In recent years, consumer products such as vacuum cleaners, power tools (such as drills, saws, and screwdrivers), and garden tools (such as lawn mowers, edgers, and lawn tractors) have transitioned from cord- and wall-powered to cordless devices and rechargeable batteries. Even bikes that were previously unpowered are now making the transition to battery-powered e-bikes and electric motorcycles.
These battery packs typically consist of a single lithium-ion, lithium-polymer, or lithium-phosphate battery, which can be dangerous if used improperly, resulting in a fire or explosion. To ensure the safe use of the battery, the electronics within the battery pack monitor the battery so that it operates only under the conditions specified by the battery manufacturer. These conditions usually include:
·Maximum allowable charging voltage.
·Maximum charge and discharge current.
·Specified charge and discharge temperature range.
Therefore, it is critical to measure key parameters within the battery pack, especially the cell voltage, current and temperature within the battery pack, as these parameters will trigger appropriate protection measures when they exceed limits.
The measurement data must be accurate so that the designer can decide how much margin to include in the design. For example, if the battery specification limits the full charge voltage to 4.3V, but the accuracy of the measurement data is ±50 mV, the designer must configure the system to disable charging when the measurement shows a voltage above 4.25V. However, since the actual battery voltage can be as low as 4.2V, in this case charging stops before the battery is fully charged, resulting in wasted application capacity and reduced battery life.
High-precision battery monitors and protectors for battery packs such as the BQ76942 and BQ76952 are designed for applications using Li-Ion, Li-Polymer, or Li-Phosphate batteries. These devices support series battery packs from 3s to 10s (BQ76942) and 16s (BQ76952), can measure battery voltage, current and temperature, and can share data with other circuits, such as a standalone microcontroller in a battery pack or in an e-bike system controller. The BQ76942 and BQ76952 can also use the data to automatically trigger battery protection, disable the battery pack to avoid operating outside of manufacturer specifications, and re-enable the battery pack when conditions permit, with or without interaction with the host or system microcontroller.
Figure 1 shows the block diagram of the BQ76952, which integrates:
Measurement and detection subsystems that monitor voltage, current, and temperature to detect when parameters exceed allowable thresholds.
Actuator that drives external protection FETs and chemical fuses.
A digital host interface subsystem that supports multiple serial communication standards in addition to pin controls for selecting functions.
Multiple voltage regulators, one for internal circuits and two for external use.
Figure 1: BQ76952 block diagram
Figure 2 shows a simplified schematic of a BQ76952 based 16s battery pack using I2C to communicate with the main microcontroller. An integrated regulator provides power rails for the microcontroller and optional external transceivers.
Figure 2: Simplified schematic diagram of 16s system based on BQ76952
The measurement subsystem in the BQ76942 and BQ76952 digitizes various voltages, currents, and temperatures within the battery pack. Since each measurement has specific requirements, these measurements are obtained in different ways. For example, temperature changes slowly, so measurements and calculations can be made slowly. However, the battery pack current may have short pulses of activity that could be missed if not sampled continuously.
Processes the values generated by the voltage and coulomb counter ADCs to provide measurement data that is used within the device and accessible to a separate processor within the battery pack, power tool, or system controller in an e-bike. These data include:
Differential voltage of a single cell and select additional system voltage.
Battery pack current and charge passed (coulomb count).
Internal die and 9 external thermistor temperature readings.
Pins that support external thermistor measurements are also available for general-purpose ADC inputs, supporting input voltages up to ~1.8V. The voltage ADC works on the measurement loop and the inputs are periodically multiplexed between multiple inputs. The measurement subsystem of the BQ76942 and BQ76952 includes several programmable options that allow optimization and trade-offs between measurement speed and accuracy.
As battery-powered consumer products become more popular, ensuring they operate within safe voltage, current, and temperature ranges is critical. A battery monitor with integrated features can help design engineers address these three key issues while improving accuracy. For more information on designing with TI battery monitors, check out the additional resources below.