As rechargeable batteries become increasingly integrated into our daily lives, ensuring they charge, store, and discharge energy efficiently and safely throughout their operational lifetime is crucial. Typically, this is achieved through a stand-alone Power Management Integrated Circuit (PMIC) or an Application-Specific Integrated Circuit (ASIC) with power management functionalities. PMICs are often used in Internet of Things (IoT) sensing devices, while ASICs are more suitable for volume-constrained active medical implanted devices. This discussion focuses on the requirements for Stereax Solid State Batteries (SSB) and the ideal traits for a battery power management system, whether in a stand-alone PMIC or within an ASIC.
In an ideal scenario, the power management system should be a versatile device capable of accepting energy from multiple sources, managing batteries with varying capacities and chemistries, and providing multiple regulated voltage outputs. Additionally, it should isolate the battery with no leakage current and provide external capacitive support for improved power delivery. Programmability through resistively-set voltage or logic inputs to define pre-set conditions, or preferably using a serial data connection for independent parameter adjustment, is essential. Furthermore, for applications with significant temperature variation, a method to measure battery temperature is needed to adjust the battery’s operating parameters accordingly.
Overcharging batteries can cause irreversible damage, making it vital to limit the maximum battery voltage to ensure optimal performance throughout their expected lifetime. For Stereax SSB, this limit is 4V at 25°C. The optimal charging method for these batteries involves a constant current with an upper voltage cut-off, enabling the maximum state of charge regardless of internal resistance limitations. Pulse charging is also suitable, but the pulse current must be limited. A voltage-limited constant current regime would only charge the battery to the maximum voltage minus the voltage drop caused by the battery’s Internal Resistance (IR).
To ensure the battery’s fullest operational lifetime, setting a limit on the minimum battery voltage is also important. When connected to a load, the battery voltage decreases towards its minimum limit, where the load should be disconnected. For Stereax SSB, this minimum battery voltage is 3V at 25°C. A warning that the load is about to be disconnected allows for a safe shutdown by the user. After the load is removed, the battery voltage recovers, possibly by several hundred millivolts. To prevent a repeating cycle of connection and disconnection, hysteresis should be built into the reconnection threshold. In situations with pulsed loads, the internal resistance of the battery causes the terminal voltage to dip during the pulse. If this dip brings the battery voltage below the minimum limit, the power management system may disconnect the load. Introducing a delay before disconnection allows the battery voltage to recover, preventing early shutdown and ensuring full battery capacity utilisation. However, this delay should be limited to less than one second to avoid detrimental over-discharge.
Modern electronic components often require multiple supply voltages. The power management solution might need to provide multiple output voltages, either fixed or programmable, depending on efficiency requirements and noise tolerance. A buck/boost converter or a Low Drop Out (LDO) regulator are both suitable, depending on the specific needs.
Microprocessor-based systems can read information from the power management circuit through logic signals. This data can be processed to provide various indications, such as incoming power, battery status, and load disconnect warnings. Optionally, the microprocessor can disconnect the load, allowing for a predefined shutdown.
When the battery discharges to the lower safe voltage limit, it should be isolated with minimal residual current drain. For the Stereax M300 battery, the disconnect level is 3V at 25°C. The timing and voltage of the disconnect event must align with the load disconnect function to prevent premature battery disconnection.
For higher power delivery needs, additional capacitance or super-capacitance can be connected to the circuit. To avoid capacitor leakage discharging the battery during low or no incoming energy periods, the additional capacitance must be disconnected while keeping the battery connected. The power management system should have an input to control this action, possibly through a microprocessor.
An ideal power management circuit should also provide a ‘sleep’ or ‘shipping’ mode, controlled by an external logic signal, to enable battery disconnection and circuit shutdown. The operating quiescent current should be as low as possible, especially with low or no incoming energy, to prolong battery life. For example, a quiescent current of 100 nA would take the Stereax M300 SSB up to 100 days to fully discharge.
PMIC and ASIC are typically used to provide the necessary features for the efficient, long-term operation of battery-powered designs. The correct power management solution depends on the specific requirements of each application and the battery specifications. When using a microprocessor-based board, the microprocessor can offer additional information on the battery’s condition and further control over the power management solution. Using this Application Note to select the ideal power management solution for your application ensures that the battery operates to its full potential for its expected lifetime.
Ilika plc (LON:IKA) is a pioneer in solid state battery technology enabling solutions for applications in Industrial IoT, MedTech, Electric Vehicles and Consumer Electronics.
