"Roger Long" wrote in newsmRTf.5154$Mj.3933
@twister.nyroc.rr.com:

That jogs my memory about the catboat stories of a few days ago. I
opened up the dead regulator that boiled the batteries nearly dry and
the inside looked like nothing more than a doorbell or some kind of
complex relay.



You're fairly close. It's a buzzer. The coil goes across the battery
terminals and has higher resistance, a calibrated resistance. As the
voltage rises, at some point, the magnetic field the fine wire creates,
which is proportional to the voltage of the battery, overcomes the spring
loading of the armature and pulls the contact, which provides field
current through a limiting resistor (that ceramic thingy on the back),
open. Of course, as soon as the voltage relay opens, battery voltage
drops until the coil can't overpower the armature spring, so the relay
closes again and field current resumes.

This happens fairly rapidly, especially when the battery has finally
charged. The pulsating DC, of variable speed and pulse width determined
by how long the battery voltage supports pulling in the armature, gets
smoothed out by the field coil's inductance into an average DC current
which, of course, sets the alternator (or generator of old) output.

There's still one in my 1973 Mercedes 220 Diesel, mounted to the right
fender inside the engine compartment. Works great. The last big diesel
starting battery lasted 6 years......er, ah, without 3-stage charging,
too!...(c;

The other relay turns on the field current when you turn on the engine
switch. Some have 3 relays. The third relay switches field resistors
(there's 2 on the back of those) to give us two charge rates depending on
how dead the battery is. This third relay pulls in at some level as
battery voltage rises and adds another resistor in series to drop the
field current to a lower level when the battery is nearer charged so we
don't heat up the battery near full charge. It drops out and simply
shorts the extra resistor at low battery voltage to charge it hard when
the battery is dead....or when there's a big load like the 120 amps my
solid state kilowatt HF linear amp draws for the big ham radio station in
the trunk....(c;