E-Mobility Engineering 014 l InoBat Auto dossier l In Conversation: Brandon Fisher l Battery monitoring focus l Supercapacitor applications insight l Green-G ecarry digest l Lithium-sulphur batteries insight l Cell-to-pack batteries focus

−2 to +5.5 V is suitable for most battery chemistries, and allows measurement of voltages across busbars, although the chips include provisions for bypassing busbars without dedicating any measurement channels. All the cells can be measured simultaneously and redundantly with two individual analogue-to-digital converters (ADCs) for each channel in the chip. The continuously operating ADCs have a sampling rate of 4.096 MHz and allow reduced external analogue filtering and aliasing-free measurement results. These ADCs are based on a sigma- delta architecture. While that is a staple in the toolkit of today’s signal acquisition and processing system designers, the different blocks in the conversion chain can be optimised to improve the accuracy and stability of battery monitoring. Typically there are two blocks in a sigma-delta converter. The sigma modulator is a single-bit, over-sampling converter that uses a higher sampling rate than theory requires to reduce the quantisation noise. This is followed by a digital low-pass filter that removes the battery monitors on separate printed circuit boards. This involves detecting individual cell over-voltage (OV) and under-voltage (UV) conditions, from 0.77 to 2.88 V for the UV settings, and OV settings from 3.7 to 4.5 V. The latest battery monitoring chips improve the accuracy and stability of the measurement of voltage and current of the cells. The latest chips typically measure up to 16 battery cells connected in series for a module, achieving a lifetime total measurement error of less than 2 mV over the full temperature range of the pack. A measurement input range of noise. The modulator loops back for the next bit, and the process allows the filtering to be changed on a bit-by-bit basis. More stable converters use a multi-stage noise shaping modulator architecture. This enables the design of stable sigma modulators by combining inherently stable lower-order loops. This allows the accuracy of the voltage and current measurement to be constantly optimised over time, compensating for changes in the system, including temperature. This is a key capability for a BMS. Higher noise reduction can be achieved by subsequent programmable infinite impulse response filters. All of this allows measurements of ±1.8 mV at 3.3 V per cell across a temperature range of -40 to +125 C. Other monitor chips use coulomb counting, capturing the accumulation of charge to provide over-current detection in both ignition-on and off states. Fully synchronised current and voltage samples from a 16-bit ADC have a maximum error of ±2 mV in the 0.5 to 4.3 V range across a temperature range of -40 to +105 C. Focus | Battery monitoring A sigma-delta converter can use additional digital signal processing to boost stability of measurements (Courtesy of Analog Devices) Balancing cell charges An additional function in battery monitoring chips is the ability to balance the charge in the cells. To allow for maximum cell usage, cell charging/discharging is passively balanced via an internal cell controller resistor network for lower balancing currents or an external network for higher balancing currents. If the monitoring chip detects a charging imbalance between cells it will connect the cells with the highest state of charge (SoC) to a resistor or a grid of resistors for better heat distribution as the load. This reduces the SOC level with controlled partial discharging of the single cell. The battery charging is interrupted during the balancing action, which is often repeated until all the cells reach the same SoC. For example, a multi-channel battery monitoring and balancing chip for up to 12 lithium-ion cells in a battery pack uses a 16-bit analogue-to-digital converter to measure the cell voltage, with ive temperature measurement sensors and cell balancing to 200 mV. It also includes diagnostic tools to ensure safe operation. This set-up monitors 12 cells in a pack, with cell balancing to 200 mV between cells (Courtesy of 0nfineon ;echnologies) 34 Summer 2022 | E-Mobility Engineering