[M] Alterpower alternator charging control switch

[Orin Eman]

> Doesn't the alternator keep the system voltage a little higher than 12.0vdc?  This
> may just be my gauge inaccuracy, but my dash voltmeter reads around 13.5vdc when
> I'm not at idle.  What I'm thinking is if you disconnect the alt at WOT to remove
> that extra little load, won't you effectively lower your spark voltage?

Maybe.

> I might be nit picking a very small difference, but I do know that allot of
> aftermarket companies make high energy coils and capacitive discharge ignition
> systems all to raise the spark voltage.  Ever wonder why they make those 8.5mm low
> impedance park plug wires??


Well, what really matters is the current in the coil when it is
interrupted.  What happens is as follows:

Coil is connected to ground.
Current in the coil rises according to V = L * di/dt.  Initially,
V is the battery voltage, so with a lower battery voltage,
di/dt is lower and the current in the coil rises more slowly.

When the ECU wishes to 'fire', it disconnects the coil from ground.
The spark voltage depends on the current in the coil at this point.

The ECU controls the length of time the coil is energised.  This time
usually depends on the battery voltage and RPM.  Depending on the
tables in the ECU, a lower battery voltage may or may not result
in a lower current thru the coil when it is interrupted in order
to make the spark.

Anyway, if there will be a problem with a lower battery voltage,
it will be at higher RPMs where there is less time for the current
in the coil to build up to the desired level.

Orin.