Rechargeable Batteries in IoT Devices

How to Avoid Pitfalls
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~ Written by Adam Jacobs, CTO

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.

  1. When using a coin cell, when voltage drops below a minimum voltage, the battery is expected to be at end of 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, 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.
  2. Voltage ramp time is typically rapid when a coin cell is put into the device. However, if a rechargeable voltage drops and needs recharging, 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.

Shelf Life

All batteries self-discharge when stored. Rechargeable batteries typically self-discharge faster than non-rechargeable types.

  1. External circuitry can significantly increase the current draw that adds up over many months. Make sure that external circuitry such as microcontrollers are in their 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. 
  2. 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 less charged to retain long term capacity. 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.

Storage Temperature

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.
 

Shipping

Be aware of shipping restrictions for Li-Ion batteries.
See http://www.iata.org/publications/store/Pages/lithium-battery-shipping-guidelines.aspx