Designing Smart Batteries into Medical Devices

• can communicate detailed diagnostic data

• can learn and recalibrate

—by David C. Baggaley, Director, Inspired Energy, Inc.

Portable electronic devices are penetrating the medical industry at an ever-increasing rate, and the success of these devices relies heavily on a successful battery design. In the case of many medical devices, a battery with built-in intelligence (Smart Battery) may be required, for which the accuracy and reliability of a fuel gauging system is crucial. Early consultation with a Smart Battery designer should yield designs with reduced component counts and lower cost assembly. 

Basic applications using NiCd cells are unlikely to require in-pack electronics. An inaccurate fuel gauge within applications like cellphones carries no dire penalty — the worst that can happen is a call cut short. However, any application (e.g. for a critical medical device) that relies on the battery functioning exactly as predicted by its fuel gauge needs a more sophisticated monitoring and gauging system. 

Choosing the correct cell chemistry and size is the first major choice facing the battery designer. Typical drivers are size, weight, cost, power delivery, form factor and environmental considerations. Specific nominal operating voltage is a common request; however, in battery-powered devices the critical voltage is the cutoff, not the nominal voltage. All cell technologies have sloping voltage curves, and hence battery packs must be designed so that their cutoff point is compatible with that of the device.

Lithium cells are rapidly becoming the main power source for portable devices. Increasing sophistication of the devices calls for higher level of communication of key battery performance parameters to the user, aside from cell protection and fuel gauging. Smart batteries can communicate detailed diagnostic data, thermal information and cycle history, and can carry out complex functions such as using the recent discharge history to calculate and provide a “remaining time to empty.”

A battery pack’s “fuel tank” shrinks and grows as the battery ages, as well as with changing temperature and rate of discharge. The accuracy of the digital fuel gauge can also drift if the battery is used in applications of frequent partial discharges. In order to provide accurate, precise fuel gauging, these conditions require a sophisticated fuel gauge with extremely high resolution and the ability to “learn” and recalibrate, in order to provide the user with dependable information.

The design process is the single most critical factor in the success of the project. Designing a smart power source for your portable electronic device is a complex assignment, the success of which is directly proportional to the amount of time and effort spent at the design stage. Early involvement with an experienced smart battery designer will result in the optimum choices in technology, electronics and battery design. This will ensure that your product has the best possible chance of success.