On 24/6/19 9:11 am, Clare Snyder wrote:
On Fri, 21 Jun 2019 13:48:56 -0700, % wrote:

On 2019-06-21 1:19 p.m., Commander Kinsey wrote:
How does a lead acid battery charger (or car alternator) know when to
switch to trickle charge?Â* I can understand it noticing a drop in
charging current if the battery is on its own, but what if a random
changing load is connected, as there is in a running car?

the catalytic converter tells it
I can tell you how an alternator does it, chargerscan do it
differently. AN alternator doesn't "switch to trickle charge". AN
alternator limits the output voltage. A battery will only accept a
certain amount of charge at any voltage. At 14.6 volts it basically
stops taking a charge. The regulator is set to limit the voltage to a
specified voltage - 13.8 or 14.2, or something similar. An alternator

The Zener will limit the voltage to something like 14.2V or thereabouts,
with temperature compensation.
The ideal rate for charging a battery lies in the range 13.8-14.1V. This
range provides the maximum battery service life and limits the heat
generated within the battery. The disadvantage is that the charge time
is much longer and there exists a risk of sulphation.
Between 14.4 and 14.7V there is less risk of sulphation but a higher
risk of grid corrosion on the positive plates and gassing. This will
require topping up of the water content of the electrolyte and can cause
severe overcharging if ambient temperatures are high.

is intrinsically current limitted so will not provide more than the
rated current, If the battery open circuit voltage is below 12 volts
it will tale pretty close to whatever the alternator can put out, as
the open circuit voltage increases, the amount of charge it will

And that voltage increase is in step with battery internal resistance.

accept decreases, untill at 14.6 volts or whatever is designed as full
charge, it will no longer accept ANY charge. As the load changes, the
alternator provides more or less current as required to maintain the
fully charged voltage.

Clear as mud???

It will only be muddy to those without a clue. ;-)

I wouldn't have thought it a particularly difficult set of concepts to
understand, especially in this day and age when there are heaps of texts
on the topic describing the operation of *all aspects* of a vehicle's
electrical system and freely available on the internet. All that is
required is an understanding of the fundamentals of electricity and,
these days, electronics.


--

Xeno


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