Analysis of active charge balancing method for Nickel Hydride No. 5 batterypacks

　　The Automotive Systems Engineering Department of Infineon Technologies AG in Munich recently received a task to develop E-Cart. The E-Cart is a drivable vehicle primarily used to demonstrate the electrical performance of hybrid vehicles. The car would be powered by a massive Nickel Hydride No. 5 batterypack, and developers realized at the time that battery management with charge balancing was an absolute necessity. In this case, active energy transfer between cells must be used instead of the traditional simple charge balancing scheme. The active charge balancing system they developed provides better performance at comparable material costs to passive solutions.

　　Battery system architecture

　　Nickel-cadmium batteries, and later nickel-metal hydride batteries, dominated the battery market for many years. Lithium-ion batteries have only recently entered the market, but their market share is growing very rapidly due to their vastly improved performance. The energy storage capacity of lithium-ion batteries is amazing, but even so, the capacity of a single battery cell is still too low in terms of voltage and current to meet the needs of a hybrid engine. Connecting multiple battery cells in parallel can increase the current provided by the battery, and connecting multiple battery cells in series can increase the voltage provided by the battery.

　　Battery assemblers often use abbreviations to describe their battery products, such as "3P50S" means that the battery pack has 3 parallel battery cells and 50 series battery cells.

　　Modular structures are ideal when managing batteries containing multiple cells connected in series. For example, in a 3P12S battery array, every 12 battery cells form a module (block) after being connected in series. These battery cells are then managed and balanced by an electronic circuit with a microcontroller at its core.

　　The output voltage of such a battery module depends on the number of battery cells connected in series and the voltage of each battery cell. The voltage of a Nickel Hydride No. 5 batterycell is usually between 3.3V and 3.6V, so the voltage of a battery module is approximately between 30V and 45V.

　　The driving of hybrid vehicles requires a DC power supply voltage of approximately 450V. In order to compensate for changes in battery cell voltage depending on the state of charge, it is appropriate to connect a DC-DC converter between the battery pack and the engine. This converter also limits the current output from the battery pack.

　　To ensure that the DC-DC converter works optimally, the battery pack voltage is required to be between 150V and 300V. Therefore, 5 to 8 battery modules need to be connected in series.

　　The need for balance

　　If the voltage exceeds the allowed range, Nickel Hydride No. 5 batterycells are easily damaged (see Figure 2). If the voltage exceeds the upper and lower limits (taking nanophosphate lithium-ion batteries as an example, the lower limit voltage is 2V and the upper limit voltage is 3.6V), the battery may be irreversibly damaged. The result is at least a faster self-discharge of the battery. The battery output voltage is stable over a wide state of charge (SOC) range, and there is little risk of the voltage deviating from the safe range. But at both ends of the safe range, the ups and downs of the charging curve are relatively steep. Therefore, voltage must be closely monitored as a precaution.

　　If the voltage reaches a critical value, the discharging or charging process must be stopped immediately. With the help of a powerful balancing circuit, the voltage of the relevant battery cell can be returned to a safe range. But to achieve this, the circuit must be able to transfer energy between cells as soon as the voltage of any cell in the battery pack begins to differ from that of other cells.

　　Charge balancing method

　　1. Traditional passive method: In a general battery management system, each battery cell is connected to a load resistor through a switch. This passive circuit can discharge individual selected cells. But this method is only suitable for suppressing the voltage rise of the strongest battery cells in charging mode. In order to limit power consumption, such circuits generally only allow discharge with a small current of about 100mA, causing the charge balance to take up to several hours.

　　2. Active balancing method: There are many active balancing methods in relevant information, all of which require a storage element for transferring energy. If capacitors are used as storage elements, a huge switch array is required to connect them to all battery cells. A more efficient method is to store energy in a magnetic field. The key component in this circuit is a transformer. The circuit prototype was developed by Infineon's development team together with VOGT Electronic Components GmbH. Its function is:

　　a. Transfer energy between battery cells

　　b. Multiplex multiple individual cell voltages to a ground-based analog-to-digital converter (ADC) input

　　The circuit is constructed according to the flyback transformer principle. This type of transformer stores energy in a magnetic field. The air gap in its ferrite core increases the magnetic resistance and therefore prevents magnetic saturation of the core material.

　　The circuits on both sides of this transformer are different:

　　a. The primary coil is connected to the entire battery pack

　　b. The secondary coil is connected to each battery unit

　　One practical model of this transformer supports up to 12 battery cells. The number of possible connections to the transformer limits the number of battery cells. The above prototype transformer has 28 pins.

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