posting history and guess what? The kindest thing I can say is that you're a f****ng idiot.

From that first "Ohm's Law" abortion to your last pitiful attempt at an insult, above, about the only thing you've done right is learned to bottom post. So, there may be hope for you yet. We'll see...

-- John Fields Professional Circuit Designer

Rich Grise wrote: : John, don't feel pregnant. I can't figure out what the OP is trying to : accomplish either. I'm guessing that he got his hands on a variac, and is : trying to use it as a 12VDC supply which would emulate a car 12V supply. --Yes, that's it.

: And, if your "charger" is designed to plug into a lighter socket, you'd : probably be better off setting your supply voltage to something like : 13.5-14 volts, where they typically sit while the engine is running --Yes, I've done this.

--
        "Steamboat Ed" Haas         :  The IRS giveth refunds and 
        Hacking the Trailing Edge!  :  Blue Cross takes them away...
                   http://www.nmpproducts.com/intro.htm
                   ---Decks a-wash in a sea of words---

--Meanwhile back at the ranch *I'm* still looking for better ways to do what I'm trying to do..

-- "Steamboat Ed" Haas : The IRS giveth refunds and Hacking the Trailing Edge! : Blue Cross takes them away...

---Decks a-wash in a sea of words---

A TOP POST. . . Now now now, Johnny boy don't get yourself in a dither.

IMO, that's largely because your requirement was vaguely specified in your original post. It wasn't until 4 days later that you became more specific, with: "What I need is to be able to charge a couple of different batteries, including motorcycle batteries and my 3 remaining 24-v packs of NiMH cells. What would be nice is a benchtop power supply that puts out 10 to 20 amps of smooth DC, but I haven't been able to find one."

Doesn't that imply that, smoothed or pulsed, a 12V ('120V'!) output from your Variac will be inadequate? I'm guessing that you really want a heavy duty DC supply capable of delivering about 26V, preferably switchable to say 15V for charging your motorcycle batteries if they are 12V (you still haven't specified that).

--
Terry Pinnell
Hobbyist, West Sussex, UK

There's nothing wrong with that method, but you neeed to monitor a couple of things; the voltage across the battery when it's charging and the current going into it while it's charging, like this:

VARIAC FWB +--------+ +-----+ ACHOT>---O--|C|

with what you

--- P.S. After making sure that mains NEUTRAL is really NEUTRAL, and before you plug in the VARIAC, you need to make sure that the rig is connected to the VARIAC _EXACTLY_ as shown, that the VARIAC's output is set to 0V, that the Variac's ON-OFF switch is set to OFF, and that the switch on the rig is set to measure current. ( That it's in the "I" position). Once that's done, connect the battery to the rig, plug in the VARIAC, and if there's no smoke and the lights are still on, place the VARIAC's ON-OFF switch to the ON position. If there's still no smoke and the lights are still on, SLOWLY rotate the VARIAC's output control while watching the voltmeter which is displaying current. If it starts to rise, then things are probably working! When you get to whatever the initial charging current is, the VARIAC's knob should be somewhere between 10 and 15 volts. Now, change the range on the voltmeter to read the battery voltage and flick the switch on the rig to the "V" position. You should be reading somewhere between 10 and 13V if the battery was fully discharged when you put it on the rig. If it is, then just monitor the battery voltage (checking current occasionally) until the battery's fully charged and you're done.

There's a little more detail to it than that, depending on whether you want to let the batteries float after they've been charged, but the best place to get that information is from the battery manufacturer's website, since they all use slightly different chemistries and their terminal voltages and currents are all a little different.

Once you've finished charging the battery, turn the VARIACS's knob down to 0V, turn it OFF, unplug it, and then (and only then) disconnect the battery from the rig.

Good luck.

-- John Fields Professional Circuit Designer

Terry Pinnell wrote: : IMO, that's largely because your requirement was vaguely specified in : your original post. It wasn't until 4 days later that you became more --Yeah, well, I'm still workin' on the "basics"...

: Doesn't that imply that, smoothed or pulsed, a 12V ('120V'!) output : from your Variac will be inadequate? I'm guessing that you really want : a heavy duty DC supply capable of delivering about 26V, preferably : switchable to say 15V for charging your motorcycle batteries if they : are 12V (you still haven't specified that). --Yes. I've been turning the dial on the Variac until the output, as measured on a voltmeter, has been around 14 to 15v, then I've hooked that up to the battery I'm trying to charge. I've *never* cranked the dial past this, for fear of damaging the battery, the house, myself, etc.

--
        "Steamboat Ed" Haas         :  I still want to travel in  
        Hacking the Trailing Edge!  :  zeppelins and flying cars...
                   http://www.nmpproducts.com/intro.htm
                   ---Decks a-wash in a sea of words---

--Thanks very much; that's going to help me a lot! :-)

-- "Steamboat Ed" Haas : I still want to travel in Hacking the Trailing Edge! : zeppelins and flying cars...

---Decks a-wash in a sea of words---