EV battery management systems improve safety
Article By : Tahar Allag
Battery monitors supervise voltage and cell temperature in battery management systems for EVs and HEVs.
The market share for electric vehicles (EVs) and hybrid electric vehicles (HEVs) could reach 30 percent during the 2020s. This market attractiveness is due to increased concerns over internal combustion engine (ICE) vehicles’ environmental impact and efforts to reduce fuel costs. So, automotive manufacturers are investing in the electrification of their fleets, leading to significant improvements in battery technologies as well as battery-pack safety.
This article explains how battery-monitoring ICs can improve the safety of EVs and HEVs.
Battery management systems in EVs/HEVs
Battery-powered vehicles replace traditional ICEs with an electric motor that does not use gasoline as a source of energy. Instead, batteries store electric energy for use by an electric motor. An EV consists of many components, including:
- an onboard charger that charges the battery directly from the grid;
- a DC/DC converter that bucks (steps down) the power to lower voltages in order to power car electronics such as the heater and automatic windows;
- a power inverter that delivers energy from the battery to the electric motor;
- battery monitors and current sensors that monitor the voltage, current, and temperature of the battery pack; and
- a host microcontroller (MCU) that acts as the “brain” and coordinates all actions within the EV.
Figure 1 illustrates a high-level architecture of an EV battery management system (BMS).
Figure 1 A battery management system facilitates monitoring and control of the high-voltage battery stack in EVs/HEVs. Source: Texas Instruments
In typical applications, the battery monitors are stacked in a daisy chain, as shown in Figure 2. Each device connects to battery cells through sense lines to monitor every cell in the pack. Every monitor in the stack transfers information through a communication line from the top of the stack to the bottom device. A bridge device is required in order to facilitate the communication between the host MCU and the stack devices.
Figure 2 Battery monitors are stacked in a daisy-chain configuration. Source: Texas Instruments
Improving safety with battery monitors
Thermal runaway is the primary cause of system safety issues within HEVs/EVs because it causes an unstoppable chain reaction. When the temperature rises rapidly to 400°C, the energy stored in the battery is suddenly released. This causes the battery to become gaseous, and a fire may erupt.
Thermal runaway can be caused by several factors:
- An internal short circuit in the battery cells if they become physically damaged after an accident or if an object penetrates the battery pack.
- An external short circuit that can cause the release of an unlimited amount of power, which heats up the cells rapidly.
- Overcharging the battery beyond its maximum allowable voltage.
- High charging and discharging currents.
To prevent these events from occurring, monitoring the battery cells is crucial. Battery monitors are designed to address all of these issues and help make EVs and HEVs safer.
Voltage monitoring
An inaccurately reported voltage can cause the MCU to overcharge the battery, potentially damaging the cell or causing thermal runaway. Additionally, measurement redundancy is critical to increase safety and prevent failure or drift over time. Implementing two completely independent analog-to-digital converters (ADCs) with two independent paths can help achieve Automotive Safety Integrity Level D (ASIL-D) compliance according to the ISO 26262 standard.
The redundancy is designed to detect any failure in one of the ADCs and serves to double-check the accuracy of the measurement from an independent ADC. In case of failure or a drift in the measurement, during the safety diagnostics, the same cell measurement will be double-checked and measured against the same reading using an auxiliary ADC that has a completely independent path and reference.
Take the example of BQ79606A-Q1 automotive precision battery monitor, balancer, and integrated protector from Texas Instruments; it has six dedicated delta-sigma ADCs for each channel and an auxiliary ADC for redundancy. The device has a set of window comparators that provides cell voltage monitoring for all six channels separately from the main acquisition path and works in parallel with the main ADC route. This comparator function is entirely separate from the ADC function; so, even if the ADC function fails, the analog comparators will still flag the crossing of under- and over-voltage comparator thresholds.
Cell temperature monitoring
Lithium-ion (Li-ion) batteries do not tolerate extreme temperatures. A typical tolerable temperature for a battery pack is between 0°C and 60°C. In addition to external factors, some of the switching elements consume power and release some of that power as heat, contributing to a thermal increase in the battery case. Monitoring and controlling the battery pack temperature is essential to maintain the health and safety of the pack and to prevent thermal runaway.
Today’s battery monitors have several general-purpose inputs/outputs (GPIOs) for temperature sensing. The BQ79606A-Q1 precision battery monitor can measure up to six thermostats in a six-channel pack with high accuracy, providing plenty of redundancy to prevent failures in temperature monitoring. The device uses an integrated window comparator to monitor the GPIO’s inputs for over- and under-temperature conditions in the cells.
When enabled, the comparator cycles through each of the temperature sense inputs and compares the voltage to programmed thresholds. This comparator function is entirely separate from the ADC function; even if the ADC function fails, the analog comparators flag the crossing of the under- and over-temperature comparator thresholds. The host MCU will be notified through fault lines immediately to trigger the cooling system and take preventive measures before reaching intolerable temperatures.
Communication robustness and speed
As mentioned earlier, battery monitors are stackable in a daisy-chain configuration. Each device passes its information through another device downstream to reach the host. The communication line between devices in the stack and the host MCU must be robust to ensure fast and full diagnostics in just few milliseconds. The MCU should have reliable communication to any device in the stack to read, configure, and perform diagnostics.
However, the noisy environment of an EV poses a real challenge for battery monitors. To counter that, TI’s battery monitors use differential signaling using two pins, COMP and COMN. As shown in Figure 3, the COMP and COMN pins of the BQ79606A-Q1 battery management chip are monitored during different noisy environments.
Figure 3 This is how daisy-chain communication performance looks in the presence of noise. Source: Texas Instruments
Across all frequencies, the signal integrity is maintained and the differential noise cancels out. The drivers can tolerate up ±20V of noise amplitude. In addition, diagnostic mechanisms built into the communication signal can help ensure that if for some reason the signal is corrupted, the device detects a communication failure. This architecture ensures robust and fast communication with the host.
Li-ion batteries are sensitive to overcharging, extreme temperatures, and physical damage. Any of these conditions can lead the cells to thermal runaway. Battery monitors have evolved to monitor cell voltages with a high degree of safety and accuracy in order to prevent overcharging. Temperature monitoring of the pack occurs through multiple redundancies to ensure that the pack temperature is well within an acceptable range. The communication between stack monitors is designed to withstand noisy environments and ensure the safe transfer of information to the main MCU.
This article was originally published on EDN.
Tahar Allag is senior systems engineer at Texas Instruments.
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