Utilizing the PXB regenerative, bidirectional power supply for battery testing applications and more
Programmable DC power supplies are used in a wide array of test applications from energy storage device validation to power source performance testing. The testing and validation of batteries and fuel cells can be a large endeavor with a sizeable infrastructure including (but not limited to) environmental chambers (e.g., temperature or explosion-proof chambers), temperature sensors, racks of programmable DC power supplies, electronics loads, and measurement racks with the supporting I/O for the automated control and monitoring of data. Regenerative power supplies combine the programmable DC power supply and electronic load into one piece of equipment, providing both a source and a sink for tests. This technology provides an alternative to the conventional approach of using resistive load banks to burn excess energy. This method saves the energy that was previously burned and recycles it for neighboring racks and test equipment.
DC power supplies perform other test functions as well. Other tests require equipment that can simulate the internal resistance of a battery over its discharge cycle and analyze a device under test’s (DUT’s) performance over emulated battery life. Programmable DC supplies with sequencing, I-V control, pulse and sine wave functions will also enable standards testing for electromagnetic compatibility (EMC), airborne equipment compliance, and the compliance of other components for industrial and medical standards. These are all functions that the Kikusui PXB series of bidirectional power supply can perform. This article dives into the PXB series power supply and discusses its benefits in various test applications.
A brief look at the PXB series power supply
The PXB series are bidirectional DC power supplies with an output that has a rated power of 20 kilowatts (kW) and voltage up to 1500 V in a compact 3U-sized frame. In most bidirectional supplies, the output power is suppressed by the input voltage. In these supplies, the full power can be used with a 480 V input, but only 50% or 67% of the sink or source is utilized with a 208 V input. With the PXB, there is no output power suppression, so the full 20 kW of sink and source capability can be utilized at any input. Up to ten units of the PXB can be paralleled for 200 kW of power. This feature, combined with its full utilization at any input voltage, allows for a highly power dense testing solution.
As shown in Table 1, each model will offer constant voltage (CV) and constant current (CC) modes with different ranges for battery charging and discharging.
Table 1: The PXB series specifications.
The power supplies offer a high level of voltage stability with no overshoot and ringing. There is also a low level of voltage and current ripple (~1420 mV peak-to-peak) in the PXB compared to DC power supplies on the market that generally have nearly four times the amount of ripple; this allows the PXB to generate a more accurate test with reliable results (Figure 1).
Figure 1: Comparing the PXB series’ current and voltage ripple (on left) with a competitor’s ripple (on right).
The utility of a bidirectional output
A bidirectional power supply can act as both a sink and a source. Previously, these functions were handled by two different pieces of equipment: a supply and a load. In the testing of energy storage devices such as fuel cells and batteries, for example, the power supply would provide the necessary energy for a charge while the load would provide the necessary load profile for a discharge. A bidirectional output allows energy to flow towards the input as well as the output; this is necessary for testing bidirectional energy systems such as bidirectional chargers that use regenerative braking to supply power from the vehicle back to the battery. This could also be used for other DUTs such as bidirectional DC power supplies or bidirectional drives.
Using the PXB for battery testing
Charging and discharging batteries
Battery performance testing is critical in understanding how to achieve the best battery performance over time (e.g., minimize power dissipation, extend lifetime). Understanding the usable battery capacity over cycles, temperature, current, and storage time is important in maximizing battery usage. In order to understand all of this, both steady-state and transient response of batteries is paramount.
Typically battery charging occurs in two phases: an initial CC phase where the battery is charged with a current (Ich) until a sufficient charging voltage is reached (Vch), and a second CV phase to maintain the charging voltage and hold it constant while the charging current is continuously reduced. This second phase is concluded when the threshold current lend is reached -- this process is known as CC-CV charging (Figure 2). Other charging profiles such as constant power (CP) charging may also be used where the charging current is initially high (or voltage initially low in some cases) and is gradually decreased. All of these profiles would be necessary in battery performance tests where batteries are charged and discharged extensively while data is collected on the DUT’s decline in capacity.
Figure 2: Standard CC-CV charging procedure for batteries.
The PXB offers the CC, CV, and CP charging profiles and source and sink capabilities for seamless charging and discharging to analyze the steady-state behavior of batteries (Figure 3).
Figure 3: Charging and discharging of a battery using the 1500V PXB model with a CC mode from -30 A to 30 A (left) and the 500 V PXB model with a CC mode from -120 A to 120 A with a rapid switch to CV of 400 µs (right).
Testing a battery’s transient response
These tests can occur at different charge and discharge rates (C-rate) to test battery performance under stress (e.g., 0.1C, 0.5C, 1C, 2C, 3C, etc.) with different charge and discharge currents. A pulsed discharge will allow users to discern the transient behavior of the battery under test. The sine/pulse feature of the CC and CV modes of the PXB series can be used to obtain a transient response analysis of the battery (Figure 4).
Figure 4: The transient behavior of the battery in response to a dynamic load can be tested by adjusting the duty cycle and frequency of the CV and CC modes to generate a step load current.
Simulating the battery’s changing IR
Ideally, a battery will exhibit a low internal resistance (IR) to deliver a high current on demand; however, a high IR can occur due to both calendar and cyclic again that will cause the battery to heat up when used, wasting energy and causing a voltage drop. The PXB series can mimic these I-V characteristics where the voltage output decreases due to an increasing IR. This diverges from the limited functionality of a constant voltage power supply where external circuits would be required to simulate this behavior (Figure 5). This feature saves both test development time and space in the test facility. These characteristics allow the PXB to emulate not only a battery, but also fuel cells, solar cells and on-board chargers (OBC).
Figure 5: The PXB series operate according to the I-V characteristics of an energy storage device that will exhibit a drop in voltage with an increasing internal resistance.
The benefits of a regenerative power supply
Most loads will burn off excess energy through resistive elements as heat. This can quickly become a costly endeavor, as these devices require convective cooling with fans—another added energy burden. This can also add significant heat to the ambient environment which can distort test accuracy without the proper ventilation. As stated earlier, the PXB series acts as both a source and a sink. However, the PXB series does not burn off the excess energy as heat. The regenerative capability of the power supply allows the power supply to redirect the power to other nearby racks (Figure 6). This capability allows test facilities and labs to easily use the excess power for their native equipment and save on energy costs.
Figure 6: The PXB series can redirect power to nearby test equipment saving power that would otherwise be dissipated as heat.
Using the PXB’s programmability for standards testing
The PXB has several programmable states outside of the I-V control, variable internal resistance and pulse and sine functions. Sequencing enables users to create and edit sequences visually with a mouse -- a feature that allows for testing of the safety and electrical parameters of equipment in accordance with standards such as IEC 61000-4 and LV123, LV124, LV148, etc (Figure 7). Standard tests can be performed using a user-generated sequence or an external signal generator. The sequence list is likely preferable for those who know exactly what their desired charge/discharge waveform is (e.g., EV lifecycle testing), and a signal generator might be preferable when the user already has a method to output the waveform they need (and the PXB can act as a power amplifier).
Figure 7: Test process for conducting standards test with the PXB series.
Summarizing the strengths of the PXB
The PXB series is a highly power dense bidirectional programmable power supply. The bidirectionality of the supply saves precious floor space, combining the functionality of a power supply and load in one piece of equipment for battery charging/discharging and for bidirectional inverters and converters. With its regenerative capability, any potentially wasted energy can be redirected to nearby equipment to both recycle power and reduce the energy consumption typically required for ventilation. The power supply can emulate the behavior of batteries, fuel cells, and solar cells to test the performance of devices that might be powered by these various forms of energy storage. The sequencing feature enables the ease of standards and compliance testing of DUTs. Finally, the modules can be paralleled for up to 200 kW of operation, widening the range of potential tests. All of these features set the PXB up as a high-performing power supply for a wide range of testing applications.