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Simplifying battery charge/discharge cycling

Posted: 09 Oct 2015 ?? ?Print Version ?Bookmark and Share

Keywords:battery? power supply? Source measure unit? current sourcing?

Researchers commonly perform charge/discharge cycling when characterising rechargeable batteries. During development, charge/discharge cycling gives researchers information about the battery, such as its internal chemistry, capacity, number of usable cycles, and lifetime. In production testing, a discharge/charge cycle is often performed to verify battery specifications and screen out defective products.

A typical battery discharge/charge test configuration often includes a programmable power supply, an electronic load, an electronic switch, a voltmeter, and an ammeter. Significant configuration time is required to synchronise all these instruments into a working test station. Another option is to use a potentiostat or galvanostat, which are popular instruments used for electrochemical studies. However, there may be disadvantages to using either of these instruments, including the limited control users may have over them.

Source measure unit (SMU) instruments, such as Keithley's Model 2450 and 2460 SourceMeter SMU instruments, are touted to offer a simpler approach to battery charge/discharge cycling. These instruments offer voltage and current sourcing and measurement ranges that are well suited for this application.

An SMU instrument can either charge a battery by setting a desired current rate or discharge a battery by dissipating power, while monitoring a battery's voltage. A single SMU instrument can also replace an entire rack of equipment, minimising equipment and integration costs.

For both the charge and discharge cycles, the SMU would be configured to source voltage and measure current. Figure 1 is a simplified circuit diagram of both the charge and discharge cycles.

Figure 1: Although the SMU is set to source voltage during both the battery charge and discharge tests, the instrument will actually source current during the charge cycle and sink current during the discharge cycle.

A battery is normally charged with a constant current. To do this with an SMU instrument, the voltage source is set to the voltage rating of the battery and the current limit of the source is set to the desired charging current. At the start of the charging cycle, the battery voltage will be less than the SMU instrument's voltage output, and current will flow into the battery.

Because the current is limited, however, the SMU instrument acts as a constant current source until the battery reaches the programmed voltage level. As the battery becomes fully charged, the current will decrease until it reaches zero or near zero. To minimise safety hazards or battery damage, avoid overcharging the battery.

When discharging a battery, the SMU sinks current because it is dissipating power rather than sourcing it. To draw current from a battery, the voltage source of the SMU instrument should be set to a lower level than the battery voltage and the SMU's current limit should be set to the desired discharge rate. When the output is enabled, current from the battery will flow into the HI terminal of the SMU instrument. As a result, the current readings will be negative. The discharge current should stay constant until the battery voltage decreases to the voltage source setting of the SMU.

Rates for constant current charging and discharging are determined by the battery's capacity, which is the amount of electrical charge that the battery can store. The capacity is specified in milliampere-hours (mAh) and should be expressed in terms of a discharge, or load, current. The rate at which the discharge current will discharge the entire battery in one hour is known as the C-rate. For example, a battery rated at 1000mAh will output 1000mA for one hour if discharged at 1C. If a 500mAh cell is discharged at 50mA, then it is discharged at one-tenth the C-rate (0.1C), so it can source 50mA for ten hours.

Making connections to the battery
To set up the test, connect the SMU to the battery as shown in figure 2. Connect the Force HI and Sense HI output terminals of the SMU to the positive (+) terminal of the battery and the Sense LO and Force LO outputs to the negative (C) terminal of the battery. This four-wire or remote sense connection eliminates the effects of the lead resistance, and improves the accuracy of the battery voltage because the sense connections are made as close as possible to its terminals.

Figure 2: Using four-wire connects from the battery to the instrument eliminates the effects of the lead resistance and allows reading the battery voltage as close as possible to its terminals./i>

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