[T3] I don't know why yet for some reason my 73 T3 charging system seems off.

[Jim Adney]

On 29 Jun 2017 at 13:17, William J wrote:

>  Why at 1000 engine RPM I read 14.2 VDC and at 800 engine RPM I read
> 14.1 and as I idle at a stop the voltage drops to 13.5 and the longer
> I sit and idle it will drop to 12.34 Volts ? As soon as I step on the
> gas to drive off it jumps back to 14.2 volts. 14,2 volts is the
> highest reading I will get even at 2500 rpm. Once I shut it off with
> DVOM still connected the battery read 13.45 volts which is surface
> charge. It settles to 12.72 Volts over night. To add I don't drive
> much or very far so I keep a charger connected on the 12 volt 2 amp
> setting on a timer set for 2 hours each day it sits and after short
> drives with no less than 4 starts just to keep the battery topped off.

Everything here is perfectly normal. Think of it this way. If not for the voltage 
regulator the generator output voltage would be proportional to generator 
rpm. It starts with zero output at zero rpm and rise in a straight line as the 
rpm rises.  

We can divide the generator operation into 3 regions: 

1) Low RPM, when the generator field windings are fully energized, but the 
output voltage is still less than the battery voltage. Under these conditions 
the battery would normally push current backwards thru the generator. The 
voltage regulator prevents this. (It's not perfect, but it's close enough to 
ignore the difference.) In this RPM range, no current comes out of the 
generator.

2) Medium RPM, when the generator output voltage is between the stable 
overnight voltage and the regulating voltage (14.1-14.4 V.) In this range, the 
voltage regulator is still fully energizing the generator field windings, 
allowing the generator to put out whatever voltage it's capable of. In this 
range, the generator will put out whatever current the car needs, plus any 
current the battery needs to bring it up to that voltage. (The voltage regulator 
may cut back on the output current if it approaches the current limit, but this 
seldom happens in normal operation.)

3) Higher RPM, when the generator has reached the RPM where it is 
capable of generating voltages higher than the regulating voltage (14.1-14.4 
V.) At this point, the voltage regulator reduces the amount of current 
supplied to the field coils, limiting the output voltage to the regulating 
voltage. At this point, the generator and the output voltage are "in control." 
This is where the charging system spends most of its time when you're 
driving. In this range, the generator provides all the current that the car 
needs, plus perhaps a bit more to bring the battery up to charge. Most of this 
time, there's NO current coming in or out of the battery under these 
conditions.  

At idle, theres no output from the generator. When you run the engine above 
idle it charges the battery. The voltage regulator limits the generator output 
voltage to something between 14.1 and 14.4 V. When you come back to idle 
the generator is no longer doing any charging, so the full battery voltage 
slowly drops back down, as the car draws power from the battery.

The battery discharge curve is not linear, so as it discharges, the voltage 
falls rather quickly from 14+ to the mid 12s. This is normal.

Unless you don't drive the car for a month or more, it's really unnecessary to 
use the charger. I finally bought a solar charger to keep my Square charged 
in the winter, when it seldom gets driven, but I had to modify the circuit 
inside it to prevent reverse discharge at night and to provide a reasonable 
indicator LED output. All it takes is 20-50 mA during the day to maintain 
itself.

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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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