> What do these alternators put out 35 amps or so?
Perhaps that is the point of confusion here ... alternators have a current
rating, but do not put out that much current all the time. Almost always,
the output is limited by voltage, not current. (The actual regulated
voltage deliberately varies somewhat with temperature.)
> Seems to me there must be something different between what a
> battery charger is doing to the battery
> and what the car's alternator is doing to the battery.
Yes ! Once again, most battery chargers are NOT voltage regulated. It's
not at all unusual for even a small charger to have an open circuit peak
voltage of 20 volts or higher.
> V=IR. Doesn't that say that when the battery's resistance goes up
> the current goes down?
V=IR applies only to passive components, like resistors and wires.
Batteries are active components, and are considerably more complicated ...
in a very real sense they are an unattended, self contained generating
station. In EE terms, a battery can be modeled as a voltage source in
series with a resistance. This resistance is what's called "internal
resistance", but it's not a fixed value. It actually goes down as the
battery is charged. And since it is extremely low, it normally only becomes
a significant factor in current calculations at very high currents, like
when turning the starter motor. The voltage is not fixed either, and rising
voltage is what's normally used to limit charging current.
> OK I am a
> dummy about electricity, but I always thought the amperage was a
> function of voltage and resistance.
It is, but you have to be careful about how you measure voltage and
resistance.
Maybe an example would help. If you take two 12v car batteries, in similar
condition, and connect them together positive to positive and negative to
negative, you'll get very little current flow. Makes no sense if you look
at the 12v, since the resistance is very low. But what _is_ important is
the _difference_ in voltage, which will be essentially zero. (Any slight
difference will rapidly fade away as one battery charges the other until
their voltages are exactly equal.)
One of the characteristics of the lead-acid batteries used in cars is that
their voltage is a function of the state of charge. The voltage goes up
relatively slowly through most of the charge range, then rises relatively
rapidly at around 100% charge. Automotive electrical systems (and smart
battery chargers) take advantage of this fact to stop charging when the
battery reaches this voltage, which is roughly 14.6 (depending somewhat on
battery temperature, age/condition and so on).
> Maybe the charger increases the voltage when the amperage is increased?
That is also frequently true. High power chargers (like over 100 amps) put
out even more voltage at low current ... it's not unusual to find 30 volts
or more at no load.
To put it another way, battery chargers also have internal resistance, and
it typically is many hundreds of times higher than the battery. In order to
deliver say, 300 amps at 15 volts through 0.1 ohm of internal resistance,
the open-circuit voltage would have to be 31.5 volts. I have seen little
"wall wart" type trickle chargers that delivered close to 30 volts open
circuit, which implies an internal resistance in the tens of ohms ...
Randall
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