Charging a car battery with 14V AC?

Hi everyone!

Any reason why AC can't be used to charge a car battery? Reason I'm asking is that I have an old Fedtro Powerhouse charger which is outputting AC.

Reply to
Gary
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More likely, the charger puts out rectified but unfiltered DC.

Put a 100mf 30 volt capacitor on the output of the charger (watch polarity) then measure the DC voltage.

If there is no DC voltage across the capacitor, then the rectifier(s) in the charger are toast.

Reply to
news

OK, except I can't see anything inside the charger that looks like a rectifier: aside from the main transformer, there is just a small coil of wire (the buzzer?) and a 6V/12V switch.

Reply to
Gary

Depending on the age of the charger, it could have a selenium rectifier (looks like a stack of square or round metal plates wih washers between them) or it could use solid state diode(s). Depending on the amp rating of the charger, a solid state diode(usually silicon) coud be stud mounted or it could look like a "blob" on a wire.

When the selenium rectifier in an old adjustable EICO 6/12 volt battery eliminator/charger (that I've had for years) finally died, I replaced it with a silicon bridge rectifier on a heat sink. The maximum output voltage is now higher, thanks to the differece in voltage drop across a silicon diode (0.6 volt reardless of load) versus the several volt drop (that increases with load) across the original selenium rectifier.

You need DC (even if it's unfiltered) to charge a battery - AC won't work.

Reply to
news

OK, I will try as you suggest. You said something about watching the polarity across the output of the charger - what exactly did you mean?

Reply to
Gary

No, not if you don't mind the battery exploding and spraying hot sulfuric acid in your face.

Reply to
William Sommerwerck

The plus output of the charger goes to the plus terminal of the battery.

I urge you NOT to do this. There is no guarantee this charger will correctly charge your battery. Find someone knowledgeable to help you. Or pay a service shop to charge the battery. Or buy a new, working charger.

Reply to
William Sommerwerck

How do you know it's putting out AC?

For an AC input, a typical voltmeter will show the voltage on an AC scale, but about zero on a DC scale.

Rectified, but unfiltered DC will show significant readings on both AC and DC scales.

This is a risky experiment. OP says he thinks it's putting out AC. If that's an accurate statement, like due to a shorted diode, be prepared for an exploded cap.

I tried to come up with an answer to the original question, "why AC can't be used to charge a car battery?"

I couldn't think of an answer that would not require a complete explanation of the difference between AC and DC. Closest thing I could think of was, "AC goes negative every cycle and batteries can't."

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Reply to
mike

Think: If you attempted to charge a battery with AC, the charge deposited when the current swung positive would be discharged when the current swung positive. All you would be doing is heating the battery, not charging. Take out your ohmmeter and look to see if there's a diode between any two points.

Also, look for modifications. When I was a sprout, we had a battery charger purported to rejuvenate the carbon zinc cells of the day. The light went out, and the charger quit charging. Not knowing any better, to make it work, I jumpered the light, which apparently acted as the charge current regulator. At which point the diode went "Sproing." Somebody might have taken your charger's diode out in the belief it would improve things.

Reply to
spamtrap1888

yeah sure, put the battery on a turn table and electrodes off the side and the spin the table at 50/60hz, what ever your country system is.

Have this magical little charger of yours make contact on the electrodes at the appropriate time. :)

or, one could simply get a bridge rectifier and maybe a resistor (ballast) for current limiting.

Jamie

Reply to
Jamie

That's what I'm getting.

I knew there was a good reason.

Reply to
Gary

If this thing is ac, you need a rectifier. Need more info. You also might need current limiter.

I'm reminded of some old chargers we had in the army, in the battery room. Big box with switched levels of pulsating voltage to control charge rate. I think the transformer had multiple taps. Charged and cycled batteries about the size of motorcycle batteries. These were for RCAT's , remote controlled aircraft targets.

Greg

Reply to
gregz

I agree ...why not just buy one? A real charger is not that expensive:

Reply to
Sofa Slug

I'm aware of that. I was asking about how the polarity of the AC output affects how the capacitor should be placed?

You're missing the point. Sometimes we play around with old, obsolete kit, just to see if it'll work.

Reply to
Gary

AC has no polarity.

Reply to
William Sommerwerck

gregz wrote in news: snipped-for-privacy@news.eternal-september.o rg:

you want to regulate the charge voltage,too. go too high and you boil your electrolyte away,ruin your battery.

--
Jim Yanik
jyanik
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Reply to
Jim Yanik

snipped-for-privacy@news.eternal-september.o

The car's electrical system is designed around a standard of 13.8 V, I was always told. That's what one should shoot for, I would think.

Reply to
spamtrap1888

We used to have regulated supplies on some battery backup gear on NASA equipment. We adjusted by the bubble method. You were supposed to cycle them once in a while .

Greg

Reply to
gregz

That's true, but any electronic item must be designed to live at 16 volts. I remember the specs on my datsun mechanical regulator, got up over 15 volts. 13.8 is maximum long term use, but you need higher voltage to fully charge or overcharge to clear contamination.

Greg

Reply to
gregz

Exactly my point. So does it matter how the capacitor is placed across the output leads?

Reply to
Gary

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