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Ground Circuits
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Welcome
to all those who have recently signed up for our email newsletter!
Ground
Part 7 of (?) Parts
Measuring
Single Battery Ground Electron Current
Engine
RUN
When the engine begins to RUN the generator takes over as
the source of electrical energy needed to operate all vehicle circuits.
The battery goes off-line and becomes a load as it receives electron
current from the generator to recharge. Generator voltage is often
referred to by the term "charging voltage" which is a higher
voltage than battery voltage. Generator electron current which replaces
battery electron current is often referred to by most technicians by
the term "charging amps" implying the electron current
provided by the generator.
If we are to test and evaluate the charging system and
vehicle electrical circuit performance it would require that we measure
both generator voltage and generator electron current underload to
verify the charging system is performing properly. Figure 8 below shows
the generator operating during engine run and the changes in electron
current that occur. But first our focus is on generator electron
current as it flows through the vehicle.
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Fig. 8
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Electrons
always travel from voltage source negative terminal to voltage source
positive terminal. Since the charging voltage created during engine run
is about 2 V higher than battery voltage, we expect the current clamp
readings to be higher during engine RUN. All electron current comes
from -GEN. Normal charging voltage will be in the range of 13.6 - 13.8
V during hot weather to as high as 15.1 V in extremely cold weather. In
extremely hot weather the charging voltage may drop slightly below 13.8
which is perfectly normal for very hot weather. For now, let's stay
focused on generator electron current because this plays a crucial role
in testing a battery that is often overlooked. Generator charging
voltage issues will be discussed in future parts of this series.
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"Smart Charging Systems"
Some vehicles employ what is known as a "smart
charging system" where an onboard computer controls the generator
output. The computer monitors battery recharge current and when it
detects battery recharge current has dropped to a predetermined level
it indicates the battery is fully charged. At this point the onboard
computer reduces the charging voltage to 13.0x V to prevent excessive
battery gassing which is important when the battery is mounted in a
confined compartment in the vehicle other than the engine compartment.
Lowering the charging voltage also reduces the generator's load on the
engine which can result in a slight boost in miles per gallon.
(The topics of charging voltage and charging amps, as well
as "smart charging" controlled by computer is covered at
length in our book "Electrical SHORTCUTS" in Section 6, which
discusses the operation of generators and testing charging system
performance based on voltage measurements.)
For
the remainder of this discussion we will consider this vehicle does NOT
employ "smart charging." The generator is not controlled by
an onboard computer but with an internal voltage regulator. With the
engine running, the generator is online and the battery is off-line.
Motor vehicle operation during engine RUN is important to understand
because this is when the majority of electrical problems show up.
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Current Clamp #1
This reading is taken on the primary, and only ground
strap, so we are assured that the reading indicates total generator
electron current output to all vehicle circuits. It does NOT show
electron current that is recharging the battery.
Current Clamp #2
If the current clamp is attached to the battery negative
cable with the engine OFF and the crank sequence is initiated, the
reading obtained is the cranking amps required by the starter motor to
crank the engine. During cranking, electrons leave the battery negative
terminal, flow down the battery cable to supply electrons to the
starter motor. This produces a high reading in the range of 95 A to a
high of 250 A for most engines.
(The topic of cranking amps - warm weather versus cold
weather cranking - is covered at length in our book "Electrical
SHORTCUTS" in Section 5, which discusses the operation and
troubleshooting of the cranking circuit based on voltage and electron
current measurements.)
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Immediately
after the engine begins to RUN, (the starter motor is disengaged) but a
significant reading is still indicated by the current clamp. This is
called battery "inrush electron current" to begin the
recharging process. During engine RUN the electron current in the
battery cable reverses direction as the generator starts to generate
electron current. Electrons flow up the battery cable and into the
negative battery terminal to recharge the battery.
As the engine continues to RUN, the battery continues to
recharge and the recharge current should slowly decrease. Once the
battery has fully recharged, that is, the energy (state of charge)
taken from the battery to start the engine has been replenished, the
current clamp reading will settle down to a "steady state"
reading which we would like to see settled down to less than 10 Amps in
5 minutes of engine RUN.
Whatever the steady-state current happens to be remains a
constant load on the generator. So, the lower the steady-state battery
recharge current the better it is for the generator. This is entirely
determined by the internal resistance of the battery. Just remember,
the lower the steady-state reading the better the battery during
recharge. High steady-state battery recharge current can burn up a
generator.
(The topics of battery recharge electron current is
covered at length in our book "Electrical SHORTCUTS" in
Section 4 and Section 6, which discusses the operation and testing of
batteries with the engine OFF and engine RUN.).
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Current Clamp #3
This reading becomes significant in evaluating the
performance of Lamp Circuit #1 since the higher generator charging
voltage drives a higher electron current through the circuit. The
higher current will likely cause any voltage drop problem to appear
which may not be noticeable at the lower battery voltage when the
engine was not running and electron current was lower.
Current Clamp #4
This reading indicates the electron current supplying all
circuits grounded to sheet metal #2.
Current Clamp #5
This reading becomes significant in evaluating the
performance of Lamp Circuit #2 since the higher generator charging
voltage drives a higher electron current through the circuit. The
higher current will likely cause any voltage drop problem to appear which
may not be noticeable at the lower battery voltage when the engine was
not running and electron current was lower.
Current Clamp #5
This reading becomes significant in evaluating the
performance of Lamp Circuit #2 since the higher generator charging voltage
drives a higher electron current through the circuit. The higher
current will likely cause any voltage drop problem to appear which may
not be so noticeable at the lower battery voltage when the engine was
not running and electron current was lower.
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In Summary So Far
We conclude that the best time to test the electron flow
in any circuit is when the engine is running and maximum electron
current is flowing through the circuit due to the higher charging
voltage driving the highest electron flow through the circuit. Total
generator electron current supplying all vehicle circuits with electron
current can be determined by adding the readings from current clamp #1
and current clamp #2.
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In Figure 8A we illustrate Current Clamp #6 is placed on
the generator cable connected to the B+ terminal or +GEN. What can we
expect from this reading? It happens to be a very common test procedure
taught in electrical classes to determine maximum generator electron
current. But this reading actually has little useful information.
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Fig. 8A
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Kirchhoff's Law of electron current states:
"At any node (junction - connection) in an electrical
circuit the sum of currents flowing into that node is equal to the sum
of currents flowing out of that node."
We can summarize Kirchhoff's law for electron
current in our generator circuit by saying that every time an electron
leaves the negative terminal of the generator (enters the circuit) an
electron must enter the positive terminal of the generator (leave the
circuit) at the same time. This indicates that the electron current
exiting the generator negative terminal must equal the electron current
entering the generator positive terminal.
So, according to correct electrical theory, during engine
RUN, the reading of Current Clamp #1 plus the reading of Current Clamp
#2 (total electrons leaving the generator) should equal the reading of
Current Clamp #6 (total electron current returning to the generator).
A Problem
The problem putting the current clamp on the generators
positive cable is that it doesn't allow you to isolate how much
recharge current is flowing through the battery. You cannot detect a battery that has excessive battery
recharge current using this method. Excessive battery recharge current
can only be detected from the reading of Current Clamp #2.
Stay tuned for more.
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