Testing battery and charging system





Slow Cranking System Test


Test Your Charging
and Electrical

A lead-acid
battery has certain
characteristics than
make it easy to
evaluate the health
of the charging
system without fancy
test gear. An in-car test is actually better than taking parts off. I hope this
helps people avoid wasting money on parts. Here’s
how we can check a
charging system with
a few simple pieces
of equipment. Basically, you just need a test light and small test meter.


I have a full diagram of the 1989 Mustang here
1989 Ford Mustang Wiring Schematic



The alternator
converts mechanical
energy into
electrical energy.
There are unavoidable power losses in an alternator. Some losses are mechanical,
primarily heating the alternator bearings and the drive belt. There are also
electrical losses. The diodes drop a little voltage, this makes the diodes get
hot. The alternator’s windings and internal wiring has resistance, and that
causes power loss and heat. The changing magnetic field also causes some losses.
Keep in mind most of the alternator’s load on the
crankshaft comes from the electrical load power drawn from the alternator.

Contrary to myths and what marketing departments and salesmen would have us believe, using
an under drive
pulley system does

free up horsepower when racing. It actually can do the opposite! It frees
up horsepower at idle, but it loads the system heavier when you rev up the
engine as the alternator tries to catch up on missing battery charge! 

alternator shaft speed is reduced, the
voltage regulator
turns up the
field current. The regulator, as long as the shaft is spinning fast enough,
increases field current and pulley torque demand until
the alternator is
back drawing exactly
the same engine horsepower power it
would have consumed turning at normal
speeds! As a
matter of fact, since
efficiency often drops
with reduced rotor
speeds, the
alternator sometimes draws more
engine horsepower and run hotter with
an underdrive
pulley system than with standard shaft speeds!

The only way to
reliably and significantly reduce
alternator drag is
to turn the
alternator off while
racing, although
turning lights and
off during racing certainly helps.
Remember that when
the vehicle is
running the
alternator tries to
supply all the load
energy. At
reasonable to
maximum engine
speeds, typically
from 1500 RPM to
redline with heavy
loads, and from idle all the way to
red line with light
electrical accessory
loads, the battery
just goes along for
the ride. It really doesn’t do anything, except wait for the alternator to fall
below operating speeds.  A
battery will consume noticeable engine power only
when the battery is
low on charge and
the missing charge
is being
replenished. A battery, when charged, is really just an electrical storage tank.

NEVER pull the
battery cable to
check the

This very crude test
method was
marginally OK when we
had vacuum tube car
radios and point-type ignitions, but
it is a very bad
idea now. The
battery stabilizes
the electrical
system and loads the
preventing high peak
voltages or voltage surges
as the alternator
adjusts magnetic
flux to produce the
same average voltage
at different current
demands. If you rev
the engine up and
pull a battery
cable, the
alternator voltage
can spike up to 100
volts or higher
before the
alternator flux dies
off enough to bring
voltage back down to 14 volts
or so. This can kill
the car’s computer
and other expensive
components. I’ve
seen headlights blow
out when a guy
opened a battery
switch while an
engine was revved
up. If you hear
anyone telling
someone this is a
way to check an
alternator in a
modern vehicle, stop

TESTING the ALTERNATOR, is it good or bad?

To charge the
battery, the
alternator voltage
output has to exceed
a minimum charging
voltage. This
minimum charging voltage is
13.8 volts dc across
the battery
terminals, or at the output of the alternator. A single
lead-acid cell
starts to charge at
anything over 2.25
volts. Since a 12
volt battery has six
cells, any 12 volt lead-acid battery needs at
least 13.8 volts to
to charge. This
voltage will be
enough to fully
charge or maintain
the battery on a
trickle charge, but
charging time will
be very long at 13.8

To fully charge in reasonable times, alternator
output  must be
14.2 V to 14.5 V
as measured

right across the
battery posts
. Above 14.5 volts charging voltage,
batteries have a greatly increased tendency to release excessive acidic vapors,
hydrogen gas, and to corrode things around the battery. The battery terminal
charging voltage must be
less than 14.7 volts to prevent excessive gassing. Charging voltages over
14.7 volts can prematurely dry the
battery by boiling out electrolyte, and increase risk of
a battery hydrogen gas explosion.

In this case the
battery charging
voltage is 14.61
volts with the
engine at high idle.
14.4 volts is the
gassing threshold.
The battery above will
slightly gas, but
not enough to be
harmful, and the
battery will get a
quick full charge
recovery after
starting. 14.8 would
start to be a worry
(there might be
liquid or corrosion
on the battery) and
15 volts would be a
real concern, but
14.6 is fine. Less
than 14.3 would be a
“weak” alternator or
regulator. Much less
than 14.2 at a fast
idle is bad wiring,
bad alternator or
regulator, or a bad
connection or fuse
link. When running at a normal slow cruising engine RPM, the voltage across the
battery terminals should
stay over 14.3 volts even
with full load, like
lights, heater
blower, and
everything else,
running. If this
system was in a
restored 1966 GT
coupe, I would
probably change the
regulator to reduce
maximum alternator
voltage. This would
deterioration of
metal around the
battery from
excessive charging
vapors. It’s OK in my daily driver as long as I watch the battery for purged
acid deposits.

If you measure battery voltage and it is anywhere over 14.2 volts and less
than 14.8 volts with the car running at slow cruising engine RPM speeds and maximum
loads on, you already have a bigger alternator than you need. If voltage is over
14.2 under maximum loads when at cruising RPM, buying a bigger
alternator or a new alternator is a waste of time and money. 

Turn the motor
off with no load
(headlights, etc. all OFF)
and read the
battery voltage.


With the engine just shut off, the battery
voltage should be
13.2 volts to 13.8
volts. The exact voltage varies with the battery, how
quickly you read it,
and the state of
battery charge. This voltage
is not too important
because the battery
will slowly and
steadily settle to a
new voltage that
indicates the true state of
battery charge, but the voltage measured right at engine shutoff is a very clear
indicator if
the alternator or charging system is charging. If the voltage is over 13.2, the
battery was just being charged.

So what happens if your battery goes dead all the time, but the alternator seems

Measuring electrical
system leakage


To test the electrical system for unwanted loads draining
power, shut everything in the car off. Do just like you would do things when
parking the car overnight. make sure all the lights and accessories are off.

Remove the
negative post wire and
check current draw
with all electrical
loads off using a
test light. (I made a test light from an old tail lamp bulb.)

The dull glow in
the light filament indicates a
current drain problem. At
this point I do


want to connect a
current meter to
check leakage
because the short
could damage the
test meter! If a small
clear test lamp like
this does not
light, then it is
generally safe to
directly measure current
drain with a test meter.

Parasitic Current

With all
electrical loads off
connect the meter,
on a low amperes
scale of about 1
ampere or so, in
series with the
battery negative
post to
ground. The positive
meter lead connects to
the car chassis, and
negative meter lead
to the
negative post of the

good electrical
system battery drain

This is measured
on the 20mA scale.
The mA scale reads
in thousandths of an
ampere. My 1989 Mustang LX,
after I changed a
bad alternator
diode, now
has about 1.73 mA of
battery drain. This
drain is all from
the EEC-IV computer
memory. Different
radios and different computers might have
different standby
drains, as might  accessories like
clocks, but in no
case should “overnight off” leakage
exceed 25 mA or so.
100 mA is like
leaving a small
interior dome light on!

My Kenwood stereo
draws 1.5 mA when
connected. If you
have a digital clock
that stays on, an
alarm, or some other
load this current
will be higher. At
75 mA, leakage could
compromise battery
life of infrequently
driven vehicles. mA is milliamperes, or one 1000th’s of an ampere.


The meter above is on the 20 mA scale, and is reading 1.73 mA. That is
nothing. The battery charge would probably last for months of sitting.

battery parasitic Drain

If the test light
lit, you will want
to locate the wire
loading the battery.
First make sure all
lights are off. You
can do this by
having someone open
and shut things with
lights, like the
trunk, and watching
for a define large
load change. You
should see a
definite load change
when shutting doors
with lights, like
the glove

Connect the test
light in series with
the negative post,
and start pulling
feed wires. The
first to check is
the heavy charging
wire from the
alternator.  A
bad or leaky diode
in an alternator is
a very common source
of overnight battery

Connect wires one
at a time to see
what lead is drawing
current. In my case
it was the
alternator lead!
Even though the
alternator was
charging fine, it
was also draining
the battery. My
problem was a bad
alternator diode.
There could be a
variety of other
problems, like a bad
voltage regulator or
a stuck relay

Download a wiring

I downloaded this

T. Moss’s website
which I find much
more useful than
other sources.
Tom Moss goes out of his way to help people, and is a genuinely nice fellow. AutoZone and others
have some free
schematics up also.


diagram (linked above)
showed me the heavy
dark green wire from
my starter relay
causing my “drain problem”
went directly to my
car’s alternator output. 
In my case one of
the diodes
(the little black “arrows”) in my car’s
alternator was bad.
This current loss also made my
alternator slightly
warm to the touch,
even when sitting
off for several hours.


Helpful Voltages

Battery voltage can be higher than 12.6 volts immediately after charging.

Open Circuit
Voltage of 12V
battery after
car is off for
one hour
12.6 V 100%
12.4 V 75%
12.2 V 50%
12.1 V 25%
Less than 12

Any open terminal
voltage below 12
volts is considered
full discharge or a
dead battery.


Testing Starters

Starters sometimes can test good out of the car, but can be bad. One common problem
with cheaply built or defective starters is starting torque loss when hot. This
usually happens because the iron loses ability to hold
magnetic flux (starter current skyrockets when hot), or because the wire is
undersized and goes up in resistance (starter current drops when hot), or the
starter seizes up (also causing high current).

The best way to test a starter is by measuring voltage and current.

To test a starter and wiring with a simple meter:

  • Clip the meter positive lead to the starter feed
    wire going into the starter
  • Clip the meter’s black negative lead to the ENGINE BLOCK
  • Make sure the meter is on volts, and set to the
    lowest voltage scale possible that reads at least 15 volts.  In other words if
    your meter has 2.5-volt, 25-volt scale, and 250-volt scales, use the
    25-volt scale. The 25-volt scale is the closest scale to 15 volts, but is not
    under 15 volts.
  • With the meter clipped across the starter, watch the meter while cranking the engine.

Make sure the battery is good. There is a voltage table above for battery
charge. The no load (everything off) battery post voltage should be at least
12.6 volts and as high as 13.8 volts.

If the starter cranking voltage
goes below 9-10 volts, you have a problem with starter current, engine
grounding, or battery.

Measure across the battery by probing directly into the
battery posts (NOT the terminals that clamp to the posts, but poke directly into the
lead posts coming out of the battery), and see how much the battery
drops when cranking. If it drops and you are sure the alternator is working, take
the battery to an automotive parts store that
tests batteries. Unlike starters, testing batteries is VERY easy and very

If the battery stays up on the posts and the starter voltage sagged down, you probably have a
bad starter wire, ground wire, or some other wiring problem. If the battery post
voltage tests good but sags abnormally with your starter, you probably have a starter problem.
You need to have starter current checked.

Cheap or poorly manufactured starters show up mostly when the starter is very
hot. Starters very often cannot be tested accurately on a bench, because they
often can fail
only when very hot. I see very few tractors, cars, and trucks that crank OK when
cold and do not crank when hot that have problems other than a starter! My
diesel tractor was a bitch to start when hot, but cranked like a dream when
cold, and it was the starter. My tractor does not have headers, either. Just the
block heat was enough to make the starter quit. I’ve had the same experience with cars. When cold,
the starters work and test good! Marginal starters can have enough oomph to
start properly when the system is cold, but fail when hot.

Bad alternators or batteries show up mostly when the car is very cold, but
both alternators and batteries can be tested reliably to see if they are good.

Installing a larger alternator will not fix a bad starter, bad battery, or
bad wiring.

      Changing to LED


LED charging indicatoridiot light