How does Alternator power battery?

[Challo]

I bid you good day folks.

Last week I had an issue, where the coolant temperature gauge stopped working.

So, Saturday morning I decided to investigate and also change the engine oil at the same opportunity.

I started the engine to warm the oil before commencing draining.

Opened the bonnet and behold!!

A wire bunch of cables started fizzing and then set on fire.

Oh bugger thought I.

Upon further investigation it appears that the coolant temperature sender cable had been caressing the block beneath the cyclone breather.

In doing so it removed its outer garments and caused the short which led to the inferno.

That also fried the battery light wire and the alternator to starter cable.

I replaced all the cables but this leads to the reason for my post.

After removing the cover on the alternator, in order to replace the cables, I actually gave some thought to what each wire does.

I worked out that I have the main alternator to starter cable.

A tachometer cable, battery charge light cable and a very thin orange wire that runs to my split charge relay.

So I had a ponder but I still can't figure out how or where the alternator charges the battery or batteries.

I have always presumed there was a cable running from the alternator to the battery.

I would appreciate it very much if a grown up would enlighten me.

Cheers

Chris.

[RedLineMike]

highly likely you have a lead from the alternator to starter, from starter to battery

[simonb]

The alternator output cable is attached to the starter motor positive cable which is in turn connected to the battery.....

[vulcan bomber]

The alternator output cable is attached to the starter motor positive cable which is in turn connected to the battery.....

And the vehicles loom.. the car runs off electric from the alt, not the battery.

[discomikey]

the alternator works by induction,

it uses an electromagnetic field, and an iron rotor, to create a 3 phase alternating current. which goes through 3 paralell diodes to regulate the current, and in turn, as diodes are like a 1 way valve, convert the current to DC, this comes out the back, in the thick brown wire, and goes to the starter wire, which is obviously connected to the battery.

EDIT: there is also a regulator, which shuts off the output when it reaches 14.4v and sets it going again when it drops to 14.0v, hence poeple say they charge at 14.2v +_0.2v

the little wire that goes in, creates the electromagnetic fields that are required to make the induction currents. this is an ignition switched 12v feed which goes through the battery charge light.

if the current down the little wire stops, the light goes on, the electromagnetic field cant be created, and the alternator cant charge.

[western]

Definitly a thread for our tech archive, addition has been done

[vulcan bomber]

if the current down the little wire stops, the light goes on, the electromagnetic field cant be created, and the alternator cant charge.

Not strictly true, with enough RPM an alternator will self excite which is why you shouldnt run an engine without the alt wired up.

[Challo]

That's brilliant chaps.

Much appreciated, i get it now.

And very promt replies too.

Thanks again.

Chris

[discomikey]

Not strictly true, with enough RPM an alternator will self excite which is why you shouldnt run an engine without the alt wired up.

yes, thanks thats right. managed to get from harper to home = 50 miles on so little battery that my radio wouldnt even switch on, but kept running and headlights as long as i kept above ~2500-3000 rpm. did make me look a bit of a noob at the lights revving its tits off.

[Ed Poore]

if the current down the little wire stops, the light goes on, the electromagnetic field cant be created, and the alternator cant charge.

Not quite true - it's when the current down the wire is running that the light goes on... they're usually wired in series with the field winding.

Essentially one side of the light is connected to the battery whilst the other is connected to the field coil of the alternator. When the ignition is turned on (but the engine is not running) the coil is literally a short to chassis ground as the coil is just a wire at this point. Once the engine starts then all the electro-magnetism stuff kicks in and the field coil then becomes an inductor. Through induction the current flowing through this coil is amplified by the mechanical rotation of the alternator and thus a larger current is induced. As a consequence (and for all this to work) a potential difference (voltage) is generated across the smaller field coil and the end that is attached to the bulb rises towards 14V, i.e. the same as the battery voltage and thus no voltage is seen across the bulb and thus no current can flow which is why the bulb goes out when the engine starts.

If the light goes on when the battery is running then it indicates that there's a short somewhere in that circuit meaning that the bulb is being pulled down to ground.

[Ed Poore]

Another point worth noting for posterities sake is that in order to convert AC to DC it's usually desirable to put a large capacitor after the diodes to smooth out the ripple current. In the case of vehicle electrics the battery acts as a colossal capacitor so you don't need that. (Google AC/DC converters and you'll see what I mean).

[Team Idris]

A bit more odd info; If you run the main charge wire direct to the battery you gain a tiny bit of charging as every joint offers some resistance. Unless the starter terminals are corroded, in which case using it as a terminal looses loads.

Sometimes other 'pop in' regulators are available with higher voltages for gel cell batteries and the like.

Standard regulators are a bit rubbish, so there are external regulators aavilable that fill the batteries right up in a shorter time.

[Blackfiesta123]

Not strictly true, with enough RPM an alternator will self excite which is why you shouldnt run an engine without the alt wired up.

You can also excite the alternator by touching the terminals together. I only found this out the other day when my dad had troubles with his beetle and someone done this for us. Tho you need to do it everytime you switch the car off. Its worth remebering.