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Powering ahead to a successful design: the market offers a plethora of choices of both primary (non-rechargeable) and secondary (rechargeable) batteries in a great variety of shapes, sizes, and chemistries. This article focuses on design considerations for primary cells, with a brief look at secondary cells.

Today's medical device designers are faced with greater challenges as functionality is increased, specifications are more stringent, and space becomes smaller. They need to determine what type of battery is needed and for what purpose during the early design phase. Primary cells come in a variety of chemistries and sizes, and can be used as the main power source or as a "backup." Some chemistries provide nominal 1.5 V open circuit voltage (OCV), while others provide nominal 3.0 V OCV. The typical voltage vs. time discharge characteristics need to also be considered in choosing a chemistry. Alkaline cells are among the most commonly used and lowest cost, but the voltage of the battery declines steadily during use until voltage cutoff. In contrast, lithium and silver oxide chemistries display an excellent and very flat voltage vs. time relationship until it reaches near end of life, after which, it quickly reaches voltage cutoff.

Designers often know the usage profile of the application and expected life of the product. With additional knowledge of the electrical current profile, along with temperature and cutoff voltage, the required "capacity" of the battery can be calculated. The capacity of a cell is largely dependent on both temperature and load of the application. Temperature is also an important consideration in choosing a battery chemistry. Batteries not only have different ranges of operating temperatures, but also perform differently at low and high temperatures.

The internal resistance of a battery is sometimes overlooked by designers but also needs attention, especially for pulsed applications. The battery itself can be modeled as an ideal voltage source with an internal resistor Ri. The model battery then powers the load resistor, R(L) (Figure 1). The result is a voltage-divider network where Ri "steals" voltage away from the load and, therefore, the closed-circuit voltage (CCV) drops as Ri gets larger.

[FIGURE 1 OMITTED]

Ri varies between battery chemistries, but for a specific chemistry, also depends on other factors. At low temperatures, the ions move slower and Ri is higher. Moreover, the smaller the battery, the higher the Ri. Lastly, as the battery is discharged, Ri increases gradually over time and increases exponentially near end of life.

From a mechanical perspective, the battery is frequently constrained to tight form factors. There may be situations, for example, where it may be more prudent to utilize two silver oxide batteries in series to provide 3V instead of one lithium coin cell. Mechanical mounting of the battery also needs consideration. For applications with a battery needing to be surgically implanted, the battery may be required to be non-magnetic. The reason for this is to prevent complications related to MRI machines that can provide magnetic fields as high as 3.0 teslas. Humidity, dust, and other environmental conditions may necessitate that the battery and associated electronics be hermetically sealed or encapsulated for extra protection.

Becoming educated about the power source and judicious planning will enable the medical device designer to optimize the operational life of the product.

Benjamin Du is a sales engineering manager at Renata Batteries/SKS. He can be reached at bdu@sykessler.com.

ONLINE

For additional information on the products and technologies discussed in this article, see MDT online at www.mdtmag.com or Renata Batteries at www.renata.com.
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Title Annotation:Emphasis On
Author:Du, Benjamin
Publication:Medical Design Technology
Date:May 1, 2010
Words:546
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