Re: Car stereo capacitor not charging


"jakdedert" <jakdedert@xxxxxxxxxxxxx> wrote in message
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Karl Uppiano wrote:
"TimPerry" <timperry@xxxxxxxxxxxxxxxxxxx> wrote in message
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"Karl Uppiano" <karl.uppiano@xxxxxxxxxxx> wrote in message
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"TimPerry" <timperry@xxxxxxxxxxxxxxxxxxx> wrote in message
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<subliznime@xxxxxxxxx> wrote in message
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I bought a 1.5 F Xpress capacitor for $50 to help stabilize my car's
electrical system when I have my loud music and inverter running. I
hooked it up exactly as the book said, using the 1000 ohm resistor
between the positive terminal of the cap and the positive battery
wire,
with the ground connected to the negative terminal of the cap. The
voltage before the resistor is my car's battery voltage, about 13
volts. And the voltage accross the capacitor went somewhat quickly
up
to about 4 volts, then seemed to completely stop charging at about 5
volts.
what are you using to measure voltage with? do you know what a voltage
divider is?


So I got pissed, drove all the way back to Fry's, and traded it
for another of the exact same cap. I hooked that up the same way
again, and it is pretty much doing the same thing. I don't know if
that brand is just crap or if I'm missing something here. I did the
calculation online and a 1.5 farad cap with a 1kohm resistor at 12
volts should take about 2 hours to fully charge, although it has been
more than 2 hours and i'm still stuck at 5 volts. Someone please
help
me out.

what good is a "stiffening" capacitor with a 1k resistor in series?

some kind of buzz filter?
That kind of energy storage could be dangerous, and probably marginally
useful at best. I wonder how many melted wiring harnesses, blown fuses,
shorted alternator rectifiers, etc., these things cause.

They shouldn't cause any of the above, if properly implemented. IMO,
there should be a relay to safely charge the unit initially, and to
disconnect if from the system when the ignition is switched off, with a
bleed resistor to take the charge off it.

these things are "de regular" in high power car audio. i think you
charge
them up with the resister to avoid huge inrush current and the
assisiated
spark. then they are premanantly attached to +12 without the resistor.

Right. But what happens when someone simply disconnects the battery
cables for some reason, and the thing discharges? The huge inrush will
occur unless big warnings are plastered everywhere. The inrush could blow
fuses and melt down wiring harnesses.

I'm trying to figure out how that could happen, even without the safety
measures I mentioned above. Why would there be a 'massive inrush' when
the battery was disco'd? There's only 12 volts (nominal) on this thing.
Sure, it can supply a lot of current in a hurry (it's intended application
in this case) but unless something is *drawing* massive current...and what
would that be?

If the system is installed correctly, there is a fairly hefty fused lead
from the battery to the cap, probably enough to supply normal load for the
vehicle, except for the starter. In the scenario you describe--assuming
someone would disconnect the battery with the engine running (and why
would they?)--the cap simply would backfeed powering the electrical system
until it was discharged...nothing massive.

Yes, but when they reconnect the battery, look out. That's what I really
meant.

If someone disconnected the battery, then dropped the positive lead to
ground, there might be a problem; but I'm assuming anyone doing such an
installation correctly would have fused the feed to the cap. I've seen
huge fuse blocks for this purpose in the sound departments at auto parts
stores.

the idea is to smooth out the voltage drop accoss the DC wire to the
battery
when a high energy pulse such as a bass beat demands a huge amount of
current.

I understand the theory, I just question the solution. Why not just run
heavier cables between the battery and the load?

This is answered later in this thread. Sure, there's a bit of 'phoolery'
in the implementation; but the theory is fairly sound. It's not intended
to supply current all of the time, only on peaks...the same reason there
are big filter caps in good audio amps.

Well designed amps have right-sized, not oversized filter caps. Oh well...

OTOH, I'm sure that people overuse--and misuse--them....

jak



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Relevant Pages

  • Re: Car stereo capacitor not charging
    ... between the positive terminal of the cap and the positive battery wire, ... with the ground connected to the negative terminal of the cap. ... to about 4 volts, then seemed to completely stop charging at about 5 ... In the scenario you describe--assuming someone would disconnect the battery with the engine running --the cap simply would backfeed powering the electrical system until it was discharged...nothing massive. ...
    (rec.audio.tech)
  • Re: 12V to 7.2V DC converter circuit?
    ... I see your point about the cap giving it the start up power. ... surface, in my hand I have a small battery back and in my real vehicle, ... complicated to get the stall ampsto start the electrical motor. ... Put a 3 ohm, 50 watt resistor ...
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  • Re: Car stereo capacitor not charging
    ... 1k resistor won't do anything. ... from the cap to the power amp to have it work. ... switched so it doesn't flatten your battery if you leave it unattended ... to about 4 volts, then seemed to completely stop charging at about 5 ...
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    ... supply topologies and in particular how a choke input works or why it ... you appear to think that a resistor will give off ... the conventions guiding SE amp design towards the desired result. ... 'cause if that cap should fail, ...
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