Battery cable came off!

Sure. The capacitance I mention is more of an apparant capacitance.
It's not true capacitance per say, but the normal operating car battery
does provide a large apparant capacitance to the system. But this would

not be the case with a non functioning battery.
I use car and deep cycle marine batteries to run radios here in the
house.
My chargers are unfiltered, but yet I have little noise to my radios.
The use of my battery as a cap is a bit different in operation vs a
true
capacitor, but the final apparant filtering is still there. If the
battery were
not acting as a cap of sorts, I would have hash and trash out the
kazoo..
I don't know if this makes any sense, as it's hard for me to describe
stuff
like this off the top of my head..
As a quite dangerous test you could try running a car radio off the
running
alternator with no battery connected. I bet it will be quite noisy,
fairly
unregulated as far as volume, etc vs rpm. IE: if the rpm dropped too
low,
the radio might totally drop out due to the low voltage.
Hook the battery up, and all is smoothed out. Both as far as
regulation,
and also filtering. If thats not acting like a large "apparent"
capacitor,
I don't know what is. The operation is different, but the end results
are
about the same. This is not something I've really thought about too
much,
but I've always considered the usual operating car battery to have many
farads of capacitance, at least as far as overall function. Maybe not
true
in the strict sense, as far as true caps go, but as far as the end
results
of placing it in the system. I dunno if this makes any sense or not..
:/
MK

 

Very good, you get a gold star.

 

Very good, you get a gold star.

 

I think this is getting pretty far afield. Does anybody feel the alternator
would be stable under varying load with the battery disconnected?

Mike

 

I think this is getting pretty far afield. Does anybody feel the alternator
would be stable under varying load with the battery disconnected?

Mike

 

Pretty sure it would, yup, because I once had to drive without one for a week
(manual tranny, obviously - you get really adept at finding even the slightest
grades to park on).

 

Pretty sure it would, yup, because I once had to drive without one for a week
(manual tranny, obviously - you get really adept at finding even the slightest
grades to park on).

 

Thanks for bringing it back, mike. I've been reading all the posts very
closely.

Anyway, as an update, no delayed effects yet, thankfully. Though I'm
not willing to repeat the "experiment" anytime soon: Seems like I have
a shorted rectifier in the alternator. (Does this mean that I'm getting
voltage spikes all the time?) I'm wondering if the battery is now
"absorbing" the spikes, so it seems to boil down again to whether the
battery is acting as a large capacitor..

Thanks again to everyone.

 

Thanks for bringing it back, mike. I've been reading all the posts very
closely.

Anyway, as an update, no delayed effects yet, thankfully. Though I'm
not willing to repeat the "experiment" anytime soon: Seems like I have
a shorted rectifier in the alternator. (Does this mean that I'm getting
voltage spikes all the time?) I'm wondering if the battery is now
"absorbing" the spikes, so it seems to boil down again to whether the
battery is acting as a large capacitor..

Thanks again to everyone.

 

In my experience, yes, you can expect to be getting ripple of about 1/2 volt
to 1 volt even with the battery connected. It should be measurable with a
DVM on AC voltage setting, measuring across the battery with the engine
running. If diodes in two phases (out of the three phases most alternators
use) fail the AC voltage can be over 1 1/2 volts with the battery connected.
On an oscilloscope it looks pretty radical. With one phase out the voltage
hangs around 14 volts and drops when the bad phase is called on to put out.
With two phases out the voltage hangs around 12 volts and spikes upward.

Mike

 

In my experience, yes, you can expect to be getting ripple of about 1/2 volt
to 1 volt even with the battery connected. It should be measurable with a
DVM on AC voltage setting, measuring across the battery with the engine
running. If diodes in two phases (out of the three phases most alternators
use) fail the AC voltage can be over 1 1/2 volts with the battery connected.
On an oscilloscope it looks pretty radical. With one phase out the voltage
hangs around 14 volts and drops when the bad phase is called on to put out.
With two phases out the voltage hangs around 12 volts and spikes upward.

Mike

 

test

 

test

 

Applying AC straight from one phase of the alternator's output would harm
the battery,causing excessive heating.On one-half of a cycle,the battery
would charge just like the other phase outputs,but on the 2nd half cycle,it
would discharge(thru the ALT),maybe even provide a path for damaging
currents to be drawn from the battery.It depends on whether the diode
failed open or short/leaky.

Open failure would just remove that phase winding's output,lowering the
alternator's total output current,and giving more ripple.

 

Applying AC straight from one phase of the alternator's output would harm
the battery,causing excessive heating.On one-half of a cycle,the battery
would charge just like the other phase outputs,but on the 2nd half cycle,it
would discharge(thru the ALT),maybe even provide a path for damaging
currents to be drawn from the battery.It depends on whether the diode
failed open or short/leaky.

Open failure would just remove that phase winding's output,lowering the
alternator's total output current,and giving more ripple.