Power
Emulating battery cells for an effective battery management system testing and development
By Ralf Oestreicher, market segment manager automotive, Rohde & Schwarz and Anja Fenske, product manager for power supplies, Rohde & Schwarz
T
he global demand for electric vehicles continuously gained momentum for fully electric, plug- in hybrid or mild hybrid vehicles. Especially here the moderate safety regulation below 60V and small additional costs make the 48V mild hybrid electric vehicles an attractive alternative to other AV technologies. But all these EV technologies have one thing in common. They require an intelligent battery management system (BMS) to maximize their power, range and efficiency. Those battery management systems actively monitor, control and manage various battery cell parameters such as voltage, current, thermal and energy management as well as cell balancing. They also monitor the state of charge and the state of health of the battery cells. A typical battery management system consists of one or more cell monitoring controllers (CMC) and a battery management controller (BMC) depending on the voltage level of the battery pack. The BMC and the CMC are set up in a master and slave architecture. The voltage limitation of the CMC respectively the number of the cells determine the number of the required CMC, for instance 14 cells per controller.
Testing challenges
To optimize the monitoring, controlling and managing of the battery cells is the greatest challenge in the BMS development and validation. For example, overcharging and deep discharging reduces the lifetime of batteries, hence correct control by the BMC must be ensured. Battery defects could lead to overheating of a battery, and even cause a fire. Therefore, stress testing by emulating real-world conditions, including error scenarios such as overload and under-voltages and complex cell profiles should be conducted. CMC suppliers need to characterize the
56 June 2023 Components in Electronics
www.cieonline.co.uk.uk
controller and perform production test with a flexible cell emulation. The emulation of predefined state of charge is also required for each single cell. To ensure the performance and safety, it is crucial to emulate the battery cells used in electric vehicles.
How to simulate batteries with power supplies
There are basically four different chemistries for rechargeable batteries. There’s Lithium-ion (Li-ion), which is the most common one, and
usually found in electric vehicles. There’s Nickel Cadmium (Ni-Cd), Nickel-Metal Hydride (Ni-MH) and Lead Acid. Each of these different battery types have different characteristics, different energy densities and different lifetimes. It is very important to choose the right battery type for your application, which depends on the energy density, the device size, durability, the costs and safety regulations.
For this reason, it is very important to start with the battery simulation very early.
The development process depends on the chosen battery. The open circuit voltage of a lithium-ion battery decreases with the state of charge, while the internal resistance is increasing with decreasing state of charge. Every battery has its individual discharging and charging curve. Capacity, open circuit voltage (Voc) and the equivalent series resistance (ESR) are important battery characteristics that depend on the state of charge. It is not possible to just use a normal voltage source to simulate batteries. It needs a dedicated battery simulation to have these different characteristics in place. The Rohde & Schwarz battery portfolio with the R&S NGM200 and the R&S NGU201 power supplies is a suitable fit for battery applications. A dedicated flexible battery emulation, their source and sink operation, a 6.5 digit resolution, fast load recovery time, high-speed data acquisition, and multiple measurement ranges ensure an excellent accuracy. These power supplies can simulate the real output performance of a battery. To create a battery model there are different options. A battery model can be created
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