| > (And they don't want the voltage to ever go higher than 14.7 volts,
| > because that burns off the electrolyte of Pb batteries. The very
| > cheapest chargers (especially the wall-warts) do go higher, and will
| > burn off your electrolyte if left on, but that's only the very
| > cheapest ones.)
| There is usually a resistor and/or transformer saturation built in to
| limit the peaks.
I'm not sure what you mean by `transformer saturation', but a resistor
by itself will not prevent a 15 volt power source from burning off the
electrolyte of a 12 volt car battery. It may slow it down, but won't
| Basically they limit the current to - say - 3-4A - and given that
| all car batteries will themsleves accept 10 times higher charge
| current than that, then despite what the open circiuit voltage is,
| the battery clamps it to around 13-14v.
Yes, but that's the catch -- if the open circuit voltage is over 14.7
volts, and the battery is fully charged, it will start burning off
electrolyte, which is bad.
It may very well be that they just don't care -- after all, you don't
usually put a high amperage charger on your car battery and leave it
there for days. (A trickle charger, on the other hand ...)
| In teh past they used copper oxide rectifiers that were nice and
| resistive, and had cooling finns on em. The one I have appears to have a
| trashy bridge rectifier, and a wirewound resistor, a transformer that I
| reckon would de at 5A continuous, and a fuse that blows at 7A or so. :D
Just for the record, a wire-wound resistor also functions as a choke.
| > There's probably also a voltage regulator in there, just to make sure
| > that they don't cook your battery with too much voltage. At least in
| > all but the very cheapest chargers.
| Your confidence in the manufactureres of cheap car battery chargres
| is sadly misplaced :-)
Could be ...
| > It also doesn't have to be a capaciator. A coil (or choke, same
| > thing) in series with the signal will have the same effect.
| Mmm. Not really.
Yes, really. A capacitator stores energy in it's electric field, and
a coil stores it in it's magnetic field. In a DC circuit with `dirty'
power, a capacitator in parallel with the circuit or a coil in series
with the circuit will both have similar effects -- to smooth out the
current. (Assuming you pick appropriate values of both, of course.)
The coil will introduce resistive losses where a capacaitor usually
doesn't, but if you want a resistor in there to limit current anyways,
might as well kill both birds with one stone.
I may have to open up my car battery charger and see what's in there.
And hook it up to the oscilliscope to see what the output looks like
with no battery connected ...
Of course, this thread has drifted pretty far off topic. :)
Doug McLaren, email@example.com
Those who live by the sword die by the arrow.
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