The capacity of lithium-ion and Li-polymer batteries declines over time due to constant use. Thus, every battery cell must be carefully regulated to maximize its power output and keep it safe when being replenished.
STMicroelectronics rolled out a new battery-management IC that offers high accuracy and flexibility to improve the longevity and promote the safety of up to 25-V battery packs. The L9961 is targeted at battery-management systems (BMS) in everything from cordless power tools, backup energy-storage systems, uninterruptible power supplies (UPS), and other portable devices with limited voltage ratings.
The controller IC incorporates a high-resolution, 12-bit analog-to-digital converter (ADC) to accurately sense the voltage of every cell to within ±15 mV as well as the pack voltage, while the 16-bit current-sense amplifier (CSA) measures the current of the pack to within 0.25%. The IC relays the data to a host microcontroller (MCU) that figures out the battery’s state of charge (SOC) and state of health (SOH).
ST said the high accuracy of the IC assists with passive cell balancing and coulomb counting, in which the discharging current of the battery and the rate it’s being depleted over time is measured to calculate the charge level.
Designed to manage charging and discharging for three to five cells in series, the L9961 integrates a dual pre-driver that can be programmed for high-side and low-side connection to control safety relays inside the battery.
(Cell) Balancing Act
Passive cell balancing, which equals out the level of charge in the cells to ensure that they’re fully replenished and safely depleted, is supported. It can be applied to all battery cells simultaneously.
The L9961 pumps out 70 mA of current for cell balancing. Such functionality is required since the stress of overcharging or undercharging the battery can cause premature charge termination and a reduction in useful lifespan.
The cells at the heart of a battery pack aren’t identical. They have slight differences in capacity, internal resistance, self-discharge rate, and other factors that are largely due to inconsistencies in manufacturing. At any given time, one cell (or a small group of cells) will have less charging capacity than the other cells in the battery pack. As a result, the "weak" cells in the system limit the overall performance of the pack. Cells also lose storage capacity over time due to the stress of repeated charging and discharging.
Despite the slight imbalances in internal resistance and storage capacity, the battery cells in a series will all maintain the same charging and load current. As a result, not all cells in the stack will be topped off at the same time during charging. Thus, charging current is shut down if one cell fully charged before the others because faults can occur when batteries are overfilled.
When it comes to discharging, the "weak" cells in the stack are emptied out first, even though charge remains in the others. Depleting the cells too deeply risks damaging the battery in ways that are impossible to fix.
With passive balancing in the L9961, when one cell hits its maximum SOC before other cells in the pack, excess energy is dissipated from the cells that filled too rapidly. In turn, the other cells continue being recharged.
High Robustness
The high accuracy of the L9961 assists with safety features including overvoltage and undervoltage detection, balance undervoltage protection, overcurrent detection, and short-circuit in-discharge protection.
The controller can be used with a thermistor to take care of pack temperature monitoring with overtemperature and undertemperature detection. Battery-pack fuse protection also is part of the package.
Thanks to the L9961’s high robustness, it’s able to handle the transient inrush currents that can happen with “hot swapping”—or “hot plugging”—in which a power supply or other component is removed from, and its replacement plugged into, an active system.
To save additional power, a pair of power-saving modes in the chip reduce current consumption to 2 μA in deep-sleep and 5 μA in standby. In both modes, the integrated 3.3-V voltage regulator with high-current capability remains active to supply the MCU in the BMS, ready to resume operation rapidly.
The I2C interface inside is used for device configuration and data transfers to the host MCU. Safety-related configurations are stored in non-volatile memory (NVM), saving the MCU from reprogramming the BMS at every startup.
Housed in a 32-pin, 5- × 5- × 1-mm VFQFPN package, the L9961 is currently in mass production. Pricing starts from $1.19 each in orders of 1,000 units.
