BATTERY MANAGEMENT OVERVIEW
Battery management encompasses two main functions: 1. Charging adapter, and 2. Battery fuel gauging and protection on the battery side. The block diagram in Figure 1 depicts the overall system. In each of these systems current sensing plays an essential role in extracting maximum performance, life, and safety from the battery.
Generic battery management system with charge adapter and battery management.
BATTERY MANAGEMENT-DEVICE SIDE CIRCUITRY
Accurate Fuel Gauging and System Management (All Images courtesy of ON SEMI)
For fuel gauging Lithium-ion batteries, charge-integration over time is a frequently used method for coulomb counting and determining SoC (State-of-Charge). In other battery technologies, open-circuit voltage has been used as fuel gauging, but Lithium-ion batteries display a much more constant voltage over the discharge cycle which does not provide reliable information on State-of-Charge, as shown in Figure 2.
Generalized discharge curve for Li-ion battery shows that determining charge by voltage is less than ideal for accuracy. Sensing current makes Coulomb counting a more accurate alternative to state-of-charge measurement.
Coulomb counting, assuming starting from a known state, integrates the discharge current to count coulombs removed from the battery and charge current to count coulombs restored. It should readily be evident that this is a process that benefits from the best possible accuracy in current sensing. It is not the intent of this article to dwell on the myriad details of coulomb counting, but mainly to focus on the role current sensing plays in battery management. (See Unknown Document 568478
Another aspect of Lithium-ion batteries is the importance of battery management. Lithium-ion batteries impose strict operating limits on voltage, current, and temperature to avoid undesirable consequences such as reduced battery life, internal short circuits, or battery damage.
Battery Management System Example
Figure 3 is a block diagram of circuitry in a typical Li-ion battery pack. It shows an example of a safety protection circuit for the Li-ion cells and a gas gauge (capacity measuring device). The safety circuitry includes a Li-ion protector that controls back-to-back FET switches. These switches can be opened to protect the pack against fault conditions such as overvoltage, undervoltage, and overcurrent. The diagram also includes a temperature sensitive three-terminal fuse that will open due to prolonged overcurrent or overtemperature, or it can be forced to open by redundant protection circuitry in case there is a fault where the primary protection circuitry fails to respond. Opening this fuse is a last resort, as it will render the pack permanently disabled.
The gas-gauge circuitry measures the charge and discharge current by measuring the current with a shunt resistor and a current sense amplifier. The current measurement is integrated to determine the change in coulometric capacity. In addition, the gauge measures temperature and voltage, evaluates gas-gauging algorithms to determine the available capacity in the battery, and computes time-to-empty and other values required by the host. The available capacity as well as other measurements and computational results are made available and transmitted to the system control.
Block diagram of device side circuitry in a typical Li-ion battery pack.