THE VIRTUAL BATTERY
The comemso battery cell simulator – the all-in-one battery management system test and development solution for (mobile) energy storage systems.
BATTERY CELLS NEED CONSTANT MONITORING
Electromobility is growing at a tremendous rate worldwide. For today’s mobile energy storage systems, that means they not only have to deliver high performance, they also need to ensure safe, reliable operation. Doing so requires constantly monitoring the voltage and temperature of every single battery cell. This is done with a special electronic monitoring system called the cell management controller (CMC).
Individual battery cells are combined into cell modules, and each cell module has its own CMC (Fig. 1). The cell management controller is an electronic component that has one or more microcontrollers and is specially designed for monitoring battery cells. The CMC is connected via measuring leads to the positive and negative poles of the cells so it can measure the voltage reliably. It is also connected to temperature sensors, which are indispensable for optimizing battery loads and efficiency.
Since the cell modules consist of multiple single cells connected in series, their internal resistance can vary greatly due to a wide range of factors such as variations in production quality or age-related fatigue. This can lead to different charging and discharging curves, resulting in critically deep discharge of the battery cells or, during charging, in the charge cut-off voltage being exceeded even though the overall voltage is still in the nominal range. Depending on the type of battery, this can lead to irreparable damage or even combustion of the battery modules!
If deviating charge levels occur (Fig. 1), the charge levels of the individual battery cells can be adjusted to match each other with an equalizing system. This process is called balancing. The equalizing system involves an electronic circuit that is normally an integral part of any battery management system and controls the uniform charging of the individual battery cells in a cell module. It uses the voltage to determine the charging level, which is called state of charge (SoC). In addition, the temperature of each cell, which depends on the chemical process involved in charging and discharging, is measured.
THE CONTROLL CENTER
The battery management system (BMS) includes the battery management unit (BMU) and all cell management controllers (Fig. 2). The BMU is the central control unit for battery modules such as those used to drive electric vehicles or all other kinds of energy storage systems. It acts as the “brain” where all of the information collected by the battery monitoring systems comes together. Using the battery cell voltages, it determines the current states of charge (SoC) and controls the overall communication between the battery and the vehicle. When necessary, it also gives the command to perform balancing so that the battery cells are not deep-discharged or overcharged as described above.
In battery-powered vehicles, it is supplied with voltage from the vehicle’s 12- or 24-volt electrical system, so it has no effect on the range when the vehicle is at rest. In contrast, the cell management controllers are generally supplied with electricity from the vehicle battery and thus cause slight charge losses. This makes it important, when developing a CMC, to ensure that its energy consumption is kept as low as possible and to implement a rest mode to minimize “self-discharging”.
BALANCING: PASSIVE AND ACTIVE
When the cell voltages are not all equal, the full cell pack ceases charging once a cell reaches an SoC of 100%. At this point and no later, balancing must be initiated. This involves discharging a fully charged cell to the level of the other cells so that all of the cells can then be charged together. There are two types of balancing: passive and active. In passive balancing, resistance is applied according to an algorithm to the cells with the highest SoC. These cells are then charged at a much lower rate, or are even discharged. The other incompletely charged battery cells in the series circuit continue to receive the full charging current until the SoC is balanced. This method is easily implemented and thus economical. However, it causes energy to be converted to heat, which reduces efficiency.
In active balancing, charge is exchanged among the battery cells, a process that is controlled by the BMU. In this process, energy is transferred from cells that are already fully charged to those neighboring battery cells that have not yet reached their SoC. There are three methods:
a) semiconductor switching (e.g. matrix with transistors),
b) capacitive charge transfer, and
c) inductive charge transfer.
The advantage here is greater efficiency since only a small portion of the energy is converted to heat. However, active balancing involves more circuitry, which results in higher development and hardware costs.
VIRTUAL BATTERY CELLS
Since the conditions for testing real batteries are dangerous, unreproducible and not automatable, the validation of battery management systems calls for configurable “virtual battery cells”. These virtual battery cells have been implemented in the battery cell simulator: Since all of their electrical characteristics can be parameterized, they can simulate all required states and faults for the battery management system.
High-precision ammeters between the cell inputs on the BMS and the cell outputs on the cell modules are used to measure quiescent current and detect unwanted leakage current, which could occur due to faulty BMS outputs or incorrect software control. This leakage current measurement enables early detection of errors during deactivation of the BMS, preventing deep discharge and damage to the battery cells. In addition, the temperature sensors on the cells are replaced by suitable galvanically isolated temperature sensor emulators, the NTC/PTC sensor simulation. All data signals from the individual cell modules can then be fed into the test bench or hardware-in-the-loop system (HiL system). The battery cells and temperature sensors from comemso emulate the real functionality so the BMS can be operated in the lab. In addition, fault simulation can be used to generate faulty system states so that component malfunctions and their detection can be verified in the lab. This capability makes a comemso battery cell simulator a key test unit for verifying all BMS functionality.
BCS test system
Block diagram of virtual battery cell
Typical users of a battery cell simulator include developers of algorithms for passive and active balancing, developers of battery management systems, manufacturers of chips for battery management systems, manufacturers of energy storage systems and vehicles who want to verify functionality before BMS integration in series production, and test and certification labs.