Date: Wed, 14 Jun 95 10:30:51 CDT
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: Joel Walker <JWALKER@ua1vm.ua.edu>
Subject: article: Alternator Charging Systems (long)
ALTERNATOR CHARGING SYSTEMS
How to avoid electrical generating problems - Motorhome, July 1995
by Joel R. Donaldson
If most of your camping is done with external AC hookups, you may never
know if your motorhome's alternator and associated charging system are
up to snuff. However, frequent forays into the sticks will quickly
point out any system shortcomings. After one or more nights of dry
camping with the television, lights and furnace running full tilt, you
just may find that your charging system is hard-pressed to replace all
those borrowed amp-hours within the next day's allotted driving time.
In a sense, an alternator has it tough compared to your motorhome's
converter/battery charger, which usually has at least several days to
replenish the power removed from a battery during an occasional dry
camping trip. However, your alternator is frequently called upon to
provide this recharge in just a few hours of driving. And, if your
electrical needs repeatedly outstrip that charging capability (either
due to malfunction or insufficient capacity), you're eventually headed
for a night of camping in a cold, dark coach!
POWER PRODUCTION PRIMER
The manner in which automotive charging systems work is pretty
straightforward (see Figure 1). A drive belt spins the alternator's
rotor, an arrangement of field coil wires that makes a rotating
electromagnet within the alternator. The resultant spinning magnetic
field induces an electrical current within the alternator's stator,
a stationary set of wire coils arranged in a circle around the rotor.
Due to the rotor's motion, this current rapidly reverses electrical
polarity -- it is an alternating current. AC is inherently unsuitable
for recharging batteries, so a set of diodes (essentially one-way
electrical check valves) is mounted within the alternator to rectify
this AC into DC.
Although a single stator coil could be used, most automotive
alternators actually have three of them, arranged so the alternating
current being produced by each coil "peaks out" at a different, yet
equal, time interval from the other two. This "three-phase" arrangement
provides a smoother flow of electrical power, in much the same manner
as an automotive crankshaft staggers the action of each piston in order
to provide a smoother flow of mechanical power. A set of two diodes is
used with each of the three stator windings (although only one is shown
in the illustration for clarity sake). The DC output of each diode set
is internally connected to the others before being delivered to the
battery and vehicle accessories through a single stator connection.
A voltage regulator is used to shut off the alternator's output as
soon as the battery is recharged. The regulator does this by constantly
measuring battery voltage, which is a fairly accurate indicator of the
battery's state of charge. As soon as the battery voltage rises to the
proper value, the regulator cuts off the electrical current flowing to
the alternator's field coil, at which point alternator output ceases.
As electrical accessories start to siphon power out of the battery, its
voltage drops to the point at which the regulator reenergizes the
alternator's field coil, and recharging resumes.
FIGURE 1 - Basic Charging System Diagram
.......................................... Chassis
: diodes : Stator Battery
:|-------------------------------->I---| : Terminal + -
:| |------------------------------>I---|---o-----------o-I---------I-|
:| | statorII------------>I---| : DC Charging | I CHASSIS I |
:| | coil II : Current --> | I BATTERY I |
:| | II______Ground : | I_________I |
:| | ...... : Field | |
:| | rotating . ...... . : Terminal | |
:| | field . . II--o /----------------o | Ground
:| | coil o____II . . field brushes: | |
:| | . ...... \_____________ : | |battery voltage
:| | .. .. .. | : | |measurement wire
:| | //---| |----\\ | : | |-----------|
:| |--------// | | \\ | : | .....................|..
:| stator Ground stator | : | : electronic | :
: coil ALTERNATOR coil | : | : switch | :
:........................................: |--:-o \o---------------| :
: |-----------<I----| :
: switch voltage :
: turn-on sensor :
: signal :
: VOLTAGE REGULATOR :
:......................:
JUST SAY "CHARGE IT!"
After driving all day only to find the house batteries half empty, the
gut reaction of most motorhome owners would be to consider installing
a higher-output alternator. However, while increasing the alternator's
maximum output current can decrease charging times, this is only
possible if the rest of the charging system is performing adequately.
Therefore, it makes sense to examine the entire charging system before
plunking down the money on any one item.
In a nutshell, the amount of driving time necessary to do a complete
battery recharge primarily depends upon five factors:
1. The size and quality of your alternator.
2. The size, state of charge, type of construction and overall
condition of your house batteries.
3. The design of your voltage regulator.
4. The quality of wiring and connections between your alternator and
house batteries.
5. The amount of power being consumed by other electrical accessories
while you're driving (e.g., headlights, air-conditioning,
refrigerator, etc.)
FIRST, the BATTERIES
No charging system can fully compensate for an inadequate house-battery
system. If your batteries are too small, too old or have been neglected
to the point where they can't meet your routine camping needs, then it's
time to go battery shopping (see "Battery Power," Motorhome, August
1994). From a practical standpoint, your battery bank is being over-
worked if it's routinely discharged to less than 20 percent of its
original rate capacity.
ALTERNATOR ALTERATIONS
The capacity of your house battery bank also plays an important role
in sizing an alternator upgrade. With virtually all batteries other
than gel-cells, the maximum charging current should never exceed 40
percent of the total amp-hour capacity of the entire house battery
bank. In other words, with two 115-amp-hour house batteries (a total
of 230 amp-hours), the maximum charging current should never be
allowed to exceed 92 amps, and only when a high-quality (read: after-
market) voltage regulator is controlling the recharge profile.
Under these nearly ideal conditons, you could expect to bring the
batteries from fully discharged up to an almost full charge in about
four hours of driving time, assuming the bulk of the alternator
capacity goes to charging batteries. For instance, if a three-way
refrigerator is on 12 volts DC (16 amps plus) and dash air-conditioning
and headlights are being used, considerably more time will be required.
For gel-cell batteries, maximum charging current should never exceed
50 percent, and again, only with a really good regulator. Due to a
gel-cell's higher maximum charging current and better charging
efficiency, you'd only need around three hours of drive time for a
full recharge under ideal conditions.
At the other extreme, an alternator with a maximum output current that
is less than 25 percent of the house battery bank's total amp-hour
capacity (wet or gel-cell) is generally considered to be under-sized.
Also, since the maximum output of an alternator can drop as much as
20 percent as it warms up, it doesn't hurt to err on the high side
when selecting the minimum acceptable size.
Among the available aftermarket alternators, most will fit GM and Ford
designs. An adapter mount may be necessary for use in Chrysler vehicles.
Bear in mind that the heavier drag created during full power ouptut
will typically mandate more belt tension than with a stock low-output
alternator, and this could shorten belt and water pump life a bit in
some applications. Consequently, it probably wouldn't hurt to carry
spares.
NO REGULAR REGULATOR
Some factory-stock voltage regulators create performance bottlenecks
for a high-output alternator. The three-step voltage regulators
available in the aftermarket, primarily from marine suppliers,
precisely meter the charging current, using battery state of charge,
temperature and type of battery construction as adjustment factors.
Consequently, the batteries live longer, in spite of being recharged
faster. These designs can also protect an alternator from damage due
to overheating, since this aggressive recharge profile typically makes
the alternator work quite a bit harder than in a stock system.
Compared to a stock regulator, they aren't cheap, but they will
frequently pay for themselves in longer battery and alternator life,
and in less engine run-time.
TIRED WIRES
In many "stock" motorhome charging systems, the wiring and connectors
are woefully inadequate. Frequently, this means that the charging
voltage that reaches the house batteries is nowhere close to the
voltage "seen" by the regulator. Consequently, the alternator stops the
recharge process long before the batteries are fully charged and
chronically undercharged batteries are the result. Unfortunately,
installing a higher-output alternator won't change things much. Proper
wire size is essential. Use a wire chart (available in The RV Handbook
from TL Enterprices, (800) 234-3450) when selecting the appropriate
wire gauge.
Adequate current-carrying capacity in the connectors is also important.
Methods for splicing and terminating wires should also be checked.
Butt-splice and ring-terminal connections that are crimped should be
avoided in favor of their soldered counterparts. Finally, don't over-
look the importance of good electrical grounds. Broken or omitted
engine-to-frame ground straps are commonplace, as are corroded ground
connections.
IN DEFENSE OF ISOLATIONISM
The purpose of any isolator is to allow the alternator to simultaneously
charge both battery banks, while preventing the chassis battery from
being discharged whenever the house batteries are being used.
Solid state isolators are probably the most popular method of
accomplishing this, consisting of a set of high-current diodes that
act as electrical check valves by only allowing current flow from
the alternator to each battery bank.
A properly installed isolator (see Figure 2) will allow the alternator
to recharge two or three battery banks independently, based on their
individual needs. However, if the isolator is not installed correctly,
the alternator will not automatically compensate for the approximate
0.8-volt loss, depending on the diode type, that occurs across diodes.
Schottky-type diodes have less voltage drop than conventional diodes.
This voltage sensing problem can usually be corrected by reconnecting
the voltage regulator sense line to the isolator output, which causes
alternator output voltage to increase about 0.8 volts, or by adjusting
the alternator up by 0.8 volts if adjustment capability is available.
FIGURE 2 - Typical Wiring Diagram Using a Solid-State Isolator
+ -
To House Electrical Applicances<---| |-----HOUSE BATTERY-----|
| | Ground
stator + - o
...o------------AMMETER---------o ISOLATOR
. . o STARTER
ALTERNATOR |------------------SOLENOID
| . o |-----------RELAY-->To
| .. \-------| |----------IGNITION------| | Starter
| field | | SWITCH |
| | | | | |
| | | | |+ |+
| VOLTAGE | VOLTMETER CHASSIS
|------------REGULATOR | |- BATTERY
Ground | | |-
| | |
To Ignition<----------------| Ground-----------------|
Finally, a solid-state isolator may limit the usefulness of any
ammeter present, depending on where the ammeter is installed. Either
the ammeter never reads discharge current, or does so for only one of
the two battery banks. Isolation relays (also called solenoids and
contactors) may temporarily eliminate all of these problems by
bridging both battery sets together whenever the vehicle engine is
running (see Figure 3). Their downside is that they aren't as reliable
as solid-state isolators. Contact points deteriorate over time,
creating resistance and voltage drop. Note that in lieu of directly
controlling the relay with the ignition swith, as indicated, special
isolator control circuits are also available that automatically activate
the relay when the main battery is charging properly, or when extra
engine-cranking current is required.
A third option is an "active" battery isolator. With this arrangement,
the alternator's output is connected directly to the house batteries,
which are in turn connected to the chassis battery through the active
isolator. This device siphons electrical power off the alternator via
the house batteries to recharge the chassis battery as needed, using
an electronic switch to prevent any power drain back in the other
direction. A disadvantage is the remote location of the house batteries;
in many motorhomes this may complicate the installation task. A second
disadvantage is that these devices are normally limited in their current
capability, and have an inherent 0.3- to 0.5-volt non-compensatable
current loss.
FIGURE 3 - Typical Wiring Diagram Using an Isolator Relay
+ -
To House Electrical Applicances<-----|-----HOUSE BATTERY-----|
ISOLATOR Ground
stator RELAY
...o-----| | |
. . | + - | |-----------------STARTER
ALTERNATOR |------|AMMETER------)--------------------SOLENOID
| . o |--------| | |-----------RELAY-->To
| ... \------| |----------IGNITION------| | Starter
| field | | SWITCH |
| | | | | |
| | | | |+ |+
| VOLTAGE | VOLTMETER CHASSIS
|------------REGULATOR | |- BATTERY
Ground | | |-
| | |
To Ignition<----------------| Ground-----------------|
HOW TO PROPERLY ACCESSORIZE
Along with an upgraded charging system, the addition of an ammeter can
be an asset, especially in confirming proper charging-system operation.
However, care must be taken to insure that inadequate wire size doesn't
introduce excessive voltage drops. Using an external-shunt ammeter will
avoid this pitfall, albeit at some extra expense.
A voltmeter also can be used to keep an eye on charging-system
performance, although in more of an after-the-fact manner.
Unfortunately, most analog-type voltmeters lack sufficient resolution
and accuracy to make them useful in determining battery state-of-charge,
regulator cutoff voltage, etc., and dashboard digital voltmeters are
not commonly found.
TROUBLESHOOTING TIPS AND TIDBITS
In determining a charging-system malfunction, the wiring diagram usually
included in the original factory service manual for the motorhome
chassis can be a big asset in identifying pertinent wires and
connectors. Most service manuals also include electrical specifications
for an alternator and regulator, as well as model-specific trouble-
shooting procedures. If you don't have a manual for your vehicle,
seriously consider purchasing one or looking for a similar manual at
a nearby library.
Before you start troubleshooting, be sure that something's really
wrong. For instance: Some stock ammeters use one of the battery cables
as an external shunt. If this cable eventually gets shortened or
replaced with a non-similar cable, the ammeter readings will be
inaccurate. This can lead the operator into believing that something
is wrong with the charging system, when in fact it's just a meter
problem. Again, the service manual can help catch this sort of error
before you're knee-deep in electrical parts.
In instances where the charging system is working (albeit poorly), use
a digital test meter to troubleshoot. Measure the voltage at the house
battery bank while the vehicle is running at a fast idle. Make sure a
significant electrical load is turned on inside the RV (all interior
lights are on, or the three-way refrigerator is set for DC with the
thermostat set in its coldest position). Compare this voltage with a
measurement directly at the alternator's output terminal. (Note: A
digital test meter is necessary for this test; analog-type meters lack
sufficient resolution. Also, the house and chassis batteries should be
fully charged before making this test.) On a normal charging system,
the voltage at the battery should be between 13.2 and 14.5 volts, and
should be within 0.6 volts of the reading at the alternator output
terminal (unless a solid-state isolator is used, in which case, an
additional difference of about 0.8 volts).
If the house-battery voltage is below the range, use the test meter
to make measurements at various points between the alternator and the
chassis battery. Do this until a poor connection, corroded connector
or undersized wire is found. Voltage drop tests must be done with the
circuit in operation. The engine should be running at a fast idle so
the alternator is charging, so use caution to avoid moving parts and
burns.
Don't overlook poor connections along the return (the chassis ground)
path, as well as the battery terminals themselves. It's possible that
no single trouble spot will be located, but rather several items that
significantly contribute to the total voltage drop. In this case, fix
the worst offenders first, until the total voltage drop is within the
values given here.
A reading above 14.5 volts indicates a problem with the voltage
regulator or its wiring to the ignition system, battery or chassis
ground. (This may not be true when a solid-state isolator is used and
the voltage reading is taken at the alternator output terminal rather
than at the output terminal on the isolator.)
Use the test meter to check each of these paths for more than 0.2 volts
of drop, with the engine running. If the charging system is completely
dead, an external regulator can usually be bypassed to determine whether
the alternator itself is at fault. To do this, use a service manual to
determine whether the alternator field terminal needs to be grounded,
connected to the battery's positive terminal or both (where an
alternator has two field terminals) to initiate charging. Disconnect
the original field wires and use test clips to make the appropriate
substitute connections between the terminals and the battery or ground.
When the vehicle is idling, completing the field terminal circuit
should make the engine "load down" slightly, and battery voltage should
quickly rise to at least 13.2 volts. If not, the alternator is suspect.
For alternators with internal regulators (and others with intermittent
or insufficient output), it's probably best to have a complete
electrical test performed at an automotive parts supplier or repair
shop. Some alternator and starter specialty shops are capable of
alternator rebuilds for considerably less than the cost of a new unit.
Rebuild parts kits are also available from some manufacturers of high-
performance aftermarket alternators. Most stock external regulators
are sealed and potted in epoxy --- they aren't adjustable or
serviceable. So, aside from cleaning the occasional dirty connector,
replacement of the entire unit is usually necessary.
Whether you do the work yourself or take it to a shop, a thorough
understanding of the charging system can help you avoid problems
and get more enjoyment from your rig.
SOURCES----------------------------------------------------------
Ample Technology Central Hi-Power
2442 N.W. Market Street, #43 1075 Nevada Street
Seattle, WA 98107 Long Beach, CA 90806
(206) 784-4255 (213) 427-6813
Heart Interface Hehr Power Systems
21440 68th Avenue S. 4616 Fairlane Avenue
Kent, WA 98032 Fort Worth, TX 76119
(206) 872-7225 (214) 663-1261
Intellitec Leisure Components
131 Eisenhower Lane N. 16730 Gridley Road
Lombard, IL 60148 Cerritos, CA 90703
(800) 251-2408 (310) 924-5763
Lestek Manufacturing Powertap Incorporated
6542 Baker Boulevard 1513 N.W. 46th Street
Fort Worth, TX 76118 Seattle, WA 98107
(817) 284-0821 (800) 541-7789
Revolution Technologies Sure Power Industries
Amptech Division 10189 S.W. Avery
2305 Montgomery Street Tualatin, OR 97062
Fort Worth, TX 76107 (503) 692-5360
(800) 364-9966
Taytronics Vanner Weldon Incorporated
430 Ritt Street 4282 Reynolds Drive
St. Peter, MN 56082 Hilliard, OH 43026
(507) 931-1406 (614) 771-2718
Wrangler Power Products
P.O. Box 12109
Prescott, AZ 86304
(800) 962-2616
