1999 Subaru WRX Just a general question, on Alternators.

Hi Nick Maple. Welcome to the forum. Running without the battery is a very bad idea and WILL lead to the death of computers eventually.

Disconnecting a cable was a trick done many years ago by people who didn't understand how to troubleshoot simple charging systems, and cars didn't have all the unnecessary, unreliable, expensive computers. Today, mechanics have to be very highly-trained computer experts and they understand how voltage spikes are dampened and absorbed by the battery. Also, due to the pulsing output of the generator, without the battery to even out the voltage, the higher voltage pulses cause a bigger magnetic field to build in the input circuit of the generator which further increases output voltage. If you increase engine speed without the battery connected, it is very easy for its output to reach 30 volts or more. That will pop any light bubs that are turned on and will destroy computers. The voltage regulator will not be able to control this runaway voltage.

Caradiodoc

OH, ... Sorry for my confusion.

It might help to think of the battery as an expansion tank on a home hydronic heating system or the overflow reservoir on your cooling system. The battery is in parallel with the generator and in parallel with the car's circuitry. Current leaving the generator can go to the car's other systems or it can go to the battery, or both.

Here's a better way of thinking of it. When my students had trouble visualizing something they couldn't see, I always got good results comparing everything to water flow which they COULD visualize. Imagine a city's water pump, a storage tower, and the pipe going to your house. When you're watering the lawn, water flows from the pump to your house. It does not go through the water tower. However, since the pump is capable of pumping more water than you need for your lawn, the extra goes up into the tower. As the level rises in the tower, the pressure goes up thanks to gravity. (Voltage is electrical pressure). Eventually when the pressure gets high enough, a pressure switch, (voltage regulator), tells the pump to turn off for a while. Since you're still watering the lawn, some of the water stored in the tower leaves and goes to your house. Water pressure goes down / battery voltage goes down. The pressure switch turns the pump back on / the voltage regulator turns the generator back on.

If you remove the tower from the picture, the pump will still deliver water to your house but without the cushion, the pressure would skyrocket to maximum instantly, then the switch would turn the pump off and pressure would go to 0 psi instantly. It would cycle very rapidly like that. The pressure switch is reacting to low pressure, but before it can react, your hose might pop due to high pressure.

Similarly, if you remove the battery from the story, the generator will still deliver current to the other circuits, one of which is its own input circuit.

It sounds like part of the confusion is related to the current going THROUGH the battery. No current goes through the battery to get to the other circuits. The current that goes through it is the electrons that are getting absorbed and stored in the plates to be released later. No water goes through the tower to get to your house. Instead, the water is absorbed into the tower to be released later.

In case you understand that description, let me make it more complicated. For current to flow, there must always be a complete path. Technically, electrons leave the negative battery post, but it is much easier and more common for us to start at the positive post. From there, current goes through the wires, fuses, connections, and splices to the load. The load has resistance that limits how much current can flow. (Your hose nozzle is the load). After current goes through the load, it goes back to the battery in the negative post. That's a complete circuit. There will also be one or more methods of switching anywhere in that circuit. That could be a physical switch you control or a transistor inside a computer module.

The generator circuit works the same way. Current leaves the output terminal, goes through the same circuit of wires, fuses, and loads, and goes back in through the case which is the negative / ground return. No battery involved, although the battery could be one of those circuits where the generator's output current is going. Without the generator, the car would operate off the current from the storage battery. Without the battery, the car would operate off the current from the generator. Each system has to be connected to the car's circuitry to work. Since the battery already is, it is common to just run the generator's output wire over to the battery's positive cable because it's convenient. Some cars, GM in particular, run the generator's output wire to the large terminal on the starter because that cable goes back to the battery too. Saves on the amount of wire needed. The generator's negative return is its case bolted to the engine. The battery is connected to that point with the large black cable that is also bolted to the engine.

Another way of looking at it is every circuit in the car has a positive side and a negative side. The battery has a positive supply terminal and a negative return terminal. The generator has a positive supply terminal and a negative return terminal. The positive terminals are all tied together from the battery, generator, and circuits. This is most commonly done at the battery cable, but it doesn't have to be. That's just done for convenience. Similarly, all of the negative terminals must be connected. On fiberglass cars that requires wires that all meet up at one common point, again, typically the battery's negative cable, or possibly the engine, but with metal car bodies, it is real convenient to use the sheet metal to connect everything together.

Caradiodoc

Sorry to get so involved. 99 percent of people don't have your understanding of electrical theory.

The battery doesn't provide the ground. It is simply connected to ground just like all of the other loads and sources. In his case, the generator is a source of system voltage just like the battery is. Think of a diesel truck with two batteries. Both have their negative cables connected to ground. Either one alone would power the systems. Now imagine you're replacing one of the batteries with the generator.

"Ground" is just a term we give to a common connection point. In the tv world we also have a name for the common power supply. That is "B+", a reference held over from the days of "portable" radios that used an A, B, and C battery, in case you care. We could also say the battery positive cable is tied to the B+ circuit, but that doesn't mean it develops the circuit, just that it is connected to it. Same for the negative side. It doesn't develop the ground, or common point, it is just connected to it.

Let me know if my descriptions made things worse.

Caradiodoc

"Ground" is just the name we give the common point. The generator also needs a complete circuit for current to flow. It just happens to be sending current through the same circuits as the battery.

Since you understand electrical terms, think of the battery and the generator as being in parallel. Either one can pump current through the circuits that are connected to them. The generator can also pump current into the battery if its voltage is higher than the battery's. If the battery's voltage is higher, as when the engine is off, it would like to send current through the generator, but there's diodes in there to prevent that.

At the risk of adding confusion, another way to look at this is to treat each load as an individual circuit. Battery, wires, fuse, switch, head lights, wires, battery. That's a complete circuit if the switch is turned on. Generator, output wire, fuse, dome light, switch, body, braided strap from firewall to engine block, generator case. That's a totally different complete circuit when a door is open. Now take the case of the generator, (actually, the engine block), and the negative battery cable, and tie them together. It hasn't changed either circuit, but we call that common point "ground". Next, connect the battery positive cable and the generator's output terminal together. That still hasn't changed either circuit, but we don't have a special name for that tie point other than "hot". The only thing that has changed is now the battery can send current to the dome light circuit and the generator can send current to the head light circuit.

Caradiodoc

"But if the battery is removed what is there to link the chassis to ground.(Nothing) So no circuit should be completed at all, which means the alternator shouldn't work after the battery is completely removed from the car."

Here's where I think the confusion is coming in. The battery doesn't link anything. If anything is linked, it would be done by having multiple wires or cables crimped together in the battery cable clamp. The large black cable goes to the engine block, and a smaller wire goes to the body. (The only reason for a separate wire to the body is the engine and transmission are mounted on rubber mounts so the engine is isolated electrically from the body). The generator DOES still have a complete circuit. Current flows through the output terminal, to the fuse box, through the fuses to the circuits, through the loads, and back to the case of the generator by way of sheet metal or the frame of the car. The battery isn't part of the story, but when it is connected, it is just another load as far as the generator is concerned.

I was thinking about handouts I developed for my students and realized I drew the entire charging circuit with lines representing wires and cables, with no mention or reference to "ground". The battery has a negative and positive terminal. The generator has a negative and positive terminal. Those two are connected in parallel. Every circuit has a starting point, (positive), and a negative, (ending point), and are in effect, also in parallel with the battery and generator. Now, ... Imagine taking one of those negative wires, breaking it, and connecting each end to a piece of sheet metal. The sheet metal replaces, or works as, a piece of wire. The only reason for doing this is to eliminate the need to run return wires for every circuit back to the voltage source, battery or generator.

I think you'll have better luck if you don't get all "wrapped around the axle" from concentrating on the concept of "ground". Instead, just look at it as feed and return paths. Also, don't over-think the multiple wires in each battery cable clamp. Older cars had a strap bolted to the engine block and to the firewall or frame instead of a small black wire from the battery to the fender. Ford was famous for using the starter solenoid on the fender as their common feed point, and there was just a single large red battery cable going to it. The generator output wire and the wire going to the fuse box were also connected to that common point.

Not that I want to add to the confusion, but when you mention needing the battery to complete the circuit, it makes me think of current going from the generator THROUGH the battery, THEN to the circuits and back. That would put the battery and generator in series. The generator develops around 14.4 volts and a fully charged battery will measure 12.6 volts. That would give 27 volts to the circuits. You know that ain't happening.

Don't let your brain turn to mush. :)

caradiodoc