Battery Power Management and Protection System for Embedded devices
Battery management, monitoring and protection are one of the critical parts of the system. It is required for the safety of the system, extend the life of the battery, protect the cell from damage and to maintain the stability of the battery in providing the required power to for application. The functionality of the battery management system includes the measurement, Communication, control and Diagnosis of all power-related requirements of the system and to ensure the safety in the usage of the device.
For designing a battery management system, we need to understand and consider the following fundamentals in depth such as the battery chemistry, battery failure modes, ESR rating, C-rating, voltage limits, current limits, characteristics of the battery, application requirement, protection required etc.,
Architecture:
BMS and Protection system forms a major part of the power management unit of a device. The basic architecture includes,
- Rechargeable Battery
- Battery internal/External protection circuit
- NTC for Thermal monitoring and regulation
- Charging IC
- Fuel Gauge
- Charging input terminal
Rechargeable Battery:
The rechargeable batteries are of different types based on the cell chemistry and chemicals used etc., These batteries are used almost in all the smart, wearable and portable embedded devices. The capacity, size, weight, Voltage rating, Ampere rating, C-rating, ESR(Equivalent series resistances, efficiency, Status of charging, status of discharging, cycle time,lifetime, aging factor etc., are the few design factors to be considered while choosing a battery for an application. The Li-ion, Ni-cd, Nickel hydroxide, Lead-acid are some of the common secondary batteries.
Battery Internal/External Protection Circuit:
The battery protection circuit is required in order to avoid overcharge, over discharge and quick discharge as the battery has specified discharge current according to the C-rating. C-rating is the amount of current that you can draw from a battery of particular capacity over a period. This basically consists of MOSFETs of high RDS and low gate voltage. The mosfet will act as the switch and opens the Charge switch whenever there is a high terminal cell voltage, the discharge will happen through the DSG diode at that time. When the cell terminal voltage is less than the minimal cut-off voltage(Under-voltage Lockout),the MOSFET Discharge switch will be opened and charging will happen through the CHG diode.
Image Source: Texas Instruments | BQ29700 Battery protection circuit IC.
NTC for Thermal monitoring and regulation :
The NTC is the temperature sensor used for thermal monitoring and regulation for a battery. This can either be in-built in a battery or can be externally attached which will come out as the third terminal for a secondary battery giving temperature data. It can be used to switch off the battery when it is beyond the nominal temperature. This will help in the protection of exploding of the battery due to high temperature.
Charging of Battery ICs:
The charging decides most of the critical parameters of a power management unit. There are different charging methods like slow charge, fast charging which has complex circuitry etc., Various charging modes are as follows
- Constant Voltage Mode
- Constant Current mode
- Pre-Charge Mode
- Trickle Charging Mode
Battery Charging IC Pinout diagram
Image Source: ST
In the above reference circuit, the value of the Rprog decides the output charging current. The charging IC comes with the internal thermal regulation feedback to reduce the input charging current if the temperature of the charging system is more than a specific threshold temperature value.
The self-discharging, Battery internal ESR calculations, Max and Min Charging percentage, charging current all has to considered while designing a charging IC circuit for a system.
Battery Charging Characteristics
The graph describes the various modes of charging of the battery. In the charging characteristics, the initial phase (from 20% to 50%) of battery charging the voltage will be increased almost linearly and the current will be constant,this is probably why the charging is faster in the initial phase and after that (above 50% or 80%) the constant current mode exploits and current start decreasing with the constant voltage supplied slows down the charging speed.
The fast charging mode for our mobile phone is done by charging the battery throughout in the constant current with the increased power supply that is a higher amount of current is fed for charging. This is supported for the batteries with higher C-ratings.
Fuel Gauge:
Fuel Gauge also called gas gauge is the ICs designed to measure the battery voltage and current. It gives the status of charge of the battery. Gas Gauge will measure and calculate the voltage and current values from the battery with various algorithms like voltage-mode, current -mode, coulomb counter mode or mixed algorithm and stores it in a 16 bit /32-bit register. Further, the data will be sent to the Microcontroller via I2C lines connected. This may also have the alarm pin that can be set to cut off/control the device power supply by giving interrupt to the MCU, Which is programmed further to shut down the system or do the required functionality.
Charging Input Terminal:
The charging input terminal is mostly given as the Micro USB input as it is generic and readily available for all. Some of the embedded system device designers also use specified type of connectors along with the input charging voltage regulators with different types of connectors such as the Molex connectors, pogo pin connectors etc.,
Hence these are the basic parameters and description about designing battery management and protection system for a rechargeable embedded device. There are lots of other criteria also to be considered that have to be calculated and tested to have an efficient and safety Battery management and protection system. As the device will be in the hands of millions and millions of human as the end product it Should not hazard the user at any cause, which will even affect the lives of human and environment. Let us take even minute things serious and work finitely without assumptions not only in engineering design but also in life!
