Rechargeable batteries are finding their way into small, portable IoT devices. Here are some pitfalls to avoid.
Rechargeable batteries are useful when designing connected devices that require higher power capacity than can be accommodated by a non-rechargeable coin cell battery. This can occur when data rates are moderate to high, connectivity is continuous, or the system requires higher power to operate.
Li-Ion is often the chemistry of choice these days. However, designers must make sure that they meet the requirements of the application. Things to watch out for include:
Power-Up Requirements on Chips
Some wireless microcontrollers are tailored for use with coin cell batteries. Problems can occur using rechargeable batteries.
- When using a coin cell, when voltage drops below a minimum voltage, the battery is expected to be at end of its life and the coin cell changed. This brings the voltage to zero and then it powers up on the new battery. However, with rechargeable batteries, the voltage can drop below the minimum threshold (but not to zero) and then be recharged slowly. Some devices then need a hard power-up or reset after voltage returns from below a certain level to operate correctly.
- Voltage ramp time is typically rapid when a coin cell is put into the device. However, if a rechargeable voltage drops and needs recharging, the voltage ramp can be slow. Not all devices accommodate this. An external power disconnect/connect may be needed to reconnect the chargeable battery once it’s above the needed threshold.
All batteries self-discharge when stored. Rechargeable batteries typically self-discharge faster than non-rechargeable types.
- External circuitry can significantly increase the current draw that adds up over many months. Make sure that external circuitry such as microcontrollers is in its very lowest power state that can be woken up with an external event, such as a button push. Alternatively, disconnect the power completely with a switch or pull tab.
- Battery capacity is reduced during long storage times, especially at elevated temperatures. There is a tradeoff between storing the battery more highly charged for longer initial shelf life, versus being less charged to retain long-term capacity. The optimal storage state of charge is about 60% of capacity to maximize the retained battery capacity after storage. For longer storage times, it may be necessary to charge it beyond this, but this may reduce the cycle life and long-term capacity of the battery. Find the sweet spot for the application requirements.
Batteries that are stored at elevated temperatures for months will lose a significant portion of their capacity. Specify requirements to minimize time at elevated temperatures throughout the supply chain to a few weeks. This allows enough time for shipping in hot containers, but not longer-term storage in warehouses.
Be aware of shipping restrictions for Li-Ion batteries.