Hi Greg,

On 30-Sep-03, Greg Mayman wrote to George White:

 GW>> That "near full" is probably 99% of full and most people would
 GW>> never notice the difference. However, nit picking, I only said
 GW>> "the batteries are never fully charged" for an
alternator sensed
 GW>> system.

 GM> Again, it is a matter of defining what is meant by "fully
 GM> charged".

Always! :-)

 GW>> When I worked on the CAV 440 regulator it had a choice of 3
 GW>> settings, from memory 12.5 (for NiCad systems), 13.5 (normal) and
 GW>> 14.5 (for heavy use/cold climate).

 GM> So for lead/acid batteries, in a halfway cold climate or for
 GM> medium heavy use the alternator could be set to 13.5 which would
 GM> leave the battery undercharged, or to 14.5 which would overcharge
 GM> it.

Again, as you say later, it's a compromise. The problem is that
battery capacity is not the important factor, it's the surface
discharge characteristics that matter for starting. Changing the
charging terminal voltage does not have too much effect on that and
given the engine compartment temperatures normally the voltage is
set below what otherwise would be considered "fully charged" to
minimise gassing and prolong overall battery life.

Consider that standard float charge is to 2.3 V/cell and cyclic 2.4V
giving 13.8V float and 14.4 cyclic terminal charging voltage.  These
are very close to the standard settings we used. A vehicle application
can normally be considered as "float" rather than
"cyclic" use, as it
normally is recharged very quickly and then spends the rest of the
journey at fully charged state. At the "cyclic" charge voltage it will
be gassing slightly, at the "float" voltage the much lower gassing
will normally recombine within the cell and have no vapour loss from
the battery.
Even these figures (the 2.3/2.4 V/cell) are subject to debate...

 GM> And did the charging system cut out completely at this voltage or
 GM> was it the point at which charge would be reduced to something
 GM> approximating a trickle charge.

That was terminal charge voltage, what current the battery would take,
it took. We actually had problems with slip rings glazing on some
vehicles (buses normally) because when he battery was fully charged
the charge current became effectivly zero and for most of the time the
field was off! They had to be using high quality batteries, but it did
happen!

 GM> And was this an exact voltage, under all conditions, or a nominal
 GM> figure?

Exact figure, under most conditions. We put a lot of design work into
matching temperature coefficients of components to get an output
voltage characteristic that matched the battery characteristics.
Oh yes, temperature range was -40 to +70 C. We tested to those limits
too...

 GM> And when the vehicle was used with a fairly substantial drain from
 GM> lights etc, say 200w (about 16A), which would cause an appreciable
 GM> voltage drop in the wiring, was this compensated for in the
 GM> regulation of the alternator? In other words, did the regulator
 GM> control the voltage at the output of the alternator or at the
 GM> battery terminals?

We designed for battery sensing. As far as we were concerned
alternator sensing was a problem, and as we had no way of controlling
the wiring there is no way we could design in any form of
compensation. I doubt if we ever even considered it... (It's over 20
years ago now so memories fade...)

 GM> In truth, it's all a compromise.

 GW>> Indeed true, however there is _always_ some volt drop, and of
 GW>> course the lower the current, the lower the temperature rise
 GW>> within the diode, so there is a balance...

 GM> Correct.

 GW>> We're agreed then :-).

 GM> Of course ;-)

:-)


George

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