While Li-ion batteries promise dramatic improvements in electric and hybrid-electric vehicles, fulfilling that promise requires a new generation of semiconductor devices. These devices must monitor and manage Li-ion cells efficiently to get the batteries’ promised performance. Otherwise, a vehicle that should go 200 km (125 mi) on a charge might coast to a stop after 180 km (112 mi).
Equally important, the new semiconductor monitoring devices must detect battery faults reliably. That means monitoring each Li-ion cell as well as monitoring the monitoring system itself.
These considerations have become familiar in the automotive industry, as electronics play an ever-increasing role. Yet Li-ion batteries introduce entirely new safety concerns which accompany use of a high-current system that contains large numbers of individual cells. The system must use multiple semiconductor monitors to measure the state of each cell continuously. To handle this task, all the monitoring devices have to communicate among themselves with absolute reliability—a tall order in a vehicle environment that has high levels of electromagnetic interference (EMI).
With these requirements in mind, the four most important criteria for a good Li-ion monitoring system are:
- Accuracy: The monitoring system has to determine each Li-ion cell’s state of charge accurately enough to get the best performance out of the battery array. How accurate is that? The answer depends on the type of cells used.
- Thorough diagnostics: In addition to monitoring each cell’s state, the system must constantly run checks on its own functionality to ensure that every part is operating at the expected accuracy.
- Robust communication: All the parts of the monitoring system have to coordinate their operation, so they must communicate reliably—a requirement that defeats most conventional communication methods in this noisy environment.
- Safety: By managing the Li-ion cells appropriately, the system avoids failures and safety issues. When faults do occur, the system must take appropriate action, while avoiding false alarms.
Although semiconductor vendors have been making devices to manage Li-ion batteries for many years, only recently have devices been developed to meet the particular demands of large automotive battery arrays. This article provides an overview of the requirements for the new generation of Li-ion monitoring devices and their vital role in enabling advanced electric and hybrid-electric vehicles.Avoid monitoring device disintegration
The figure below shows the basic layout of a Li-ion battery array for a hybrid vehicle. In this example, the array consists of eight packs of 12 cells each for a total of 96 Li-ion cells. The cells in a pack are connected in series, and each cell in the pack also connects to a monitoring device.
Though a microcontroller may be used for overall management, these series-connected battery packs can have voltages over 50V—too high for a microcontroller to handle directly. Additionally, the battery system can generate transient voltages with values several times that of the normal pack voltage. Cell monitoring circuits must survive these over-voltage events and sometimes operate under fairly onerous conditions.
Such assaults begin the moment the battery is first connected. Because cost considerations typically prohibit the provision of connection sequencing within the connector design, the first thing the monitoring devices see is a random connection event. While the monitoring devices can easily withstand the voltages in this event, the currents can be another matter.
The battery currents that charge any external capacitors in an unprotected system can range to several amperes. These currents inevitably flow through some part of the electronics, usually the electrostatic discharge (ESD) protection circuits inside connected ICs. The result can be a spectacular disintegration of the IC. In most circumstances, the only practical way to limit these currents is to place resistors in line with the battery terminals.
Unfortunately, the use of these resistors has traditionally limited the accuracy of monitoring devices because leakage current through the resistors creates uncertainty in the monitoring device’s voltage measurements. To ensure high measurement accuracy, the leakage currents need to be kept at low and predictable values—a difficult task with conventional types of bipolar-transistor input circuits. The latest generation of monitoring devices use a new charge-transfer architecture that makes the leakage current more predictable and thus dramatically improves measurement accuracy.