| Standard Multi Meter
A multimeter is a very valuable diagnostic tool which,
because of its mobility and multi-function capability can
provide information that a stationary distribution /meter
box cannot. A multimeter is ideal for:
! Measuring Voltage of Individual Battery Cells
! Measuring Voltage Drop in Cable and Connectors
! Measuring Charge Rate of Individual Solar Panels
! Measuring Power Consumption of Individual Lights
and Appliances
! Checking Calibration of Meters on the Control Board
! Checking Light Bulbs and Diodes
In the following few pages we will discuss how to
measure volts and amps and calculate watts and amphours.
We will also discuss how to test if a circuit is
complete or not (continuity test).
Buying a Multimeter
It would be advisable to get a multimeter with either a
10 amp range or a 20 amp range. A DC clamp meter that
can register several hundred amps would be particularly
useful for measuring power consumption of inverters
and the output of solar arrays and large battery chargers.
The voltage range of the multimeter would preferably be
0-15 volts for a 12 volt battery bank or 0-30 volts for a
24 volt battery bank. It would also be good to have a 0-3
volt range for testing individual cells of a lead-acid
battery or a 0-2 volt range for testing individual cells of
a nicad battery. For our purposes the ohms scale isn't so
important other than for testing continuity.
It is also recommended to purchase yourself a set of
insulated slip-on Alligator Clips.
If what you are attempting to measure is constantly
fluctuating an analog meter (which has a needle pointing
to a scale of numbers) is easier to read than a digital
meter. For measuring the voltage of a battery bank and
DC currents and voltages generally, a digital meter
(which has an LCD display similar to the display of a
calculator) is preferred. |
DC Clamp Meter
HINTS
1. When using the meter, pay particular attention to
polarities and check positive and negative points.
The red lead connects to positive and the black lead
to negative.
2. It is generally good practice to position one probe
first (usually the negative probe), and get it secured
with an alligator clip or by finger tightening a screw
onto the probe before testing or probing with the
other probe. This makes it easier to concentrate on
only one probe.
3. If you are checking unknown currents and voltage,
use highest range first, then next lower range, and so
on until readings can be obtained.
4. For most accurate readings, keep the meter lying flat
on a non-metallic surface. Also, use a range setting
that results in a reading in the upper third of the
meter scale.
5. With an analog meter, for exact readings, look at the
scale from the point where the pointer and its
reflection on the mirror behind the pointer come
together; otherwise a reading error may result due to
parallax.
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WARNINGS
1. Do not apply voltage to probes while the range
switch is in current (amps) or ohms position. When
using the clamp on a digital clampmeter, this is not
a concern.
2. Testing AC wiring circuits can be dangerous. Never
clamp on to a 'hot' wire (usually red or brown) since
if you did so and then touched the other probe, you
could receive an electric shock. For your own safety
leave the AC diagnostics to a qualified electrician
and just concentrate on the DC circuitry.
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How to use a Multimeter
It is recommended to read the instruction manual of your multimeter before reading the following: |
DC Voltage Measurement
Select the required DC voltage range (if in doubt start
from the highest range and work your way down until a
reading can be obtained) with the probes connected in
parallel (+ve to +ve, -ve to -ve) to the points to be
measured.
Open Circuit Voltage
Open Circuit voltage (OCV) is the terminal voltage of
a battery while at rest. This means that there is no
charge or discharge of that battery. OCV is the most
meaningful voltage of a battery as this can indicate state
of charge. Each cell of a fully charged lead-acid battery
should have an OCV of around 2.1 volts. At 50%
discharge the OCV will be about 2.0 volts per cell. At
around 1.8 volts per cell or less the battery is considered
discharged.
It is good practice to occasionally compare the OCV of
the component cells of a battery bank (if the intercell
connectors are accessible). This will allow you to
identify the sluggish cells. The sluggish cells should be
given an identifying mark and used to regularly monitor
the battery bank. The sluggish cells can then be used to
identify when next to apply a boost charge to the battery
bank. You never want a variation between the best and
the worst cell of more than 0.05 volts.
A NiCad battery has an OCV of about 1.25 volts per cell
and its variation between charged and discharged is
difficult to measure as the voltage varies so little.
Charging Voltage
The voltage of a battery being charged can give you an
indication of when that battery has reached full charge.
This is NOT an OCV.
Whilst charging the voltage of a battery may not vary
much for most of the charge and then rise quite
dramatically once the battery is full. A Lead Acid
battery voltage will rise to between 2.3 and 2.4 volts per
cell when fully charged. If a Lead Acid battery has been
left in a state of partial or total discharge for a long
period of time (months) it may be sulphated (definition
on page 136) and have a very high internal resistance in
which case the charging voltage may behave as if the
battery is full when in fact it's not. Taking a specific
gravity reading with a hydrometer will then tell you that
in fact the battery is not fully charged (see battery
section of "Energy from Nature").
Whilst a NiCad battery is being charged the voltage may
rise to 1.62 volts per cell. A NiCad battery never suffers
from sulphation and the charging voltage can be used
very reliably to determine that it is fully charged. The
multimeter is not a reliable indicator of the state of
charge up until charging is completed.
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Measuring Voltage Drop
A voltage drop will only occur whilst there is a current
flowing. Voltage drop is directly proportional to the
amount of current flowing and the cable length. By
comparing the voltage reading at one end of a cable to
the reading taken at the other end you can obtain the
voltage drop (subtract the lower reading from the higher
reading).
To reduce the voltage drop you may need to increase the
cable size and improve the connections.
DC Current Measurement
Select the required DC current range (if in doubt start
from the highest range and work your way down until a
reading can be obtained) with the test leads connected to
the points to be measured. Amps are usually measured
by breaking the continuity of the positive line and
connecting an amp meter between these two points (ie
in series), whereas with a DC clampmeter you need to
isolate a single conductor (either positive or negative),
open the clamp jaws so as to place that single conductor
inside the jaws before closing them and reading the
display.
An amp-meter on a distribution/meter box to measure
discharge rate needs to able to read the power
consumption of the maximum number of things that
may be turned on at once. Such a meter would hardly
register and hence would be almost useless in measuring
the consumption if it is very low. A 12 volt electric
fence energiser and a battery powered radio are two
examples of appliances that are usually on for long
periods of time whose power consumption is quite low.
If an appliance is on continuously for a long period of
time even a small power consumption will accumulate
to be quite significant and from that point of view it is
good to be able to measure it.
Testing the Current Consumption of a Light or Appliance
Make sure that the appliance or whatever that you are
about to measure is turned off. If you have all your
positive connections made at one common link it may
be easiest to break the continuity at this point. Links
often have numbers stamped into the brass to identify
the wire locations. Simply undo the screws that hold the
wire in question. Finger tighten the screws back onto
your positive probe, fix an alligator clip onto the
negative probe to hold onto the end of the wire that just
came out of the link. Once all your connections are
secure you can turn the appliance on and check its
current consumption. |
Checking the Charging Rate of a Solar Panel
Again you need to break the continuity of the positive
line. This time you don't need to turn anything off first.
This time the positive probe connects to a point that
connects back to the panel and the negative probe
connects to a point that goes on to the battery bank. You
can isolate and measure individual solar panels by
measuring on the solar panels directly or you can
measure the output of all the solar panels combined by
removing the solar fuse on the distribution/meter box
and using the fuse contacts as your test points.
Power (Watts) versus Current (Amps)
To calculate the power consumption of an appliance or
the power output of a solar panel, simply multiply the
measured current by the measured voltage.
Power Loss (Watts)
The power loss of cable and connectors is calculated by
multiplying the measured voltage drop by the measured
current flow (see 'Measuring Voltage Drop' and 'Testing
the Current Consumption of a Light or Appliance' -
above).
Amp-Hours and Watt-Hours
Amp-hours is calculated by multiplying the current
(amps) by the number of hours that that current has been
flowing for. To calculate watt-hours, multiply amphours
by measured volts.
Testing for Continuity
In order to measure continuity you need to have a
voltage source.
If there is a poor connection or a break in the house
wiring it can often be located by tracing the wires from
the battery bank outwards and using the battery bank as
your voltage source.
With the meter on the appropriate voltage scale start by
measuring the voltage at the battery. Now move to the
next location where you can connect your probes as you
head towards the possible location of the fault.
If at any point you measure no voltage then there is a
break in the wiring between the previous test point and
this one.
If you measure a drastic voltage drop (particularly with
a small load turned on) this may indicate a poor
connection such as a wire that is almost broken,
corrosion in a connector or a wire, or it may be due to
undersized wiring.
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Testing if a light bulb is OK
This test can only be applied to incandescent type light
bulbs. Fluorescent lights will not respond to this test.
It would be easier in this case to use one of the ohms
scales on the meter or to use the continuity function if it
has one. To make these functions work the multimeter
should have an internal battery.
Some multimeters have a built-in continuity function
which often sounds a buzzer. Test this by selecting
continuity on the range switch and touching the two
probes together. If it buzzes try holding the probes onto
the two contacts of the light bulb and see if it buzzes - if
it does the light bulb is OK.
Using Ohms (S) for Continuity
If you do not have a continuity function on your
multi-meter you can use one of the ohms scales.
If you select an ohms scale and touch the probes
together you should see the needle of an analog meter
move right across the scale and a digital meter should
change from reading maximum resistance to zero. Most
digital meters will show a high number which flashes
(over range) when the circuit is broken (no continuity).
If you get the appropriate response from your meter,
hold the two probes onto the light bulb contacts. If the
needle of the analog meter moves across the scale or if
the digital meter reads zero or a low number then there
is continuity and the light bulb is OK.
Testing if a diode is OK
A diode is like a one-way valve. It should allow the
current to only flow in one direction and prevent the
current from flowing in the other direction. A good
diode should show continuity in one direction and no
continuity (or over range) in the other.
Do not test the diode whilst there is an external voltage
(eg solar panel) connected as this will effect the
outcome and possibly damage the meter.
Connect the probes to the device you want to check and
note the meter reading. Reverse the probes and note the
second reading. If the one reading shows some value
and the other is overrange, the device is good. If both
readings are overrange, the device is faulty (open
circuit). If both readings are very small or zero, the
device is also faulty (short circuit). |
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