To reduce malfunctions, it is necessary to accurately detect the battery's voltage, current, and temperature, preprocess the results, calculate the charging and operating status, send the results to the engine control unit (ECU), and control the charging function.
The modern automobile was born in the early 20th century. The first automobile relied on manual starting, which required a great deal of force and posed a high risk; this hand-cranked crank caused many fatal accidents. In 1902, the first battery-powered starter motor was successfully developed, and by 1920, all automobiles had adopted electric starting.
Initially, dry cell batteries were used, which had to be replaced when their power was depleted. Soon after, liquid batteries (the older lead-acid batteries) replaced dry cell batteries. The advantage of lead-acid batteries was that they could be recharged while the engine was running.
In the last century, lead-acid batteries remained largely unchanged, with the last major improvement being their sealing. What truly changed was the demand for them. Initially, batteries were used only to start cars, sound the horn, and power the lights. Today, they power all of a car's electrical systems before ignition.
The surge in new electronic devices goes beyond consumer electronics like GPS and DVD players. Today, vehicle-mounted electronic systems such as engine control units (ECUs), power windows, and power seats are standard features in many basic car models. This trend is exponential.
The increased load on the battery has already had a serious impact, as evidenced by the growing number of electrical system-related failures. According to statistics from ADAC and RAC, nearly 36% of all automotive failures can be attributed to electrical faults. Further analysis of this figure reveals that over 50% of these failures are caused by the lead-acid battery.
Assess the health of the battery
The following two key characteristics can reflect the health status of a lead-acid battery:
(1) State of Charge (SoC): SoC indicates how much charge the battery can supply, expressed as a percentage of the battery’s rated capacity (i.e., the SoC of a new battery).
(2) Operating Status (SoH): SoH indicates how much charge the battery can store.
Charging status
The charging status indicator is like a fuel gauge for a battery. There are many methods for calculating the System State of Charge (SoC), among which two are the most commonly used: open-circuit voltage measurement and coulomb counting (also known as coulomb counting).
(1) Open circuit voltage (VOC) measurement method: The open circuit voltage of the battery under no-load conditions has a linear relationship with its state of charge. This calculation method has two basic limitations: first, in order to calculate the SoC, the battery must be open-circuited and not connected to a load; second, this measurement is only accurate after a fairly long stabilization period.
These limitations make the VOC method unsuitable for online SoC calculations. This method is typically used in auto repair shops, where the battery is removed and the voltage between the positive and negative terminals can be measured with a voltmeter.
(2) Coulomb measurement method: This method uses coulomb counting to calculate the integral of current over time, thereby determining the SoC. This method can be used to calculate the SoC in real time, even when the battery is under load. However, the error of the coulomb measurement method increases over time.
Generally, the state of charge of a battery is calculated by combining open-circuit voltage and coulomb counting.
Running status
State of Operation (SOH) reflects the general state of a battery and its ability to store charge compared to a new battery. Due to the inherent nature of batteries, SOH calculations are highly complex and rely on knowledge of battery chemistry and the surrounding environment. A battery's SOH is influenced by many factors, including charge acceptance, internal impedance, voltage, self-discharge, and temperature.
These factors are generally considered difficult to measure in real time in an environment like a car. During the startup phase (engine start-up), the battery is under maximum load, which best reflects its SoH (Solar Hours).
The SoC and SoH calculation methods actually used by leading automotive battery sensor developers such as Bosch and Hella are highly confidential and often protected by patents. As the owners of the intellectual property, they typically work closely with battery manufacturers such as Varta and Moll to develop these algorithms.
Figure 1 shows a commonly used discrete circuit for battery testing.
Figure 1: Solutions for Discrete Battery Testing
This circuit can be divided into three parts:
(1) Battery Testing: The battery voltage is detected using a resistive attenuator directly connected to the positive terminal of the battery. To detect the current, a sensing resistor (typically 100mΩ for 12V applications) is placed between the negative terminal of the battery and ground. In this configuration, the car's metal chassis is typically grounded, and the sensing resistor is installed in the battery's current loop. In other configurations, the negative terminal of the battery is grounded. Regarding SoH calculations, the battery temperature must also be measured.
(2) Microcontroller: A microcontroller, or MCU, primarily performs two tasks. The first task is to process the results of the analog-to-digital converter (ADC). This task can be simple, such as performing only basic filtering; or it can be complex, such as calculating the SoC and SOH. The actual functionality depends on the MCU's processing power and the needs of the automaker. The second task is to send the processed data to the ECU via a communication interface.
(3) Communication Interface: Currently, the Local Interconnect Network (LIN) interface is the most commonly used communication interface between battery sensors and ECUs. LIN is a well-known single-wire, low-cost alternative to the CAN protocol.
This is the simplest configuration for battery detection. However, most sophisticated battery detection algorithms require simultaneous sampling of battery voltage and current, or battery voltage, current, and temperature. To perform synchronous sampling, up to two additional analog-to-digital converters (ADCs) are needed. Furthermore, the ADC and MCU require power supply regulation for proper operation, increasing circuit complexity. This has been addressed by LIN transceiver manufacturers through integrated power supply regulation.
The next step in precision battery testing for automobiles is the integration of ADCs, MCUs, and LIN transceivers, such as Analog Devices' ADuC703x series precision analog microcontrollers. The ADuC703x is supplied with two or three 8ksps, 16-bit Σ-ADCs, and one 20.48...
The ADuC703x series features an MHz ARM7TDMI MCU and an integrated LINv2.0 compatible transceiver. It also integrates a low-dropout regulator, allowing direct power from a lead-acid battery.
To meet the needs of automotive battery testing, the front end includes the following components: a voltage attenuator for monitoring battery voltage; and a programmable gain amplifier that, when used with a 100mΩ resistor, supports full-scale measurements from below 1A to 1500A.
The chip includes a current meter; an accumulator that supports coulomb counting without software monitoring; and an on-chip temperature sensor.
Figure 2 shows the solution using this integrated device.
Figure 2: Example of a solution using integrated devices
A few years ago, only high-end cars were equipped with battery sensors. Now, more and more mid-range and low-end cars are featuring these small electronic devices, whereas a decade ago they were only found in high-end models. As a result, the number of malfunctions caused by lead-acid batteries has been steadily increasing. In a few years, every car will have a battery sensor, thus reducing the risk of malfunctions caused by the increasing number of electronic devices.
