Battery management unit (bmu) and battery protection
Cell material developments are ongoing to increase thermal runaway temperature. On the other hand, although the battery must pass stringent UL safety tests such as UL1642, it is always going to be a responsibility of the system designer to provide correct charging conditions and be well-prepared for possibility of multiple failures of electronic components. The system should not cause battery catastrophic failures due to over-voltage, over-current, short circuit, over-temperature conditions and external discrete component failures. This means redundant protection should be implemented — having at least two independent protection circuits or mechanisms in the same battery pack. It is also desirable to have the electronics circuit to detect battery internal micro-short to prevent battery failures.
Figure 1 shows the battery management unit block diagram in the battery pack, which consists of gas gauge integrated circuit (IC), analog front end (AFE) circuit, and independent second level safety protection circuit.
Figure 1. Battery Management Unit
The gas gauge circuit is designed to accurately report available capacity of Li-Ion batteries. Its unique algorithm allows for real-time tracking of battery capacity change, battery impedance, voltage, current, temperature, and other critical information of the battery pack. The gas gauge automatically accounts for charge and discharge rate, self-discharge, and cell aging, resulting in excellent gas-gauging accuracy even when the battery ages. For example, a family of patented Impedance Track gas gauges such as bq20z70, bq20z80, and bq20z90 can provide up to one percent gauging accuracy over battery lifetime. A thermistor is used to monitor the Li-Ion cell temperature for cell over temperature protection, and for charge and discharge qualification. For example, the battery is usually not allowed to charge when the cell temperature is below 0°C or above 45°C, and is not allowed to discharge when the cell temperature is above 65°C. When over voltage, over current, or over temperature conditions are detected, the Gas Gauge IC will command the AFE to turn off the charge and discharge MOSFETs Q1 and Q2. When cell under-voltage is detected, it will command the AFE to turn off the discharge MOSFET Q2 while keeping the charge MOSFET on so that battery charging is allowed.
The main task of the AFE is overload, short circuit detection and protection of the charge and discharge MOSFETs, cells, and any other inline components from excessive current conditions. The overload detection is used to detect excessive over currents (OC) in the battery discharge direction, while the short-circuit (SC) detection is used to detect excessive current in either the charge or discharge direction. The AFE threshold and delay time of overload and short-circuit can be programmed through the gas gauge data flash settings. When an overload or short-circuit is detected and a programmed delay time has expired, both charge and discharge MOSFETs Q1 and Q2 are turned off and the details of the condition are reported in the status register of AFE so that the gas gauge can read and investigate causes of the failure.