Alternator and Battery Charging
On an a 12 volt automotive system which way does an
alternator charge back to the battery? #1 Through the positive, #2 the
negative #3 both?
First, be sure to remember that any current that flows in one terminal
flows out the other terminal. Electrical charge is conserved -- it is not
created or destroyed.
Second, you should realize that the direction of current flow is a
convention. The current is carried by electrons moving. Unfortunately,
due to an unlucky guess by Benjamin Franklin (electrons had not yet been
discovered), the conventional current flows in the opposite direction to
the electrons. That is because electrons carry a negative charge.
The conventional current when the battery is being charged flows into the
positive terminal (making it even more positive) and out the negative
terminal. Electrons, of course, flow into the negative terminal and out
the positive terminal.
I am sorry for this confusion, but it would be an enormous disaster to try
to change what we call positive charge at this late date!
Best, Dick Plano
Professor of Physics emeritus, Rutgers University
Every electric current needs a full loop circuit.
That means two wires between source and load:
one wire carries current one way, other carries it back in exact
We could put a DC-rated "amp-clamp" current meter around the ground wire
of the battery,
and then around the "hot" wire, and they would read the same level of
current at all times.
So in principle the answer to your question is #3 "through both".
The usual confusion might be due to our customary habit of calling one wire
"ground" and the other "hot" or "power" or whatever.
This convention refers not to the current in the wires, but to their voltage.
Remember that voltage is like the pressure of water in a pipe (i.e. PSI),
and current is like volume-flow (i.e. gallons per minute). Two different
All voltages are relative, only meaningful as a difference between point A
and point B.
To make conversations simpler we like to designate one "point B" for all
so measurements on various "point A"'s are directly comparable without
and so we don't have to specify two points in every sentence. Much simpler
In a DC system we most commonly we choose the wire with negative voltage
to consider our "zero volts" point.
Even though we will always call the voltage on that wire zero volts,
it still has substantial current in it, equal and opposite to whatever
current is flowing in the "hot" wire.
So the alternator pushes electrons through the car's chassis into the
battery through the negative terminal,
and pulls them back out through the positive terminal, always with a 12 to
15 volt pressure difference
The net result, filling the battery with energy, is akin to winding up a
The energy put into the battery is the product of voltage and current and
E = V * I * t
=~ 12volts * 10amps * 5 hours = 600 watt-hours = 0.6 kw-hr
= 12v * 10A * (5*3600sec) = 2.16 mega-joules = 2160 kw-sec
We do not often measure the current in the ground-wire of various units,
and the voltage is always defined as zero,
so we tend to stop thinking of the ground's participation in the action.
The voltage is lower, 12-13.5v, when the battery is being used,
and then the alternator increases the voltage there to 14-15v when charging.
So it is easy to start thinking in "one-port" terms,
as if the alternator just pushes charge in through the hot wire and
nothing else is happening.
This kind of thinking will work to a certain extent,
but it may be limited when new situations arise, such as ground-system issues.
Okay, answering that question will require I make a couple of points first.
1) An alternator generates AC voltages. So it has to go through a rectifier
before it can be used to charge anything.
2) To say one side or the other is charging the battery is a bit of an
exercise in semantics. Relative to the chassis of the car, the alternator
is/rectifier is only pushing the one side, but that is because the other
terminal is 'grounded' to the chassis of the vehicle itself.
Really, the best answer I can give you is "both", because the battery will
be unable to charge without both terminals attached. It is the difference
between the two that causes the charging to take place.
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Update: June 2012