On 23/6/19 12:58 am, trader_4 wrote:
On Saturday, June 22, 2019 at 5:12:48 AM UTC-4, Xeno wrote:
On 22/6/19 7:18 am, trader_4 wrote:
On Friday, June 21, 2019 at 4:54:51 PM UTC-4, Rod Speed wrote:
Commander Kinsey wrote

How does a lead acid battery charger (or car alternator) know when to
switch to trickle charge?

From the current the battery takes.

Except that the alternator doesn't know how much current is going
into the battery and how much is being used to power the car.
At least not in any car I've had. The alternator is tied directly
to the battery and that common point supplies the car. The alternator
can't switch to trickle charge either. EVery one I've seen, with
the car running normally, the voltage at the alternator/battery is
about 13.5 - 14V

The alternator knows jack **** about either the current or the voltage.

That depends on how you define alternator. On many cars today, the voltage
regulator is part of the alternator, when you buy a replacement it comes
with it.

I was considering the regulator to be a separate discrete device, even
where it is built into the back of the alternator. It can be removed as
a discrete entity



Assuming a constant excitation current at the alternator field coils,
the alternator voltage is determined by the *speed* the alternator runs
at and the *load* that is applied to it.

What does control the alternator is the *voltage regulator*. This
regulator can be external or, most often these days, built into the
alternator's rear housing. The regulator has the job of maintaining the
alternator voltage and, therefore, the vehicle system voltage.

+1

Exactly what I've said too.

The
regulator controls the level of the field excitation current in the
rotor. By doing this it controls the intensity of the magnetic field in
the rotor as a function of the voltage generated in the alternator. In
this way the regulator keeps the system voltage constant at around 14V.
To do this the regulator switches the rotor field current off and on
very rapidly to average it out so that alternator generated voltage
doesn't exceed set limits. A Zener diode is used as the alternators
set-point generator. A second diode, with a negative temperature
coefficient, is used for temperature compensation. In low temperatures
the 2nd diode keeps the alternator output a little higher and at high
temperatures reduces it slightly to prevent battery gassing.

When a load is added to the circuit, it causes the voltage to drop and
this will increase the alternator output current. Same with the battery,
as the state of charge increases, the voltage will rise and the
regulator will begin reducing the alternator output commensurately. When
the battery is fully charged its voltage will be high so alternator
charge output will be reduced to a *trickle*, enough to keep a little
current flowing through the battery and supply any other power needs of
the system.

Agree, except it;s obviously not a trickle. Todays cars have a lot of
loads in them to be powered, which is why they use 100 amp alternators.

Poorly worded. My bad. I meant/should have written the battery will only
be *taking* a trickle, only what it needs to maintain it's charge. The
rest of the circuit(s) will be taking whatever it needs. I made that
point in another post on the topic.




In the old days of coil ignition, a vehicle's alternator
output could be as little as 5 amps - 4 amps for the ignition coil and 1
amp to keep the battery happy after it's fully charged and all that was
needed was a 40 amp unit. Not so these days where an alternator could be
supplying 25 amps and more to all the electrical circuits that are
required. 60 and 80 amp alternators are now common on cars to cope with
this *extra loading*.

In summary, the *regulator* is sensing *voltage* and will run up the
alternator current output to whatever maximum limit it has in order to
reach that voltage target of 14.2-14.4 volts. That could be 40-80 amps.
This is the reason why, when you fully flatten a battery, it is best to
remove it and slow charge it using 10% of the rated capacity. ie. for a
40 AH battery, you charge it at 4 amps. You can charge it a little
faster than that, say 8-10 amps, if you're in a rush but putting a fast
charger on it, which is what the car's alternator effectively is, will
create a lot of heat in the battery and can damage or buckle plates.
Certainly fast charging will abbreviate the battery's life.


--

Xeno


Agree with you.
Now maybe you can explain that to the folks that are talking about computers, OBD2, and the alternator monitoring current output.


The only thing worth saying about that is that the voltage regulators on
some vehicles are computer controlled. Too recent for me to have much
experience with them since I retired 18 years ago.

--

Xeno


Nothing astonishes Noddy so much as common sense and plain dealing.
(with apologies to Ralph Waldo Emerson)