Agilent AN 372-2
Battery Testing
Application Note
An electronic load can be used to discharge batteries of various
chemistries to determine actual capacity, capacity retention,
and internal impedance.
2
Increasing demand for portable DC
power has risen from improvements
in battery and motor design technology. More than ever before, portable
DC powered products have become
available in many diverse applications.
Rechargeable batteries appear in all
types of products from analytical
electronic equipment to power tools
and toys. In some instances, these
diverse applications pose different
requirements on the source of DC
Power. Fortunately, availability of
many types of battery chemistries
yield unique characteristics. Table 1
contains just some of the different
battery types and their advantages.
Whether testing batteries in R&D or
production environments, the test
requirements for each of the different
battery types are basically the same.
Figure 1 shows a common test configuration. In general, the testing of a
battery involves discharging it over
a period of time to determine several
specifications. This application note
will concentrate on the test of secondary batteries because they require
additional tests involving recharging.
Nickel-cadmium batteries, in particular, are discussed because they are
the most universally used type of
secondary battery in today’s demanding applications.
Introduction
Table 1. Characteristics and Applications of Different Battery Types
Nickel- Gelled Lead Lithium Carbon Alkaline Silver Mercuric
Cadmium Acid Zinc Oxide Oxide
Volts/Cell 1.2 2.0 1.5 to 1 1.5 1.5 1.5 1.4
Applications portable standby service, memory backup, average use good general button-sized button-sized cells
equipment, rechargeable pacemakers, purpose battery cells for watches for watches and
rechargeable electronic and hearing aids hearing aids
door locks,
emergency
locator
transmitters
Charge CC CV, float charge N/A N/A N/A N/A N/A
Method
Cycle Life 500+ cycles 200 cycles N/A N/A N/A N/A N/A
Life 3 mos. 1 year 5 to 10+ years 1 to 5 years 5% loss/yr. 6% loss/yr. 4% loss/yr.
(Charged) (–2%/day)
Operating 20°C to 70°C –20°C to 65°C –55°C to 75°C –5°C to 55°C –30°C to 55°C –20°C to 55°C –10°C to 55°C
Temp.
Performance high discharge high capacity flat discharge, low cost, good energy flat discharge, flat discharge
Comments rate, quick long life, wide sloping density, more energy
charge rate temperature discharge, sloping per unit
range, good low energy discharge volume than
energy density density mercuric oxide
Figure 1. Common Test Configuration
3
Seven standard test procedures1are
used to verify certain electrical characteristics 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 procedures 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 current 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 ambient may require supplemental cooling
to prevent battery performance degradation due to elevated temperatures.
1. As specified in ANSI® C18.2-1984,
American National Standards
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 minimum termination voltage (volts).
See Figure 2. A battery is considered
completely discharged when it attains
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.
Application Overview and Test Implementation
Figure 2. Typical Discharge Curve
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