How to use a Multimeter
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 amp-hours. 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 analogue 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 Voltage Measurement
It is recommended to read the instruction manual of your multimeter before reading the following:
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.
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 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..
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.
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 clamp-meter 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 be 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 amp-hours 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.
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 analogue 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 analogue 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 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.
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).