To meet the higher current requirements of next-generation Li-ion-powered equipment, Texas Instruments has introduced a fully integrated battery protection and cell-balancing solution for high power battery packs. The bq77910 battery management/protection IC (Fig. 1.) is intended for Cordless Power Tools, Cordless Lawn Equipment, Electric Bikes, UPS, Medical Equipment, and Light Electric Vehicles (LEVs), etc.
To see the previous measures employed to protect li-ion battery-based systems, we’ll look at the protection circuit for lower power battery packs. This circuit protects the pack and optimizes its service life. It also protects the user against unsafe operating conditions. Typically, these first generation protection circuits:
- Limit the battery pack’s maximum charge and discharge current, which protects against overvoltage, undervoltage, overcharge current, and overdischarge current.
- Associated battery pack usually consists of one to four cells.
- Usually employ high-side power MOSFET switches on-chip to disconnect the battery pack if there is a fault. These MOSFETs must carry the load current during discharge and charge.
- Monitor cell temperature to protect against possible hazardous overheating conditions.
- Ensure complete charging and utilization of the pack without violating safety thresholds.
- Ideally, should consume minimum current when the battery-powered load is turned off.
These first generation protection circuits are intended for smartphones, laptop computers, and other portable devices with low to moderate power consumption. These protection circuits usually handle up to a few amps at a steady rate.
Now, a new generation of Li-ion battery-based equipment can be confronted with loads in tens of amperes. For example, a portable electric power tool like a drill or saw has a very different load profile when compared to a notebook PC. This higher load requires an entirely new type of protection circuit with the appropriate specifications. The protection circuit is embedded within the battery pack. Among its characteristics:
- It must handle very high currents on discharge, and lower currents on charge.
- Discharge current can cause uneven self-heating of battery cells, which can cause an imbalance of cell voltages that make the battery unusable. This requires an ability to monitor the output voltages of all the cells to ensure all support an equal share of the total load.
- In some applications, the load and battery are physically separated, as in the case of a string trimmer where motor is about three feet away. That motor can produce inductive spikes that affect the protection circuit.
- Power MOSFETs are employed to disconnect the battery in case of a fault, so they must be sized for the charge and discharge current paths. Primarily, this involves their on-resistance, RDS(ON), that must be low enough to minimize the voltage drop for the expected discharge current. In addition, the MOSFETs must turn off fast if there is a fault.
- MOSFETs that can operate with the higher discharge currents require gate drives with the appropriate drive current to turn off the MOSFET if there is a fault.
- Even with the higher current capability, the protection circuitís quiescent current must be as low as possible so that it doesn’t affect battery life.
Meeting the requirements for these high power, battery-based systems is the bq77910 from Texas Instruments. Fig. 2 shows the simplified circuit of the bq77910 that protects high-powered Li-ion battery pack-based equipment. One of this IC’s functions is monitoring individual cell voltages. It also drives two external N-channel power MOSFETs to disconnect the battery pack during fault conditions. Fault detection and recovery criteria for the device are fully programmable in non-volatile memory, which can suit all types of lithium battery systems.
The bq77910 protection circuit in Fig. 2 has separate negative charge and discharge lines. Separate lines allow the use of a high current MOSFET for discharging (driving the load) and a lower power MOFET for charging the battery pack.
- As shown in Fig. 2, individual cell monitoring and balancing cells use integrated FETs that help maximize battery pack service life and performance,.
- Can be configured for operation with packs from 4- to 10- series cells and 50V max input with a 10-cell configuration
- 50 µA typical quiescent current (2.5 µA in shutdown mode) minimizes battery discharge during storage or idle periods, which optimizes battery life.
- Supports additional lithium battery chemistries: Programmable (EEPROM) fault detection thresholds and time delays make the bq77910 adaptable to all variations of lithium systems, including LiCoO2 and LiFePO4.
- A standalone integrated solution does not require an external controller or processor (such as those employed with some lower power protection circuits).
This protection IC also provides all necessary battery pack protection functions. It operates as a standalone IC using primarily analog circuits to minimize power consumption. It checks:
- Cell overvoltage
- Cell undervoltage
- Charge short circuit
- Discharge short circuit/overcurrent
- Pack temperature
- Open/shorted thermistor (for temperature sensing) detection and open-cell detection
- External power MOSFET overheating
- You can pre-set safety thresholds and time delays (programmed in non-volatile memory) to match any type of Li-Ion battery system, such as LiCoO2, LiFePO4, LiMn2O4, etc.
- The protection circuit can be programmed during pack manufacturing and then can operate as a standalone protector within the pack.
- Internal high power gate drivers control the external low-side NMOS power switches. Use of external MOSFETs provides a lower on-resistance than integrated, on-chip MOSFETs. This is lower cost and more efficient for high voltage/high current applications.
- Programmable gain allows accurate current sensing with external 1 mΩ or 5 mΩ sense resistor. Fig. 2 has an external 1 mΩ current sense resistor.
The bq77910 implements an internal cell-balance control circuit and power FET structure. High cell count battery systems are more likely to see imbalance due to temperature gradients and cell self-heating at high discharge rates.
A fully integrated cell-balancing capability with internal balancing FETs is shown within dashed lines in Fig. 2. Cell balancing is important because mismatched cells internal to the pack affect overall battery pack performance. And, this cell imbalance can accumulate over time.
Automatic cell balancing uses integrated 50-mA cell circuits. Balance current must be limited using external resistance. Resistive component sizes limit the balance current as the return current flows through the 50Ω resistors. These values are relatively low (to allow sufficient balance current), so it may be necessary to maximize external capacitor sizes, depending on the filtering requirements.
A CPU to control a complex balancing algorithm is not required; instead the bq77910 implements a simple and robust hardware algorithm. It uses a separate comparator to check if cells have reached the balancing threshold to start balancing (i.e., does not use the overvoltage trip comparator). Cell-balancing options are programmable, including balancing threshold, when to balance (always, only during charge, or never), and how long to balance.
A mechanical or assembly fault in the pack can cause a high-impedance or broken connection between the IC cell sense pins and the actual cells. Therefore, the bq77910 periodically checks individual cell voltages by applying a micropower pulsed load across each cell. If the connection between the pin and the cell is open, the IC will detect a fault (permanent failure) condition. The open cell detection reading is taken at a time interval. Because an open-cell fault may be considered as a permanent failure, the fault detection logic must detect two consecutive open-cell conditions before activating the protection condition for an open-cell fault. Due to the nature of open-cell fault conditions, other apparent faults may be observed during an open-cell condition.
Also available is a bq77910 evaluation module (EVM) allows testing with different FET and pack configurations (Fig. 3). It can be used to set EEPROM thresholds for fault detection values. GUI software (PC-based) allows easy inspection and programming of fault and recovery thresholds.
The bq77910 evaluation module consists of a bq77910 circuit module and a resistor cell simulator module that can be used for simple evaluation of the bq77910 functions. The circuit module includes one bq77910, sense resistor, power FETs and all other onboard components necessary to protect the cells from overcharge, overdischarge, short circuit, and over current discharge in a 10 series cell Li-ion or Li-polymer battery pack.
The circuit module connects directly across the cells in a battery. With a compatible interface board and Windows™-based PC software, the user can view the bq77910 registers and program the IC configuration and protection limits.