Battery Monitoring System Improves Battery Efficiency

Batteries play an important role in a variety of applications, as shown in their rapid development in electric vehicle field and use in the storage of renewable energy for smart grids, where the battery monitoring system is of great significance. This article will show you the battery monitoring system from ADI and related solutions.

Control chip can monitor and protect battery power supply

Today, lithium-based chemistry is the advanced battery technology used in a variety of markets, including the automotive market, industrial market and healthcare market. Different advantages available with different types of lithium batteries enable them to better meet the power needs of various applications and product designs. For example, LiCoO2 (lithium cobalt oxide) with high specific energy is ideal for portable products; LiMn2O4 (lithium manganese oxide) with low internal resistance, fast charging and large current discharge is an ideal choice for peak regulation and energy storage applications; LiFePO4 (lithium iron phosphate) has increased resistance to fully charged conditions and can be maintained at high voltages for extended periods, making it the best choice for large energy storage systems that need to operate during power failures. The disadvantage of the battery is that it has a higher rate of self-discharge, but this does not make much difference in the above storage practices.

Different applications require different battery types; for example, automotive applications require high reliability and excellent charging and discharging rates, while healthcare applications require high peak current sustainability to improve efficiency and service life. What all these solutions have in common is a very flat discharge curve across the nominal voltage range for the various lithium chemical compositions. The voltage drop range for standard batteries is 500 mV to 1 V, whereas in advanced lithium batteries such as LiFePO4 or LiCoO2, the discharge curve is showed as a flat area with a voltage drop ranging from 50 mV to 200 mV.

In all of these applications, precise and efficient semiconductors are needed for monitoring, balancing, protection and communication of battery power supply. The first-class battery monitoring system (including battery balance and isolated communication network) will improve reliability by using innovative integrated circuits, which can extend the service life of batteries by 30% especially for the energy storage systems (ESS), so as to take full advantage of the chemical advantages of new lithium batteries.

Article image - BD 

Charging and health of batteries are critical

In the power management chain of the IC connected to the battery voltage rail, the great advantage of the flatness of the voltage curve allows the design of DC-DC converters to operate at maximum efficiency points over small input voltage range. After converting from a known VIN to a very close VOUT, the power chain of the system can be designed to have an ideal duty cycle for the buck-boost converters to achieve 99% efficiency under all operating conditions.

The main drawback of a flat discharge curve is greater difficulty in determining the state of charge (SOC) and state of health (SOH) ratings of the battery. The SOC must be calculated with high precision to ensure batteries’ proper charging and discharging. The safety problems associated with overcharging and resulting chemical degradation and short circuit can result in fire and gas hazards. Excessive discharging can damage the battery, reducing its life by more than 50%.

In the best conditions, accurate and reliable SOC and SOH calculations help to extend battery life from 10 to 20 years, with an increase of 30% in general, and can reduce the total cost of ownership of an energy storage system by more than 30% when maintenance costs are taken into account. Combined with more accurate SOC information, the rapid depletion of the battery due to overcharging or over-discharging can be prevented, and it is also possible to minimize the possibility of short circuit, fire, and other hazards, help maximize the use of all the power of the battery, and enable the battery to be charged in the best and most efficient way possible.

The most efficient and reliable battery monitoring system

ADI’s LTC6813 battery management solution (BMS) can be used in healthcare devices such as portable ultrasound machines and in large scale (megawatt/hours) energy storage systems (for hospitals, factories, grid stabilization, electric vehicle charging infrastructure, and residential units), as well as in industrial robots and vehicles. The portability of ADI technology brings terrific advantages in reliability and safety, as it’s designed to work in different, harsh environments and is compliant to various functional safety standards, from the Automotive ASIL to Industrial SIL (for example, VDE AR 2510-2/-50, IEC EN 61508, and others).

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This new and unique solution for having the most efficient and reliable battery monitoring system involves the combination of an 18-cell monitor and balance IC with a microcontroller to SPI slave isolated interface. A multicell battery stack monitor measures up to 18 series connected battery cells with a total measurement error of less than 2.2 mV. The cell measurement range of 0 V to 5 V makes it suitable for most battery chemistries. All 18 cells can be measured in 290 μs, and lower data acquisition rates can be selected for high noise reduction. Multiple stack monitor devices can be connected in series, permitting simultaneous cell monitoring of long, high voltage battery strings. Each stack monitor has an isoSPI™ interface for high speed, RF immune, long distance communications. Multiple devices are connected in a daisy chain with one host processor connection for all devices. This daisy chain can be operated bidirectionally, ensuring communication integrity, even in the event of a fault along the communication path. The IC can be powered directly from the battery stack or from an isolated supply. The IC includes passive balancing for each cell, with individual PWM duty cycle control for each cell. Other features include an onboard 5 V regulator, nine general-purpose I/O lines, and a sleep mode where current consumption is reduced to 6 μA.

ADI’s LTC681x and LTC680x families represent the state of the art for battery stack monitors. The 18-channel version is called LTC6813. To support customers in designing their final products, ADI provides a full range of evaluation systems and platforms for the battery monitor devices, as well as a complete portfolio of variants to adapt to all needs.

Up to 18 battery units in series are monitored simultaneously

Now ADI’s newly launched ADBMS1818 is a multi-cell battery stack monitor compatible with LTC6813 mentioned in this article, capable of measuring up to 18 battery cells in series, with a total measurement error of less than 3.0 mV. The ADBMS1818 has a battery measurement range of 0 V to 5 V, making it suitable for most battery chemistry applications. All 18 battery cells can be measured within 290 μs, and a lower data acquisition rate may be selected for noise reduction.

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In practice, multiple ADBMS1818 devices can be connected in series to monitor a long high-voltage battery string simultaneously. isoSPI™ interfaces are built into each of the ADBMS1818's stackable architectures for high-voltage systems, enabling 1Mb isolated serial communication and RF-free high-speed communication over long distances using a single twisted pair wire up to 100 m long. Multiple devices are connected in a daisy chain, and all devices are connected to a single host processor. The daisy chain's bi-directional operation ability ensures communication integrity even in an incorrect communication path.

The battery stack can supply power to ADBMS1818 either directly or with an isolated power supply. ADBMS1818 can control over passive balance and individual PWM duty cycle per battery cell. Other features include an onboard 5 V regulator, 9 general purpose I/O lines and sleep mode (power consumption down to 6 μA).

ADBMS1818's low EMI susceptibility and radiation and support for bi-directional disconnection protection allow it to measure all cells in the system within only 290 μs. Synchronous voltage and current measurements are possible. Its 16-bit Δ-Σ ADC with programmable third-order noise filter supports passive battery balance at a maximum current of 200 mA. It has programmable pulse width modulation, and 9 general purpose digital I/O or analog inputs for temperature or other sensor inputs. It can be configured as I2C or SPI master device with power current of 6μA in sleep mode and is packaged in a 64-pin eLQFP. ADBMS1818 can be used in standby battery system, grid energy storage, residential energy storage, uninterruptible power supply (UPS), high-power portable equipment and other fields.

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Evaluation board speeds up product development

In addition to ADBMS1818, the EVAL-ADBMS1818 evaluation board is launched as a multi-cell battery monitor demo circuit that contains ADBMS1818 to serve as an 18-cell monitor on EVAL-ADBMS1818. Multiple boards can be connected via a 2-wire isolated serial interface (isoSPI), enabling monitoring of any number of batteries in the stack. The demo circuit has a reversible isoSPI which makes a fully redundant communication path possible.

EVAL-ADBMS1818 can communicate with a PC by connecting directly to DC2026 Linduino® One. To control the battery pack monitor IC and receive data through the USB serial port, DC2026 must load the appropriate program (called "sketch"). DC2792/DC1941 can be connected to DC2026, providing a fully isolated isoSPI interface for EVAL-ADBMS1818.

EVAL-ADBMS1818 can be controlled by the DC2026 Linduino One board. DC2026 is part of the Arduino-compatible Linduino platform, which provides sample code to demonstrate the ways to control the multi-cell battery monitor IC. Compared with most Arduino-compatible microcontroller boards, the DC2026 offers various convenient features hoping to correctly connect the battery monitor IC open-drain SDO, for example, it can be connected to the isolated USB of PC and has the built-in SPI MISO line pull-up resistor. It features a simple ribbon cable connection for SPI communication via EVAL-ADBMS1818 14-pin QuikEval J3 connector.

Conclusion

Battery stack monitors with balance and communication capabilities enable quick and accurate control of battery state to optimize the charging-discharging process. ADI's solution endows the battery stack monitor with outstanding accuracy and precision that will help optimize existing good designs. Moreover, ADI provides a full range of evaluation systems and platforms suitable for battery monitoring devices that will meet the various requirements of customers.

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