On Fri, 27 Nov 2015 11:29:04 -0800, mike wrote:

On 11/27/2015 10:28 AM, wrote:
On Thu, 26 Nov 2015 23:16:55 -0800, mike wrote:

On 11/26/2015 8:54 PM, wrote:


With reverse polarity protection you don't get 12.2 volts open
circuit. You get NOTHING. If you have a battery charger putting out
12.2 volts open circuit, it won't put out more into a load.

Well, that may be the way you would have designed it.
But, you probably didn't design it.

I don't have a problem with assumptions that you verify.
I have a problem with people who state, with certainty,
stuff they have not experienced on unspecified equipment
in unknown circumstances.

Doesn't it trouble you the least little bit that it's
12.2V? What's the failure mode that results in that
open circuit voltage reading?

Could be several different failure modes, including a bad rectifier,
as most simple battery chargers are very poorly regulated open
circuit, with the impedence/resistancde of the windings limiting
urrent under load. Just like a cheap 12 volt wall wart that may put
out 18 volts open circuit.
You're mixing parameters. The wall wart is filtered, so the open
circuit voltage is likely near the peak value of the rectified waveform.
If you put a cap on the output of a battery charger, you'll read
the peak value on your DC voltmeter. Without the cap, the reading
is more like the average value of the waveform.

My first check would be the bridge rectifier, which is
repairable.replaceable - second would be a shorted secondary winding
in the transformer, which means it is junk.
I'd have to agree with that diagnostic method, but after you determine
that it's actually broke.


If it was a sophisticated automatic charger, it could be anything -
but being listed as a dual current rated charger, it is NOT a smart
charger.

Like I said - not rocket science

Good thing that rocket scientists don't think they KNOW everything.

If you understand batteries and electricity you don't have to know
everything to be sure of many things.

Yep, I used to manage very bright engineers with similar arrogance.
Spent much of my time cleaning up after them.

Let's just save the thread the excruciating protracted attempts
to prove how smart you are and turn it into a thought experiment.

Take a random car battery charger.
Plug it into your oscilloscope with no other load.
What would you expect to see on the screen?
A choppy DC - diferrent depending on what kind of rectifier circuit t
has - center tapped full wave, full wave bridge, oe half wave.

If the OP had stated what make and model the charger is, it would be
easier to give a definitive answer - just like if he told us what kind
of "scooterz' he was working on.

and engineers are a lot different breed than those of us who have to
fix their mistakes - or have to work on what they design.

Take your random DC voltmeter and think about how it would
respond to what you see on the screen.

It would read HIGHER than actual voltage if anything.

You are so smart, you explain to me how a DC voltmeter would reaf
lower on an unfiltered rectified voltage than it would when connected
to a "filtering" load like a battery.
Youbetcha.
At the risk of repeating myself...
What does a DC voltmeter read when you apply something that's not DC?
The answer is that it depends.
The meter that most people have will read something like average...it
depends.
And, because I didn't expect you to capitulate, I did the experiment
before I posted. Created a pulse waveform with my function generator
and stuck the Fluke on it. Reads something approximating average.
And that's less than peak. And peak is more relevant to what the
charger can deliver to the load.
Try it, it ain't rocket science...just an understanding of electricity ;-)

Cheapo battery chargers often use the leakage inductance of the
transformer to limit current pulses to a safe value. It's not unusual
to have a SCR in series and a zener diode so that the SCR can't turn
on when the battery is over the desired charge termination voltage.
Smarter ones have hysteresis that drops the voltage to the float level.

When you measure the battery voltage during charging, you're doing just
that,
measuring the battery voltage. The charger is putting current pulses into
a rather stiff voltage source, so the output is whatever the battery allows.
Measured when disconnected, the charger output is often full-wave
rectified AC. The number on the display of your DC voltmeter is
less than the peak. Average, or thereabouts, is likely what
the typical DC voltmeter displays.
If you want to know if the battery is charging, you need to measure
the current.

Full wave rectified AC is DC. It is pulsating DC, but not nearly as
pulsating as half wave, and bridge rectified is not a pulsating as
center tapped full wave (2 diode). A DC voltmeter will read the DC
(average) voltage on an unloaded circuit, and will always read
somewhat lower under load. The battery "sees" the average voltage.

You still have not explained how a battery charger would put out less
open circuit voltage than the voltage it would put into a load or a
battery - sorry.

If you can adequately explain it, I'll accept your criticism
I was trying to get you to rethink your criticism, but that
was going nowhere.
How'd I do?


You still didn't explain how the voltage could be lower under no load
than under load.
I agree that understanding of electricity would help...;-)

Real easy way to check for a shorted diode is put an AC meter on it.
If you have a couple volts of AC it's pretty well a dead giveaway, and
it would definitely drop the output. And many wall warts ARE
unfiltered. All they have in them is a transformer and a diode bridge
- or on the real cheap-assed ones, a center tapped transformer and 2
diodes.

I've even seen them with single diode half wave rectification.