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Technical Datasheets and Real Power Sources.

Picture 1: Discharge data for a Duracell AA Alkaline Battery

I have been thinking for some time now about talking about technical data sheets. Luckily, while researching the net for some information about batteries for the electrical theory post, I found a very good example.  This also lets us get back into talking about “real things” instead of just theory.

Picture 1 is from a data sheet on a Duracell Alkaline AA size battery. Most companies making technical components have data sheets in pdf form available on the net.  Duracell and Energizer are no exceptions.

Picture 2: A battery powered simple circuit.

In Picture 2 I have drawn a simple circuit very similar to those shown before except I replaced the Ideal Power Source with a battery symbol. Using Ohms law and the commonly stated Voltage for the battery we should be measuring 0.25 A flowing in this circuit. However,there is a very high chance that we would find less current flowing. For example, we might measure 0.23 A current. At first glance, we might think the battery is discharged and is “putting out less Volts”. This is really a correct assumption but if we disconnect the battery and measure the Voltage, the battery would probably be producing 1.5 and possibly even more Voltage. So what happened?

First, the 1.5 V stated value is considered a nominal value.  Just like in the case of lumber and pipe sizes nominal means a commonly stated value but the actual value may be approximately that value.  However, that would not explain the graphs in Picture 1.  The main reason for the difference is some internal resistance in the battery.

Picture 3: A model of a real battery

A real battery can be conceptualised as an ideal power source with some internal resistance as shown in Picture 3.  The ideal source Voltage may be in the case of an alkaline battery about 1.6 V.  However, as soon as we connect a load on the battery, the current flowing through the internal resistance will create a Voltage drop and the Voltage at the terminals will be less.

Picture 4: Terminal Voltage and Internal Resistance of the Battery while being discharged.

The model shown in Picture 3 only tells part of the story. Picture 4 shows the actual internal resistance (orange line) and terminal Voltage of a test battery under load.  As shown in this graph, some other things affect the internal resistance, I.R.  The I.R. is mostly a function of how active the chemistry in the battery is at the time of the test.  As the battery becomes discharged, the chemicals are being used and it is harder for fresh chemicals to react and the internal resistance rises making less Voltage available at the battery terminals.   Another factor affecting the chemical activity is the temperature of the battery at the time of the test.  The higher the temperature, the more active the chemical reaction and the less the resistance.  That is why the test is specified a 70 deg F. (21 deg C.)  That is also why in the case of automotive batteries the concern is the ability to provide enough current at cold temperatures and there is a specified value called “Cold Cranking Amps” in United States.

Picture 5: Dimensional diagram also from the Duracell data sheet.

Some electronic power sources may not experience this voltage drop at the terminals and look as if they have zero internal resistance. This is because internal circuits in the power supply monitors the terminal Voltage and automatically compensates for the Voltage drop by increasing the source voltage.
The final part of the data sheet I want to share with you is  that almost all data sheets have a diagram showing the physical dimensions. Again, this is no different. Those of you outside the US borders will probably delight in the fact that the dimensions are shown in metric and it is following an ANSI (American National Standard Institute) standard.

Gary

Note:  Pictures 1, 4 and 5 were screen shots from Duracell data sheet MN1500_US_CT.

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