Analysis of battery charging and discharging test process using Keithley 2400 series digital source meter

With the development of electronic technology, batteries are widely used in mobile phones, hearing aids, electronic tools, and even satellites. Depending on the industry in which the battery is used, its testing also differs based on its chemical characteristics, size, and special use. Secondary batteries (rechargeable batteries) generally require a discharge-charge test process. The discharge characteristics of secondary batteries are important indicators of their capacity and life. During product testing, charging and discharging can be used to ensure its quality. At the same time, it should be ensured that the test battery is not short-circuited.

Typical battery charge and discharge tests should include programmable power supply, electronic load, voltmeter, and ammeter. Since the Keithley 2400 series digital source meter can excite and test current and voltage, only one instrument can complete the testing process, thus saving space and programming time.

Test principle:

Secondary batteries can be charged and discharged using different methods according to different applications. This article uses constant voltage or constant current methods.

Using the constant voltage method, a programmable voltage source provides a fixed voltage that is equal to the battery voltage. The current of the voltage source can be limited to a safe charging current value. When the battery is full, the current decreases until it is zero (or close to zero). For battery safety reasons, do not overcharge. Overcharging causes the voltage to fall below a specified limit, which may extend the charging time or damage the battery.

Using the constant current method, the programmable current source can provide a rated current. The current source will continuously charge and discharge the battery until it reaches a voltage value.

The constant current method can be used to define battery capacity (C). The capacity is the integral of the battery outflow current over time, starting from t=0 to the specified cut-off voltage. The expression is: C=∫ tid t, the unit is ampere hours, and the load current is noted. For example: a 500mA.hour battery is discharged at 50mA and discharged to 0.1C (or C/10). A 500mAh@50mA battery can power a 10mA load for 50 hours. Factors that affect battery capacity include battery size, chemistry, temperature, discharge rate, etc.

Testing process:

Use the source meter as a constant voltage source to charge and discharge so that the source meter is in constant voltage mode and can test current or voltage at the same time. The voltage source can be set to the required value according to the battery requirements, and the clamping current value can also be set appropriately. When the SourceMeter charges or discharges the battery to a corresponding voltage value, the instrument is in a current clamp state until the battery reaches the required voltage.

Use the source meter as a constant current source to charge and discharge

When the source meter is at constant current, it can measure voltage/current simultaneously. First, set the current output of the instrument. When charging, positive current is used; when discharging, negative current is used. The clamping voltage can also be set appropriately.

In charging mode, the clamp voltage is set to the battery voltage, and the source meter outputs as a constant current until the set voltage value is reached. At this point, the instrument is clamped and becomes a constant voltage source. In discharge mode, the clamping voltage should be higher than the battery voltage. If it is less than or equal to the battery voltage, the source meter will clamp and the current will increase.

Automatic test of discharge process:

Since the charging and discharging process takes several hours, a computer programmable source meter is used to automatically complete the test.

Charge and discharge using pulse current

In mobile phones and pagers, the load current of the secondary battery is a digital signal during use, so the battery requires pulse current charging and discharging.

Use the 2400 Series SourceMeter to generate pulsed current

Example 1: Generate pulse current and monitor voltage

In this example, the peak current is -1A (6.7ms), the sleep current is -0.2A (13.3ms), and the interval is 20ms. In this example, the voltage is monitored and compared to a set value, and once the voltage reaches the specified value, the output current is turned off.

Example 2: Testing mobile phone batteries according to GSM standards

According to the GSM protocol, the pulse current generated by the battery is a few microseconds or hundreds of milliseconds. The source meter can output this pulse current waveform, and another source meter is required for voltage testing.

Multiple battery tests

Since the battery charging and discharging process takes several hours, several batteries need to be tested simultaneously. In Figure 1, 40 cells can be tested.

An independent voltmeter is used to monitor each battery. The source meter output is 10mA and supplies power to 40 batteries at the same time. The Keithley2000 meter (6{1}over{2}DMM) monitors the voltage of each battery and can compare the measured value with the set value. At the same time, a computer can be used to implement automatic testing.