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Measuring Electricity

Range Selection

Multimeter connected to battery for voltage measurementLets assume we wish to measure some DC voltage(electrical pressure) to get started. I'm going to select a flashlight battery just as an example. We look on the battery label and find that it says it is a 1.2 volt cell (don't confuse the voltage with the mAh ratings). This is a common voltage level for rechargeable NiMh and NiCd batteries.

A typical meter will have ranges such as 2v, 20v, 200v, 500v, and 1000v. You should select the voltage that is comfortably higher than your expected voltage. For our example battery and given the available ranges listed above - we would select the 2v range. In the case of auto-ranging meters (typically digital), this selection will automatically be made for you - though you might have a manual override option button depending on your model of meter.

You will want to select the lowest suitable range in order to obtain the most accuracy from your meter; but if the voltage you are expecting is actually higher than the range you select - you may damage your meter. Many digital meters have protection mechanisms, and will simply display an error - but better safe than sorry! Most analog(needle style) meters will not have such a feature, so use extra caution! When in doubt, select a higher range and then adjust downwards for the final reading.

So, to summarize for our battery example, our meter should now be set to the 2 volts range of the DC scale. If your meter doesn't have a 2 volt setting, select the nearest option above 2 volts.

Connections for voltage measurements

Voltage measurements are the easiest connections to make; and that's good news because they are also the most common general purpose measurement you'll make.

You will simply need to connect the Red(+) test lead to the + terminal of the battery and the Black(-) test lead to the (-) terminal of the battery. Most batteries will be labeled with the plus and minus sign; but on standard flashlight style batteries the end with the raised button end will be the positive.

This may sound familiar to a parallel circuit; and that's exactly what it is. Voltmeters are always connected in parallel with the circuit or component being tested.

Allow the reading to stabilize, and read the voltage.

Interpreting the reading you obtain

In the case of our example battery, you might expect to see 1.2 volts on your meter. Well, not so fast. In the real world, you probably won't get that exact number. The real voltage will change depending on how much charge is in the battery. Thus, we just uncovered one reason we might want to check the voltage of a battery!

To judge the charge level on the battery as a general guide, anything above the 1.2 is close to fully charged; 1.0-1.2 would probably still have some power available; and less than one volt means its time to charge it up. While interpretting the voltage of a battery can get more complex (and more accurate), these numbers should suffice for a quick test.

Just one other note, as some may overlook it. The numbers given above are for a single cell NiMh rechargeable battery. Non-rechargeable dry cell and alkaline batteries should measure about 0.3 volts higher. Battery 'packs' are simply groups of cells connected together, usually in series. Thus, you can normally just multiply the voltages above by the number of cells to obtain appropriate voltage levels. Other types, such as lithium, will have different voltages altogether.

While we used a battery for an example, the process is the same for any voltage measurements: Find the specifications for what the voltage should be, measure what it actually is, and verify that the two numbers match within some level of tolerance.

In the next section, we will discuss measuring current (amps) ...

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