5. HOW DO I KNOW IF MY CHARGING SYSTEM IS OK OR LARGE ENOUGH?
Last Updated on July 11, 2004
INDEX:
5.1. What If My "Battery" or "Alternator" Light Is
On? (Or the Gauge Is Not Showing a "Charging" Condition?)
Charging System Functional Diagram
Alternator Output Graph
Vehicle Charging Voltage Graph
5.2. What If I Cannot Keep My Battery Charged and the
Battery Tests OK?
5.3. How Can I Test To Determine If Charging System
Large Enough?
5.1. What If My "Battery" or "Alternator" Light
Is On? (Or the Gauge Is Not Showing a "Charging" Condition?)
The "Battery" or "Alternator" light
is an indication that there is a significant mismatch between
the voltage that the charging system is producing and
the battery voltage. Some vehicles use a voltmeter
or current meter to indicate if the charging system
is working. The battery and charging system must work
together to provide the electrical power for the vehicle
and to keep the battery recharged so it can restart
the engine. The most common causes, in the order of
priority, are:
- Low electrolyte levels
- Slipping or broken alternator belt
- Corrosion between the battery posts and
the battery cable terminals
- Faulty charging system
- Defective battery
If the electrolyte levels, alternator
belt is OK and the battery terminal connections are
free from corrosion, then take your vehicle to an auto
parts or battery store, and have the battery and charging
system tested (highly recommended) or use the troubleshooting
guide below. Some stores like Auto Zone, Sears, Wal-Mart,
Pep Boys, etc. in the U.S. will test them for free.
One of the first three simple faults in the list above
has caused many a good battery to be replaced. A new
battery can cause a weak alternator or starter to fail.
Referring to Dan Masters' diagram
below, a vehicle's charging system is composed of an
alternator (or DC generator), voltage regulator, battery,
and indicator light or gauge. While the engine is running,
the charging system's primary purpose is to
provide power for the car's electrical load, for example,
ignition, lighting, audio system, accessories, etc.,
and to recharge your vehicle's battery. Its output
capacity is directly proportional to the RPM of the
engine and alternator temperature. Charging systems
are normally sized by the car manufacturers to provide
at least 125% (when operating at high RPM) of the worst-case
OEM (Original Equipment Manufacturer) electrical load,
so that the car battery can be recharged. That is the
reason that short, stop and go driving at night or
in bad weather might not keep the battery fully recharged,
especially if the electrical load has been increased
with after market accessories, such as high power audio
equipment.
CHARGING SYSTEM FUNCTIONAL DIAGRAM
[Source: Vintage Triumph Register]
In the Balmar Alternator Output
diagram below, the power output curves are shown
65 amp and 85 amp alternator. Note that the 65 amp
alternator in this example, produces more current
output (power) at a lower RPM that does the larger
alternator until approximately 3300 RPM. Also note
the difference that the crankshaft pulley size makes.
A larger crankshaft pulley will create a higher alternator
RPM; thus, causing the alternator to produce more
power at a lower engine RPM. An alternation require
one horsepower on a diesel engine to produce 20 to
25 amps and for gas engine 10 to 15 amps.
ALTERNATOR OUTPUT GRAPH
[Source: Balmar]
When the charging system fails,
usually a "battery" or "alternator" warning indicator
or light will come on or the voltage (or amp) gauge
will not register "good". If you increase the engine
speed and the alternator light becomes brighter,
then the battery needs to be fully recharged and tested.
If the light becomes dimmer then the problem
is most likely in the charging system. The indicator
(also known as an "idiot") light is a direct comparison
between the voltage output of charging system and the
voltage output of the battery. The next test requires
use of a known-to-be-good, fully charged battery. Temporarily
replace the old battery with this battery and run the
engine at 2500 RPM or more for two minutes. Depending
on the load and ambient temperature, the voltage should
increase to between 13.0 and 15.1 volts during this
period. Most vehicles with good charging systems will
measure between 13.8 and 14.8 volts on a warm day,
depending on the battery type that the charging system
was designed for.
As in the Bosch Voltage Regulator
example below, most voltage regulators are temperature
compensated to properly charge the battery under
different environmental conditions. As the ambient
temperature decreases below 77° F (25° C),
the charging voltage is increased to overcome the
higher battery resistance. Conversely, as the ambient
temperature increases above 77° F (25° C),
the charging voltage is decreased. Other factors
affecting the charging voltage are the alternator
temperature, battery's condition, State-of-Charge
(SoC), sulfation, electrical load and electrolyte
purity.
VEHICLE CHARGING VOLTAGE GRAPH
[Source: Bosch]
If a battery terminal's voltage
is below 13.0 volts with the engine running and the
battery tests good after being recharged or if you
are still having problems keeping the car battery charged,
then have the charging system's output voltage and
load tested. Also, have the car's parasitic load, the
electrical load with the ignition key turned off, tested.
(Please see Section 10.)
A slipping alternator belt or open diode will significantly
reduce the alternator's output capacity. If the output
voltage is above 15.1 volts with the ambient temperature
above freezing, if the battery's electrolyte level
is frequently low, "boiling", or if there is a "rotten
egg" odor present around the battery, then the battery
is being overcharged and the vehicle's charging system
should be tested.
[back to Index]
5.2. What If I Cannot Keep
My Battery Charged and the Battery Tests OK?
The vehicle's electrical load is
normally satisfied first by the charging system
and then any remaining power is used to recharge the
battery. For example, if the total electrical load
is 14 amps and the charging system is producing 35
amps at 2500 RPM, then up to 11 amps will be available
for recharging the battery, which will take approximately
six minutes. If the charging system is operating at
say a maximum capacity of 90 amps at 5000 RPM, then
the battery usually will be recharged within two minutes.
Now let us assume that the engine is idling and the
charging system is only capable of producing 10 amps.
Four amps from the car battery are required to make
up the difference to satisfy the 14 amp electrical
load and the battery is being discharged further. This
is why making short trips, driving in stop-and-go traffic,
or during bad weather, the starting battery may never
get recharged and may even become "completely" discharged.
Using the example above, let's
assume that an after-market, high-power audio system,
electric winch or lights is installed that adds an
additional 20 amps of load. With a total electrical
load of 34 amps, at RPM below 2500, the battery will never be
recharged with an 90 amp system. While the engine is
running in this case, the battery must make up the
deficit. The solution is to upgrade the charging system
to 125% or more of the new worst-case load.
In this example and based on stop-and-go driving habits,
a high output charging system capable of 105 amps or
more would be required to keep the battery fully charged.
High alternator temperatures can further reduce the
maximum output of a charging system, so cooling and
sizing based on the continuous load matters. Heat kills
alternators, so Bosch, for example, has water cooled
models available.
[back to Index]
5.3. How Can I Test To
Determine If Charging System Large Enough?
A simple test to determine if the
charging system is large enough is to check the battery's
State-of-Charge after the surface charge has been removed. If the State-of-Charge is consistently above 95%, then
the charging system is fully recharging the car battery
based on your driving habits and electrical load. If
is is consistently below 80%, then you will want to
consider upgrading your charging system to produce
more current. There are several possibilities to increase
the capacity of your charging system to include changing
the pulley diameters, replacing the voltage regulator,
upgrading the alternator, adding a second charging
system (for a dual battery
set up), etc. An auto electric or alternator rebuilding
shop can assist you. If the SoC is inconsistent, the
you might consider using a temperature compensated, "smart" charger with a quick disconnector to "top
off" your battery. If consistently under or overcharged,
a lead-acid battery will lose capacity and prematurely
fail.
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