[Locomotive Tech]

Not exactly.

The alternator is what is providing the current to run everything when your car is running. Everything from the ecu to the headlights to the stereo.

The battery acts as a reservoir/buffer in the electrical system. If you demand more than your alternator can provide while the car is running, you will draw reserve power from the battery.

When the car is not running the battery is supplying all power, from running accessories to cranking the motor.

The power that is drawn from reserve in the battery is replaced by the alternator once the car is running again.

This is why you can actually remove the battery once your car is running. NOT RECOMMENDED. Remember the battery also acts as a buffer to prevent voltage spikes from the alternator damaging sensitive electrical equipment on the car like the ecu and other computers.

Not sure I agree with this, a car will run with either the battery or the alternator disconnected, the car doesn't care "too much" where the power is coming from. Although running the car with battery disconnected is a bad idea, running with an alt. disconnected or a weak alt is not as bad.

My car has a 105 amp alt. and I have recorded 160 amps +/- 10 amps on full acceleration and all the load I could apply during the acceleration, factory radio high, A/C (which shuts off under hard acceleration) fan on high, aggressive steering. In Fact, when I measured this The alt didn't even turn on for the five seconds of this excersize, it ran soley from the battery.

Furthermore, I have ran my car around town for a maximum of 41 minutes before the alt. started the charging cycle.

The battery is the primary source of supply power in a car. Think of it this way, if your battery is fully charged, (1) there is no bypass circuit that allows the car to run soley on the alternator. (2) Charging demand or requests (if you will), are based on voltage not current, at least in my car.

The battery and alternator work together and if you have a need to increase the output of your alternator then you should also increase the capacity of the battery. An alternator cannot deliver current on demand like a battery can. A battery has it, an alternator has to make it. If you have an HO alt. and a stock, tiny battery, you will be increasing the duty cycle of the battery charging cycle. When a batteries capacity has deteriorated it is difficult to tell, usually you will begin to notice weak starts, if this goes on even for a few weeks you have significantly decreased the service life of the alternator....heat......and continous operation that is compensating for a weak battery kills alternators. Increased and heavy/long charging cycles kills batteries. The two go hand in hand.

 

[Locomotive Tech]

A car will run without an alternator until the battery is depleted to a point where sufficient spark can no longer be generated.

A car will run with the battery disconnected forever....not forever in reality, but certainly much longer than sans alternator.

Both situations assuming an infinite supply of gas.

True, but if you ran a 100 amp alternator at 80 amps continuously, it won't last more than a few weeks, at best. The heat generated by the Alt under this demand will cause an increase of resistance causing a higher amperage draw , causing more heat. A thermal runaway situation will ensue and end the alternator. They work together

 

[Locomotive Tech]

So how does an alternator know when to produce more (or less) current?

 

[Locomotive Tech]

Let me try to help clear some of the confusion here. Alternators are controlled by voltage regulators. These are not current regulators. They sense voltage. An alternators output will continue to rise to load until the specified voltage is achieved. As more load is applied, the more voltage drop you will have, hence, the voltage regulator will apply more voltage signal to the commutator, until such point it reaches it's maximum output. When an alternator reaches it's intended set point for voltage, it decreases voltage to the commutator, thus decreasing output. If you have overvoltage issues, then you have either lost voltage sense, or the regulator has failed.

One point of suggestion to all who read this, the body of the vehicle itself, provides the lowest resistance for all grounding. Run your very large gauge cable, as short as it is possible from the negative post to the body itself, and at your amps ground with shortest large gauge cable directly to the body. Also, do the same from a place on the block to the body. Positive feeds must home-run to its intended feed. Just a word to the wise.

Exactly what I was struggling to convey, current will be based on the demand (the capacity of the battery or the load of the car). An alternaor cannot react to instantaneous demands (especially at idle) therefore the battery will supply the load and the Alt. will replenish the battery as needed.

And your advice about large gauge grounds being as short as possible is spot on!

A car does not run soley on one or the other, they work together. I can pull 120 amps with the system turned up and the car demands high, if my car was running soley on the alternator it would stall