Battery Stack Monitor Maximizes Performance of Li-Ion Batteries in Hybrid and Electric Vehicles

May 05, 2020 //By Cosimo Carriero, Analog Devices
Battery Stack Monitor Maximizes Performance of Li-Ion Batteries in Hybrid and Electric Vehicles
Lithium-ion (Li-Ion) batteries offer a high energy density, but to maximize performance, a battery monitoring system (BMS) is mandatory. A state-of-the-art BMS not only allows you to extract the highest quantity of charge from your battery pack, but also lets you manage the charge and discharge cycles in a safer way, which results in an extended life.

Accurately measuring a battery’s state of charge (SOC) increases battery run time or decreases weight. A precise and stable device does not require factory calibration after PCB assembly. Stability over time improves safety and avoids warranty problems. A self-diagnostics feature helps reach the right automotive safety integrity level (ASIL). A battery pack is a challenging environment for electromagnetic interferences (EMI), so special care has been put into designing the data communication link in order to ensure robust and reliable communication between the measurement chips and the system controller. Cables and connectors are among the main causes of failures in battery systems, so wireless solutions are presented here. Wireless communication designs increase reliability and reduce total system weight, which in turn increases mileage per charge.

Introduction

An energy storage unit has to provide high capacity and the ability to release the energy in a controlled manner. Storage and release of energy, if not properly controlled, can result in a catastrophic failure of the battery and ultimately fire. Batteries can fail for several reasons, most of them related to inappropriate use. Failure can come from mechanical stress or damage, electrical overstress in the forms of deep discharge, overcharging, overcurrent, and thermal overstress. In order to reach the highest levels of efficiency and safety, a battery monitoring system is required.

The main function of the BMS is to keep any single cell of the battery pack inside its safe operating area (SOA) by monitoring the following physical quantities: stack charge and discharge current, single cell voltage, and battery pack temperature. Based on these quantities, not only can the battery be operated safely, but also SOC and state of health (SOH) can be computed.

Another important feature provided by the BMS is cell balancing. In a battery stack, single cells can be arranged in parallel and in series in order to achieve the required capacity and operating voltage (up to 1 kV or higher). Battery manufacturers attempt to provide stacks with identical cells, but this is not physically possible. Even small differences lead to different charge or discharge levels, with the weakest cell in the stack disproportionately affecting overall stack performance. Accurate cell balancing is a significant feature in a BMS, enabling safe operation of a battery system at its highest capacity.

BMS Architectures

An electric vehicle battery consists of several cells stacked in series. A typical stack—with 96 cells in series—when charged at 4.2 V can develop a total voltage in excess of 400 V. Higher voltages can be reached by stacking more cells. Charge and discharge current are the same for all the cells, but voltages have to be monitored on every single cell. To accommodate the large quantity of cells required for high powered automotive systems, batteries are often divided into modules, and distributed through-out available spaces in the vehicle. With 10 cells to 24 cells in a typical module, modules can be assembled in different configurations to suit multiple vehicle platforms. A modular design can be used as the basis for very large battery stacks. It allows battery packs to be distributed over larger areas for more effective use of space.

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