Fall 2002 Issue

Figure 2. Battery-charge tester.

With all of the different types of batteries on the market, one wonders which battery is best to use for a particular application. Bob Shrader, W6BNB, gives us some idea of what to expect from our faithful batteries, and tells us just how faithful they can be.
 

By Bob Shrader, W6BNB
 

When you recharge a run-down battery—one in a Handie-Talkie, for instance—how do you know when the battery is fully charged? In too many cases, there is no way of knowing whether the battery has been over charged, fully charged, or under charged.

Sometimes, if a charger was made for the equipment in which a battery is being used, the charger either may stop charging when the battery comes up to full charge, or it may drop to a very low trickle charging rate. In too many situations, however, chargers will over charge a battery and shorten its life if the charger is left on for too long.

The Resistance to Charge

To determine the state of charge of some of my HT batteries I devised the following method of stopping the charging at the correct time.

First, when dealing with a battery, consider it as always having some value of internal resistance. The more discharged a battery is, the greater its internal resistance becomes.
The symbol for a battery is normally a series of short and long lines (figure 1A). To be truly accurate, it should be diagramed with a variable resistor inside, because this is the way it actually works (figure 1B).

A “voltage drop” (by Ohm’s Law, E = IR, where E = volts, I = current in amperes, R = resistance in ohms) will always develop across the internal resistance of a battery whenever current is flowing through it, as when the HT is connected across it. The amount of internal voltage drop will determine the actual terminal voltage that is being provided by the battery to the load connected to it.

When no current is being delivered to a load, the voltage across the terminals of a battery will be at a maximum. For example, when measured with a DC voltmeter, six fully charged nickel-cadmium (NiCd, or NiCad) dry cells in series form a battery that should read 7.2 volts across it (1.2 volts per cell). Unfortunately, it is rare to have two voltmeters read exactly the same. Therefore, always use the same voltmeter when making battery voltage measurements.

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