P38A charging

Hi all
I know the basics about how alternators work, the windings create an AC current, which is turned to DC through the rectifier. The regulator then turns the voltage down to a point, often 14.7v.
Depending on demand, the alternator will switch 'on and off' to provide the required output.
That's where my knowledge ends for a traditional 'non smart' alternator.
If i watch the voltage output while driving, i see it fluctuate all over the shop depending electrical demand from the vehicle and the state of charge of the battery.
I would however expect to see a constant voltage with varying amperage, though i don't have any method to measure amps at any point.
Is the alternator on a P38 in fact somewhere between a traditional alternator and a smart alternator with output managed by the ecu? If so, how does it monitor the state of charge of the battery?
I'm confused but trying to understand and learn so sorry if any/all of the above is wrong!!

kermit_rr said:

Hi all
I know the basics about how alternators work, the windings create an AC current, which is turned to DC through the rectifier. The regulator then turns the voltage down to a point, often 14.7v.
Depending on demand, the alternator will switch 'on and off' to provide the required output.
That's where my knowledge ends for a traditional 'non smart' alternator.
If i watch the voltage output while driving, i see it fluctuate all over the shop depending electrical demand from the vehicle and the state of charge of the battery.
I would however expect to see a constant voltage with varying amperage, though i don't have any method to measure amps at any point.
Is the alternator on a P38 in fact somewhere between a traditional alternator and a smart alternator with output managed by the ecu? If so, how does it monitor the state of charge of the battery?
I'm confused but trying to understand and learn so sorry if any/all of the above is wrong!!

Click to expand...

I am not really an electrical expert, but from memory, there is little difference in the workings of a voltmeter and an ammeter, the dials are just marked differently.
And volts and amps rise and fall all the time, depending on the actual power drawn by the system at any time.
So if a high load comes on the system, the volts will drop a bit and the amps rise a bit.

That isn't a very complete explanation, but it seems to be the way it works in practice, with no visible problems.
Maybe someone with more electrical expertise will comment in due course.

Any modern alternator is effectively a smart charging device. The old generators tended to rely on a regulator box with coils and points that cut in and out, basically switching the generator “on and off” in quick succession to give the required current at any given time.

Your alternator is digitally controlled to maintain a certain voltage set point. As load is placed on the electrical system, the voltage falls. the alternator senses this and produces more current in order to meet the load demand. As the demand is met, the system voltage will return to “normal”. When the load is removed, the voltage will start to rise so the alternator reduces its current output to compensate, so the battery voltage stays stable.

This is predicated on the alternator spinning fast enough to produce enough current. At idle, most modern alternators will produce enough current to cover the engine running loads (so fuel pump, computers and ignition) so the car doesn’t die from lack of electrical power if left idling. However, to cover major loads, it needs to be turning faster. Most produce their maximum output at around 3000 to 4000 RPM (alternator speed).

As for ammeters and voltmeters, an ammeter is wired so the current passes through it. It has no resistance and measures the full current flow. A voltmeter on the other hand has almost infinite resistance. It measures the potential difference between tow point on a circuit. If to hook it up to two places on the same wire, it will likely show zero as there is no potential difference between those tow spots, hook it a roof a battery and it will give you the batter reading. You will note if you do that, there is no spark or anything else as the voltmeter draws basically no current. Try it with an ammeter and see how you go (take clean undies with you )

That's pretty much what i thought, the alternator should try to meet the set point of the regulator assuming the battery is healthy and the demand isn't higher than the limit of the alternator.
I have no reason to think my new alternator is faulty as it's exactly the same as i saw from my old one, but I'm just trying to understand what I'm seeing.
I have seen 14.7v but mostly it's between 13.5-13.9v with engine running, no accessories turned on and makes no difference whether idling or 2000rpm

Mine sits at 13.7, which for a lead acid battery is more than enough.

Modern is 14.7v
p38s had the lower set point for the older batteries.
I bought a modern battery so changed the regulator to bump up the set point. 13 odd wouldn’t fare well on the then new battery.

Not sure if there was a changeover point. A lot changed in the industry during their production mainly introduction of MAF and emissions for which they bodged whatever way they could (now industry standard )

Well worth a look at Datateks posts/threads if you want to know of alternators and electrickery

Paperwork with the new one said it was 14.7

Will find his posts ta

Your meter disagrees

If your alternator is new should push out the set point. Could it be the battery is not capable of taking it?

It's a new, healthy and fully charged battery

It shouldn’t be with 13 odd volts?
Sounds dodgy

Borrow a second meter to check yours is calibrated?

It's not calibrated but it's close enough. I've got another volt reading tool which gives the same reading, but I've only used that with the engine off.
Think i need to do some more investigations