Constant Voltage and Current in Li-Ion Cell and Battery Test

Constant Voltage and Current in Li-Ion Cell and Battery Test

In a previous post of mine "Characteristics of DC Source Priority Modes" (click on link to review) I talked about constant voltage (CV) and constant current (CC) operation and priority modes of DC power sources. Virtually all DC power sources, and electronic loads, feature CV and CC operation. CV and CC operation is useful for lithium-ion cell and battery testing. Standard charging uses both CC and CV operation while standard discharging uses negative CC operation. Here we will explore how the characteristics of cell or battery interact with the power source's CV and CC operation, leading to the standard charging and discharging profiles over time that we are accustomed to seeing.

While not standard for most power sources, another mode of operation particularly applicable to cell and battery testing is constant power (CP) operation. In a future article we will delve into why CP operating mode is useful for cell and battery testing, and how it impacts their charging and discharging profiles over time.

Constant Current (CC) and Constant Voltage (CV) Operation

While a separate DC source can be used for charging, and an electronic load can be used for discharging, it is more practical and efficient, and therefore more common, to use a single two-quadrant DC source that combines charging and discharging. As an example of two-quadrant operation with CC and CV operating modes are our Keysight N6900/N7900 Series DC sources, depicted in Figure 1. Note that they can sink up to 10% of rated capacity stand alone, or up to 100% capacity when an optional dissipator unit is attached, which would be standard practice for cell and battery testing.

The N6900/N7900 exhibit standard rectangular output current-voltage (I-V) characteristics. That is, they provide either constant voltage, indicated by operating along the horizontal voltage limit boundary, or constant current, indicated by operating along a vertical current limit boundary. The CV, +CC, and -CC limits are programmable so that desired operating levels can be set for charging and discharging, while the ratings are the maximum voltage and current limits the power source is capable of.

Figure 1: Keysight N6900/N7900 Series DC sources two-quadrant operation

Standard CCCV charging for lithium-ion cells.

While all the discussion going forward is for a cell, it is equally applicable to a battery, which, in simplest terms, is a series stack of cells to produce higher voltage. The power source just requires a proportionally higher voltage rating to match the battery. Looking at a cell's basic electrical model and associated I-V characteristics is a good starting point for understanding cell charging and discharging. As illustrated in Figure 2, being primarily a voltage source, the cell's I-V characteristics are a nearly horizontal line, indicating constant voltage. Additionally, it has a slight amount of slope due to the cell's internal equivalent series resistance (ESR).

Figure 2: Lithium-ion cell basic electrical model and I-V characteristic plot.

The standard regimen for charging lithium-ion cells is CCCV charging. The charging DC source is set to the desired charging current rate and voltage level set to equal to the cell's fully charged voltage. This gives a rectangular I-V characteristic plot for the positive quadrant, like that previously shown in Figure 1, now shown in Figure 3.

Figure 3: CC phase charging for a cell at 0% SoC.

The cell's 0% SoC level is defined for when the cell has been discharged down to its minimum allowable voltage. When the cell's I-V characteristic for 0% SoC is superimposed onto the charging source's I-V characteristic, the operating point is where the two characteristic plots intersect, as shown in Figure 3. For zero and low %SoC the cell is charging at the CC limit setting of the charging DC source.

As the cell approaches 100% SoC, its voltage rises and charging transitions from the CC charging phase to the CV charging phase. This is illustrated in Figure 4.

Figure 4: CV phase charging for a cell reaching 100% SoC.

When the charging initially transitions from CC to CV phase, the cell I-V characteristic load line intersects the charging DC source's I-V characteristic at the CC to CV crossover point and charging is taking place at maximum current, voltage, and power. The cell is not yet fully charged at this point. As the cell continues to accumulate more charge in the CV charging phase, the current exponentially tapers off as the cell's load line intersects the charging DC source's I-V characteristic on its CV limit closer to zero current. Typically, charging cutoff in the CV charging phase is when the cell's current drops to a pre-defined level, often about 3% of the CC charging current level, as illustrated in Figure 4. At this point, the charge is terminated.

Looking at the resulting charging voltage and current overtime is shown in Figure 5, as the cell goes from 0 to 100% SoC. Full charging can take from under 2, to up to many hours, depending on the charging current applied to the cell.

Figure 5: Lithium-ion cell CCCV charging over time.

Standard CC Discharging for lithium-ion cells

Standard discharging for a lithium-ion cell is more straightforward compared to CCCV charging. When the cell is at 100% SoC a CC discharge is applied to the cell, as shown in Figure 6. The operating point is where the cell's 100% SoC characteristic I-V load line intersects the discharging DC source's second quadrant I-V characteristic. Note that the DC source second quadrant characteristic is usually device-specific on how it may treat its voltage limits, so careful review of the user's guide is important. Here, the voltage limit is a minimum limit which must be set lower than the cell's minimum voltage for it to fully discharge the cell.

Figure 6: Standard CC discharging for a cell starting at 100% SoC.

As the cell discharges, its voltage drops. The cell's 0% SoC level is defined to be when the cell drops to it minimum allowable voltage while being discharged. This is illustrated in Figure 7. In this example the cell's minimum voltage is 3 volts. At this point discharging is terminated to prevent over-discharging the cell.

Figure 7: Standard CC discharging for a cell reaching 0% SoC.

This predicably produces a discharge voltage and current characteristic over time and % SoC, as depicted in Figure 8.

Figure 8: CC discharge of lithium-ion cell over time

In Closing

Most all DC power sources, and DC electronic loads, feature constant voltage (CV) and constant current (CC) operation. For rechargeable cell, module, and battery pack testing, both charging (sourcing) and discharging (loading) are needed, so both are virtually always incorporated into a single two-quadrant DC power source, as it is more efficient and cost effective to do so.

CC and CV operation are useful and necessary for charging and discharging cells, modules, and battery packs during tests. The standard regimen for lithium-ion charging is CCCV charging. During the initial CC phase, the cell is charged with constant current up to its maximum voltage. At that point, the charging automatically transitions to CV phase, where the balance of charging takes place, bringing the cell up to 100% SoC. Conversely, the standard discharging regimen for lithium-ion cells uses only CC operation. The cell is taken from its full voltage at 100 %SoC, down to 0% SoC where discharge is terminated at the cell's minimum voltage. Illustrated here were the power source's I-V characteristics and how they interact with the cell's I-V characteristics, to relate how these characteristics determine what the cell's charging and discharging characteristics over time look like, that we are accustomed to seeing.

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