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.
Introduction 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.
Application Overview and Test Implementation Seven standard test procedures1 are used to verify certain electrical characteristics of secondary batteries: 1. 2. 3. 4. Rated capacity Capacity retention Effective internal resistance Discharge rate effect on capacity at –20°C 5. Discharge rate effect on capacity at 23°C 6. Life cycle performance 7. Extended overcharge Rated Capacity The principal measurement of a battery’s performance is its rated capacity.
Capacity varies with the rate of discharge as shown in Figure 3. Testing for how discharge rate affects capacity is discussed later in more detail. Generally, lower discharge rates over longer periods of time yield higher values of total capacity. It is important to realize that since discharge rate affects how the value of C is determined, battery manufacturers must decide on a standard time of discharge.
Effective Internal Impedance Battery impedance is dependent on temperature, its state of charge, and the load frequency. The effective internal impedance is lower for a fully charged battery than it is for a discharged one. Having a low internal resistance is very important when the battery must support a high current for a short time. Low temperature, use, and long storage periods all increase a battery’s internal resistance. Nickelcadmium batteries also have a high effective capacitance.
Miscellaneous Tests In addition to the tests already mentioned, there are also other miscellaneous tests performed on nickelcadmium batteries. These tests usually involve high rate charge and/or discharge. High rate discharge and charge of nickel-cadmium batteries is possible with today’s new and better designed cells having advanced plate and cell construction. The low internal resistance of nickel-cadmium batteries yields high discharge currents.
Test Equipment Requirements From the various tests described so far, we can see some common requirements for test equipment. All the tests require a discharge cycle using a constant current. A constant discharge current cannot be attained with a simple resistor because the battery voltage changes as current is drawn from it. An active device is required, such as an electronic load with a constant current mode of operation.
Battery Testing with Agilent Electronic Loads Agilent Technologies Electronic Loads are ideally suited for battery test applications. Their many features make the test system easy to configure and provide safe, reliable, and repeatable operation. The Agilent 6060A Electronic Load and 6050A Electronic Load mainframe have the required constant-current modes as well as constant-resistance and constant-voltage modes.
