Battery charger only puts out 11.5 volts

Are you sure it's the transformer? What does the secondary read when disconnected from the rest of the charging circuit?

Rectifier bridge(a module or 4 diodes pack) can have a leg open or short.

No, it was just a thought. The alligator clips were all corroded, so I used a wire brush on a bench grinder and cleaned them off well, incuding the edge of the jaws. I don't think they could be the problem.

No way to open the thing without bending a bunch of little metal tabs. I'm sure they'll break so I want to avoid that if I can.

I have another 15 amp charger, but I don't think I've needed it for 2 years. No a friend brought a battery over summer of 2014, so it's got to be around here somewhere.

For some reason I started another thread and apparently the voltage has gone up, 12.5 on ten amps, 11.5 on two amps.

Open circuit voltage is meaningless

Except the voltage is very unlikely to be higher into a load - like charging a battery. (assuming it is not a "polarity protected" charger)

The old car-battery charges I've opened up were all rather simple. I'd go and break the tabs, and see what's the various voltages inside.

Worst case, you've had a learning moment, and go buy another charger for about $30.

- . Christopher A. Young learn more about Jesus .

First thing to do is determine if it's broke. If you have a typical meter, it likely has a current function that can measure an amp. If not, put a 12V light bulb in series. It should be off when the charger is unplugged from the wall and glow somewhat when the charger is plugged into the wall...depending on the design of the unspecified random charger and the bulb.

The output may not be filtered and therefore a DC reading Meter might not read the actual peak voltage. Use an o scope or connect it to an actual battery and see If the battery voltage increases.

Indeed. Many battery charger use "pulsed DC" which can confuse lots of meters...

This charger is nothing fancy, though you inspire me to see if I can find it online and learn something about it. Didnt' take the time today to test it on my battery, or a lightbulb like was suggested.

I mentioned that a friend was over this summer to get his battery charged, but now I'm wondering if I actually charged it. I dug out my hydrometer, and I can't remember details but I got some strange readings. Why would I have dug it out if his voltage hadn't gone up? I had the thing for two days.

I'll bet this charger has been broken since I got it. I assumed it worked because the Freecycle guy who gave it to me said it did. But maybe he said I think it does.

You could also put a capacitor on output to read peak. The charger might not be working as new, but may still charge.

Greg

No need for it to be fancy. In fact, the cheapest ones might very well be just a transformer to reduce line voltage to a nominal 13 VDC (but see below)... along with a diode to (mostly) just keep the current going "forward", so to speak, into the battery.

(or go up one step to a diode bridge...)

In any event, thse will be outputting a sinusidal wave form, just at 13V instead of 120. However, that means that part of the time it'll be at 18 or so volts, other times it'll be under 12 and dropping to zero...

OK - I'll eat some crow. You can even leave the feathers on.

I did some real world testing on a couple of chargers I have sitting around that work. One of them has a center tapped secondary with 2 plate rectifiers. It is an old non-automatic charger with 6 and 12 volt, 2, 15 and 100 amp boost positions. With it not connected to anything it reads 10.2 volts on my high impedence digital voltmeter. If I put any ac load on it, including a capacitor, it jumps to 14.6 volts and on the capacitor (7800ufd 75 volt electrolytic) it behaves the same as on a fully charged batteery, cycling between 12.2 and 14.6 volts.

On my bridge rectified charger it is 14.6 open circuit, and on my automatic it is zero volts until connected to a voltage source = either battery or charged capacitor (artly or fully - reading over about 5.5 volts) it outputs the voltage required to force a charge into the battery.

The dumb chager limits itself to about 14.4 volts and the smart ones shut down at 14.6, then come back on, alternating between 12.2 (or whatever the battery voltage is) and 14.6 (in other words, not charging, then charging at full voltage/little or no current)

SO - a center tapped transformer with 2 plate rectifies (I believe they are metal oxide - not selenium, but not silicon)- it IS possible for a working battery charger to put out less than expected voltage into an open circuit. Interestingly there is a resistor between the common and one stud on the plate rectifier, which might have something to do with the unexpected voltage.

In the case ofmine - significantly below nominal battery voltage.

snipped-for-privacy@snyder.on.ca posted for all of us...

You realize you just blew all the atta-boys for the year?

I think I was wrong about this. I assumed it worked, but I just looked for and found the Freecycle ad, for 20 months ago, and it says

"This is an AC powered battery charger for charging vehicle or boat batteries. I measured 12V DC on it but you will need to clean up the clamps a bit."

"BUT", like the first part wasn't a problem. I jumped to the conclusion that meant it was good. Usually, if something fails it has less than 12 volts, but today I measured my 1 amp charger that I got 50 (50) years ago and it has over 16 volts with no load.

This bad one will light the scooter's tail light, or its brake light, but I dont' think it will charge the battery. Measured voltage with a Kelvin meter, and it was 0.1v lower, and you know that's a good brand because Kelvin was a great man. Discovered warm weather, iirc. They still sell this, Century 87102C Battery Charger, $74 including shipping at Amazon, Lowes, and HD, but only $58.50 inc. shipping at the manufacturer's site, but it's not smart at all. The smart ones are pretty darn expensive.

I remember the guy. He had a messy house, loads of stuff to get rid of, so he could move, I think, but nothing else I wanted.

Sorry for stirring up the tussle between two of you. Glad you made up.

I'll try to fix it soon.

I just measured the voltage on one of my battery chargers. Using my analog meter, it shows slightly less that 12V with no load. It charges batteries just fine!

Hmm. Is it a smart charger in any way?

I could/should/will connect it to my car. 43^ and rain Monday and rain for 2 more days

NO, just a basic OLD 10A charger. Probably from the 1970s.

I bet yours will charge just fine!!!!!

Maybe you're right. If it doesn't, I found the email from Freecycle where it was offered, and even though it's 20 months later, I'm going to write the guy and ask him if it worked. Maybe he'll remember. I'm grateful for a free charger either way, and I'll tell him that. One advantage of email, if he was moving, his email should stay the same.

OTOH, the email providers give a lot more trouble than my house does. Verizon jus announced if you dont' read your mail for 6 months, they'll throw it away and close your account, even if seems if you're paying your monthly bill to Verizon.