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Name: Michael
Status: other
Grade: other
Location: TX 
Country: N/A
Date: 11/29/2005

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?

Hi Michael,

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 compensation. 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 things.

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 measurements so measurements on various "point A"'s are directly comparable without further thought, and so we don't have to specify two points in every sentence. Much simpler for us.

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 clock-spring. The energy put into the battery is the product of voltage and current and duration:

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.

Jim Swenson

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.

Ryan Belscamper

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