Ampmeters (ammeters) versus
Voltmeters
(and Voltmeter Problems)
©
Copyright, 2011, R. Fleischer
amp&voltmeters.htm-33
I do not have ammeters normally
permanently installed on my BMW Airhead motorcycles, BUT I
HAVE instrumented-up many an Airhead with them to do charging system maintenance or engineering work.
Quick summary:
I
see little reason in having having an ammeter permanently installed.
I foresee MANY drawbacks, not the least of which
is a potential fire hazard, besides the usual situation of LESS reliability when things are modified
and added. Voltmeters CAN be very useful.
This
article will not address in any great detail the potential wiring problems of an
ammeter installation. This article will cover the relative merits of ammeter vs. voltmeter,
and possibly using both. A good mention of wiring problems
will be made, and I will discuss voltmeter reading problems with the stock... or
aftermarket... voltmeters.
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 if you have the equipment. There are plenty of other types of ammeters,
such as the shunt
in-circuit type (especially if standard calibrated shunts are
used); the proximity type; and the clamp-around-probe types
with digital readouts. Clamp-around probes are very good in
general, but the DC types are rather pricey.
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; I see NO advantage unless testing the starter motor for
current drain. Using a millivoltmeter across the battery
grounding cable type would be such a way, a very safe way too,
but you have to find a way to calibrate it, not very easy
generally.
There is nothing inherently wrong with the use of an ammeter, it WAS the method used for decades on cars. Ammeters of the simple car
types are very cheap; generally voltmeters cost more. But car
manufacturer's have gone to slightly more costly voltmeters for a
reason, they
give more truly usable information and eliminate the need for costly large
diameter cables into the passenger compartment, carrying large currents and
electrical noises, that might get into radios, cassette players, etc.
Thus, over-all, the cost of the voltmeter is well-offset, and gives better results
in several ways.
Voltmeters REALLY DO give more over-all information, if one knows how to
use their information.
They do not tell you directly if the system is draining or charging, although
that can certainly be identified from the voltage reading.
Ammeters are available in several
popular types.
One of the best is an old-fashioned
type is called a FOUR-connection shunt, with a millivoltmeter
whose scale reads in current, not voltage, wired to the shunt.
A good one of these is almost a laboratory type device.
A simple, yet adequate type for most uses, although not generally
capable of very tiny current measurements, is the old fashioned
types used in old cars and trucks. Those either have studs
and nuts posts and have some
sort of real measuring d'arsonval mechanism inside, or are really
cheap, and are simply a clip that the wire passes through, with
no metallic connection, the 'meter' operating on the
magnetic field in the wire. Those types of meters are
inexpensive and still widely available. They are excellent for
testing purposes, as the clip types in particular are quite rugged.
They usually have a zero current centered needle indication, showing a discharge, or a charge, if wired into the battery
circuit. They can not be wired into the starter
motor circuit, as the current drain is too high. Typical of
these meters, their face may show figures like these:
-30--0--+30; or they may be 60 or 50 amperes instead of 30.
One can place these in your bike's alternator circuit if you wish
to do it this way, to just measure the alternator output;
OR, you could place it in the circuit after the ignition switch. The ideal place for a permanent
installation, as it will tell you if the system is charging or
discharging the battery.... is in series with the NON-starter
wire in the battery circuit.
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 or there was a problem in the alternator system. A large discharge....and
possibly a lighted GEN lamp, would probably 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 then goes off at some rpm around 1500, you can probably rightly assume that the system is working.
For the majority of airhead owners, this is
REALLY enough. A voltmeter will give some indication of
the state of charge of the battery, and the system performance. An
Ammeter will give a bit more information, mostly unnecessary.
Some may want all three: GEN lamp, voltmeter, ammeter.
If you have a voltmeter, in particular an accurate one, it will
tell you if the battery voltage is high enough for you 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.
You can wire an ammeter to show ONLY alternator output.
A normal ammeter indication
(in the smaller red + battery lead) in a BMW Airhead motorcycle would be a discharge at idle
rpm. That discharge would be from current draws from the headlight,
taillight, running and indicating lights, clothing, radio, and the ignition.
~15 amperes would be common for just the stock ignition and
lamps, especially if the brake lamp was on. As rpm rose, that
discharge indication would
decrease and become a charge, and with rpm high enough, perhaps 3000+ rpm, 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
reasonably high enough, that charge value would
slowly decrease to a smaller charge amount.
If the discharge was 15 amperes at that stop light
and you sat there for ONE minute, you could expect MUCH longer than
one minute at, for
example, 4000 rpm, to replenish the power taken out of
the battery, due to inefficiency of charging (the battery is
certainly not 100% efficient as a converter of electricity
applied to chemical change). 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 brighter as soon as rpm increases from idle,
giving some indication of charging (or, at least LESS
discharging).
NOTE that in the situation of a voltmeter, the voltmeter indicates the system voltage
at the point where it is connected.
This is NOT necessarily the true battery terminals voltage.
In fact, in the stock BMW Airhead motorcycle, with a factory dash
voltmeter, that meter will indicate roughly 0.3 volts less than
battery terminal voltage, due to circuit voltage drops.
Due to a battery
being a CHEMICAL CHANGE storage device, batteries are 'charged' during riding to a voltage higher than
absolutely needed to maintain a full charge.
This is a required function. 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
and possibly damaged. If the voltage is only 13.5, that is
rather marginal. The value also INcreases as temperature
DEcreases, due to temperature regulation provided internally, on
purpose, in the design of the voltage regulator.
If the voltmeter falls BELOW maybe 10.5
or 11 perhaps, during cranking....or sitting at idle with the headlight
on, then something (connections or switch contacts?) in the system is bad and/or
MAYBE the battery is getting ready to fail or is just very heavily discharged. A
truly large variation when the blinkers (turn signal lamps
flashing) 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 motorcycle airhead voltmeter, be sure to get the motorcycle version. If
you do not care about having the original part, I HIGHLY suggest
you use a DIGITAL voltmeter in that BMW dash. A
digital meter can be extra-useful, even can be used to adjust the
VR.
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 no potentially failing connections carrying
high currents are needed. There are some other reasons not to use
an ammeter, such as avoiding alternator whine noise in your
radios, but I am not going to get into that here, and it was
mentioned earlier.
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, either, at least
not in all instances. With one or more failing single cells in a battery, of the
shorting-type failure (not all that uncommon), an ammeter
might well show a wonderfully nice charging 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, not wanting to
charge up to as high a voltage as it should.
This also can come from a single bad diode
in the diode board, a failing regulator, bad brushes, etc.
Batteries can also fail in such a way that they seem to charge up
to a quite decent voltage, yet can not start the bike. 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.
NOTE, however, that on our
bikes after the /5 series, the headlight is automatically turned off during cranking, and the
headlight is a 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...as a resistor if you will, and read the millivolts drop across it.
This was noted earlier in this article. Kept
calibrated, this works, and nothing is needed but relatively thin wires to the millivoltmeter.
It is almost always very SAFE to do it this way too.
After all, all the parts are just about at ground/engine-case
voltage, which is essentially zero. This method tells you if the battery is being discharged, or charged, and if
the millivoltmeter is
calibrated in actual equivalent amperes in that cable, by how much in amperes.
If you are interested in a very cheap way 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 has no terminals, and the only connection is the
wire passing through a rear circular clip or similar. There are
versions that simply snap over a wire.
This type of ammeter can be connected in your Airhead in
several places:
1). In series connection with the smaller RED WIRE of the
battery + terminal...or from the starter relay...or from the
ignition switch.
2) In series connection with the largish red wire
from the alternator diode board... that means the LARGE spade lug on the right side of the diode board, as you face the board from the
front.
This type of zero-center no-posts 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. You can multiply the sensitivity of the meter by looping the wire more times through/by the meter if it has the place or room to do so. More loops, more sensitivity. Thus a 60-0-60 meter can be made relatively useful, and you could make it far more sensitive. A rough idea of the ammeter sensitivity to try for when it is all connected, would be ..perhaps 25-0-25 or even 20-0-20 or tad less. The installation of a commercial shunt and associated meter is rather costly, most will never want to do this, including me, except when I need laboratory quality measurements.
By FAR the most convenient method, for
temporary
testing, is the clamp-on DC current probe. These are
generally much pricier than the AC
current clamp-on probe meters. These clamp-on types can be digital,
and be accurate, and be very sensitive...or insensitive...depending on your
selection of the current range on them. These are instruments for the serious
technician. They are not for permanent installation.
****I caution...greatly....that if one
installes wired-in ammeters that it CAN BE a big fire danger.
YOU MUST do things with thought, and neatness.
I feel that a voltmeter is by far best, and that BMW got it right
when it began installing them on some models.
VOLTMETER PROBLEMS:
Except for the 'only fair' quality of the mechanicals in the stock expanded-scale meter, they are
reasonably 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), as they age.
I have seen some that had very little dampening that WERE
motorcycle types...the dampening mechanism failed.
That is rare. The stock Airhead voltmeter, if
you have one, are similar to
the ones 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 (signal
flashers) connections would cause a lot of dancing too. Do NOT use a car
unit, and if you have your unit repaired, be SURE to specify the damped
motorcycle movement. The 2-1/16th inch size fits
nicely. I prefer a digital. Summit Racing sells one
similar to the one I designed and offered some years ago...and it
fits in the Airhead dash in place of the original. It is
self-powered, the original lamp is not needed. Digits are
available in at least two sizes, and two colors (green, red).
Some have fancy pushbutton functions to capture maximum or other
readings. You do NOT need those functions, and the buttons
can let moisture into the interior.
An excellent substitute for the BMW fairing voltmeter is the VDO 332103, available from Summit Racing,
and many other places. Standard 2-1/16th inch size;
black face, red pointer, white numerals, 8-16 volts,
accurate, backlit, reliable, and MUCH cheaper than from BMW.
This is NOT the digital type I referred to in the prior
paragraph.
Digital replacements of my own design are no longer available from me.
The typical failure mode for the stock BMW 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 AGAIN 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
from the alternator...in fact, that
charging route BYPASSES the ignition switch! ...and is UNfused. BTW, the headlights on most Airheads are
not fused.
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 flashers 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.
Typically, once this sort of thing starts up (voltmeter OK, connections poor),
it is only a matter of awhile 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
01/16/2009: checked for clarity and accuracy
05/12/2011: Updated for easier reading and clarity.
© Copyright, 2011, R. Fleischer
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