Ampmeters (ammeters) v.
Voltmeters
(and Voltmeter Problems)
©
amp&voltmeters.htm-33
I do not have, nor use, ammeters on my airheads normally, BUT I
HAVE instrumented-up an airhead to do charging system maintenance or engineering work.
I am biased AGAINST the use of ammeters on our airheads. I
see no point in having one permanently installed, hardly see much reason to EVER have one, and see MANY drawbacks, not the least of which
is a potential fire hazard, besides the usual situation of LESS reliability when things are modified, added, etc. This
article will not address in any great detail the potential wiring problems of an
ammeter installation; rather, it will be on the relative merits of ammeter vs. voltmeter, although some good mention of wiring problems
will be made, and I will discuss voltmeter reading problems with the stock...or
aftermarket...voltmeter. It is possible to make a crude ampmeter (ammeter)
by connecting a sensitive millivoltmeter across the battery ground cable, using
the resistance of the ground cable and fittings/connections, for the voltage
drop to drive the meter. It needs to be calibrated, but that is fairly
simple.
I do NOT think an ammeter tells you everything; nor enough. A person well-versed in electrical charging systems will get more
information from an ammeter than someone else.
An ammeter placed in the usual and customary position in a vehicle's electrical system, monitors all the current flow to and from the
battery except any starter motor drain. It IS possible for one to be made up that monitors even the starter motor drain when it is in
use. The battery grounding cable type would be such an
installation.
There is nothing inherently wrong with the use of an ammeter, it WAS the method used for decades on cars. Ammeters of the very simple car
types are very cheap, and generally voltmeters cost a small amount more. But, car
manufacturer's have gone to slightly more costly voltmeters for a reason...it
gives more truly usable information....and eliminates the need for more costly large
diameter cables into the passenger compartment, carrying large currents, and
electrical noise, that might get into radios, cassette players, etc.
Thus, over-all, the cost of the voltmeter is well-offset, and gives better results.
Ammeters are of several popular types. The most
accurate is a pricey digital meter wired in series with the load and generating
system, which automatically senses polarity. A very old-fashioned
type is called a FOUR-connection shunt, with a millivoltmeter wired to the
shunt. A simple, yet adequate type, is the old fashioned types used in old
cars and trucks. Those either have studs and nuts posts, or simply a clip
that the wire passes through, with no metallic connection, it operating on the
magnetic field in the wire. Those types of meters are
relatively inexpensive and still widely available. They are excellent for
testing purposes, as they are quite rugged. They are usually of the type
that has a zero current centered needle indication.
On your airhead, with an ammeter, if you were riding along and saw that the ammeter indicated discharging when it normally should show
a slight charging, then you would probably conclude that the electrical load was too high. Of course, there would be many possible causes
including a bad battery or excessive load. A large discharge....and a lighted GEN lamp would likely indicate a total charging system
failure.
***NOTE that the GEN lamp used on all airheads actually tells you quite a bit about the charging system. When that lamp is lit at idle,
and off at rpm over maybe 1500, you can probably rightly assume that the system is working. For the majority of airhead owners, this is
REALLY enough. A voltmeter will, however, give some indication of
the state of charge of the battery, and the system performance.
If you have a voltmeter, in particular an accurate one, it will tell you if the battery voltage is high enough to assume it is charged, or
not. An ammeter will tell you if current is flowing into, or out of the battery,
but will not indicate more than that about the battery. More about that later herein.
The ONE thing an ammeter will tell you a bit more clearly, is if you have a
single diode in the diode board fail, and you will see high charging rate (but a
voltmeter will then show a LOW system voltage!).
A normal ammeter indication in an airhead would be a discharge at idle. That discharge would be from current draws from the headlight,
taillight, running and indicating lights, clothing, radio, and the ignition. 10-15 amperes would be common. As rpm rose, that would
decrease and become a charge, and with rpm high enough, perhaps 4000, that charge value would be initially high, perhaps 10 amperes if you
have the alternator capacity, and later as the battery was replenished and rpm continued at maybe 3500 or more, that charge value would
slowly DEcrease to a small charge amount. If the discharge was 10 amperes at that stop light
and you sat there for a minute, you could expect longer than a minute at 4000 rpm to replenish the power taken out of
the battery, due to inefficiency of charging. Your only indication of the replenishment occurring is
the slow decrease of the ammeter reading....or a voltmeter rising. Granted that the headlight on the road
at night will be a bit brighter as soon as rpm increases.
In the situation of a voltmeter, the voltmeter indicates the system voltage at the point it is connected. A system being discharged will
show a decrease in voltmeter reading, and a system being charged will show the reading
going up. If the stock voltmeter indicates around 12.5 or higher, the battery is probably fully charged, as the BMW airhead
voltmeter will actually read a few tenths of a volt less than the voltage at the battery terminals. Due to how a battery works, it
being a CHEMICAL CHANGE storage device, batteries are 'charged' during riding to a voltage higher than
absolutely needed to maintain a full charge. But, there are limits to this. If the voltmeter reads over about
14.5 (14.9 on some types of batteries), the battery is being overcharged. If the voltmeter falls BELOW maybe 10.5 during cranking....or sitting at idle with the headlight
on, then something (connections or switch contacts?) in the system is bad and
MAYBE the battery is getting ready to fail or is just very heavily discharged. A
truly large variation when the blinkers are operating might indicate a bad battery or poor connections in the system, or a car
voltmeter, not a bike voltmeter. BMW bike voltmeters are dampened ...that is...smoothed/averaged...in reading
any very sudden electrical spike
changes in voltage value from the blinkers. So, if you are replacing
a BMW airhead voltmeter, be sure to get the motorcycle version.
By watching the voltage during various actions: idle, lights on, cranking, recharging, etc.....one can get a pretty good idea of what
is going on in the system. Used with the GEN lamp, even more information is available.
One particular advantage of the voltmeter method is that no large diameter cables and potentially failing connections carrying high
current are needed. There are some other reasons not to use an ammeter on modern vehicles, like alternator whine noise in radios, but
I am not going to get into that here, it was mentioned earlier herein.
The voltmeter tells you what is really going on much better than an ammeter. An ammeter has NO way of telling you the actual state of
charge in the battery. In all honesty, the voltmeter can not, in all instances, either. With a failing single cell, or more, of the
shorting type failure (common), the ammeter may well show a wonderfully nice charge whilst under way, yet the battery is about to
fail. In this case, a voltmeter would indicate a decreased voltage, indicating something amiss.
It is QUITE common to see a battery with a failing cell or two, to not want to
charge up to as high a voltage as it should. This
also can come from a single bad diode in the diode board.
In THAT situation, one little hint here is
that the battery voltage WILL charge up to normal if the headlight is off (and
any special loads like heated clothing).
If the battery had high resistance in one or more than one cell (another common failure mode, and sometimes this is a sudden total &
catastrophic battery failure), the bike would not start well, if at all, and the voltmeter would go down big time during starting. In
this type of battery failure the voltmeter might indicate just fine at decent rpm,
but the voltage will sag a bunch, and fast too, when the alternator is not charging, even from the lights load. Sometimes
with this type of battery failure, the lights are OK, but they dim, greatly or
completely, when the
starter is used. On most of our bikes the headlight is automatically turned off during cranking, and the headlight is a
far larger load than a taillight, so this is often not seen exactly as stated.
The major drawbacks of an ammeter are usually involved with the method of attachment, particularly if you want a real amperes readout.
Some folks use the existing battery negative cable as a shunt...a resistor if you will, and read the millivolts drop across it. Kept
calibrated, this works, and nothing is needed but relatively thin wires to the millivoltmeter. The only thing this sort of
thing tells you is if the battery is being discharged, or charged, and if
calibrated, by how much in amperes. If you are interested in an truly
interesting way...and very cheap... of doing this sort of thing, that can be moderately well calibrated and
expected to remain so over time, and needs NO connections, try to find an ammeter, the zero center car
type, that is a clip-on. This type of ammeter is used by having the battery wire (or alternator output wire
from the LARGE spade lug on the right side of the diode board, as you face the board from the
front....or other place in your bike's system)...pass by, or loop through, the ammeter. NO direct
connection. This type of ammeter was used in some old cars and trucks, and is still being manufactured. It works by
the magnetic field surrounding a wire...any wire....that has current flowing through it.
It is really just a compass! You can multiply the sensitivity of the meter by
looping more times through/by the meter if it has a place to do so. More loops,
more sensitivity. Thus a 60-0-60 meter can be made relatively useful, you could make it far
more sensitive. STILL you must arrange how that wire GETS to this meter, if the meter
is not right at the battery. AND, the meters usually hardly allow such a large wire as the stock battery negative cable,
certainly not to be looped. There is, of course, plenty of room inside the front cover of the engine to allow a commercial 'shunt' to be
installed, which requires a pricey millivoltmeter readout.
****I caution...greatly....that if one does anything like installing
ammeters anyplace (especially hard wired types!!!) that a big fire danger, and other niceties, is commonly a problem.
I feel that a voltmeter is by far best, and that BMW got it right.
VOLTMETER PROBLEMS:
Except for the lousy quality of the mechanicals in the stock meter, an expanded scale meter, they are reliable. And fairly accurate. They
read about 0.3 volt less than battery voltage, due to voltage drops in wiring and switches. They tend to have needle bearing problems, get
sticky (try tapping on the glass with a finger). The voltmeter, if you have one, in the stock airheads, are similar to
the one in a BMW car (and others) except that a DAMPED mechanism is used. That dampening averages out the needles swings...because
motorcycles vibrate...and the dash vibrates a LOT...and the blinker connection would cause a lot of dancing too. Do NOT use a car
unit, and if you have your unit repaired...by such as Palo Alto Speedometer...be SURE
to specify the damped motorcycle movement. Digital replacements are available, sorry, no longer from me...maybe
in the future.
The typical failure for these stock BMW instrument voltmeters is for the needle bearing to act up, causing the needle to stick someplace
for a short while....and you can almost always prove that situation is correct by tapping on the face, and if the needle jumps to a new,
usually more correct reading, it is failing. It is very rare for these to not read correctly if mechanically they are OK.
NOTE that the reading is typically 0.3 or so volt LESS than the ACTUAL voltage at the battery. The reason is where the voltmeter is
connected, downstream from the battery, and the battery has a more direct charging route...in fact, that
charging route BYPASSES the switch!
Voltmeters previously OK, and now swinging, may be the meter (try tapping)...or a
circuit problem. The best way to prove that situation...especially if the use of the turn lights (blinkers) is causing a
much larger voltmeter swing than it used to, is to use a non-digital fairly fast
reacting cheapo voltmeter, and measure the voltage, during the blinking, at the battery posts themselves, and then at various other
places...such as the engine side of the ignition switch, ending up at the voltmeters own input wire. Don't use the lamp wire on the
voltmeter. If you have a fairly fast reacting digital meter, it may well be adequate
to show voltage swinging.
If the connections, switch, etc., on your bike are getting faulty, such as wearing out or corroded from lack of maintenance, you will
find it.
Typically, once this sort of thing starts up (voltmeter OK, connections poor), it is only a matter of a short while before you
will have problems with charging, lights, etc.
Find out if the problem is the meter...or a connection!
Revisions:
05/11/2003: add .htm title
05/14/2003: clarifications; hints