Agilent Technologies AN 372-2 manual Application Overview and Test Implementation, Rated Capacity

Application Overview and Test Implementation

Seven standard test procedures1 are used to verify certain electrical char- acteristics of secondary batteries:

1.Rated capacity

2.Capacity retention

3.Effective internal resistance

4.Discharge rate effect on capacity at –20°C

5.Discharge rate effect on capacity at 23°C

6.Life cycle performance

7.Extended overcharge

Other miscellaneous tests and proce- dures also involve discharging a battery such as: start-up voltage test, forced-discharge test, timed fast charge and dump-timed charge. Most battery tests typically require only about 1% accuracy unless otherwise specified. While battery tests do not require high accuracy, the tests must be very repeatable. Battery characteristics change with temperature so it is important to be able to control and monitor the temperature, usually to within ±2 degrees C. Other equipment requirements to consider are: a cur- rent source for charging secondary batteries, a voltage monitor, a current monitor, a load for discharge current, and a time keeping device. More information about test equipment

is given in the “Test Equipment Requirements” section later in this application note.

Note that a battery temperature rise of more than 5 degrees C above ambi- ent may require supplemental cooling to prevent battery performance degra- dation due to elevated temperatures.

Rated Capacity

The principal measurement of a battery’s performance is its rated capacity. Capacity ratings are attained in an accelerated test approximating the battery’s capacity in typical use. The capacity of a fully charged battery, at a fixed temperature, is defined as the product of the rated discharge current (in amperes) and the discharge time (in hours) to a specified mini- mum termination voltage (volts).

See Figure 2. A battery is considered completely discharged when it attains

Figure 2. Typical Discharge Curve

the specified minimum voltage called the “end of discharge voltage” (EODV). The EODV for nickel-cadmium batteries is typically 1.1 to 0.9 Volts.

The term C, or C-rate, is used to define the discharge current rate (in amperes), and is numerically equal to rated capacity, which is expressed in ampere-hours. The term 1C is defined as the rate of discharge that allows a battery to provide its rated current over a period of one hour.

1.As specified in ANSI® C18.2-1984, American National Standards

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