Selecting power source technology for mission critical devices is crucial to ensure success. Whether it is a monitor at the bottom of the ocean, a drill system at 30,000 feet into the Earth’s crust, or hand warmers on an astronaut in Earth orbit, the cost of failure in these situations far outweighs the cost of a battery. Equipment used in these and other mission- critical situations must perform under environmental conditions that would destroy most commercially available components and energy sources. Every element must be capable of operating in environments where extremes in temperature, pressure, shock, vibration, and corrosive exposure are the norm. Selecting batteries for these vital activities must include consideration for the high level of reliability and performance required to ensure these significant and often costly programs stay on target.
There are various battery technologies available to consider when designing devices for mission-critical applications. One of the first considerations that must be evaluated is whether a primary (non-rechargeable) or secondary (rechargeable) system will best suit the application. In making this assessment, consideration must be given to planned deployment duration, battery accessibility, logistical support for recharging or replacement, and battery system monitoring. Is it a remote application or within easy reach for recharging or changing batteries? Is there a viable power source and means to recharge if the device is isolated? Is the mission a “set it and forget it” assignment where maximum volumetric energy density is required? Full consideration of these application issues will start you on the decision path towards either a primary or secondary battery type.
Once a decision has been made with respect to primary or rechargeable systems, additional technical application requirements must be considered. These include volumetric or mass energy density, storage and operating temperatures and duration, safety, disposal, and construction features for some of the most popular battery systems utilized in mission-critical applications, including lead acid, lithium-ion, alkaline, and lithium.
Lead acid is a tried and true rechargeable power system, providing a costeffective and readily available solution. Despite its lower energy density, lead acid often is utilized in critical applications where size and weight is not an issue, such as oceanographic surveying devices and military vehicle applications. Lead acid batteries operate most efficiently at moderate temperatures so deployments in cold or deep ocean environments or where extended exposure to elevated temperatures can be expected may result in reduced capability.
Lead acid batteries are moderately robust to shock and vibration induced by ocean’s waves; however, the system eventually can yield to continuous pounding. The environmental impact of lead acid also must be considered – it is highly toxic and remains so forever after the battery is used. Lead acid batteries must be recycled and all of the materials tightly controlled for health and environmental reasons, so a mission-critical device utilizing lead acid batteries must be positioned in an ultimately retrievable location.
Lithium-ion (Li-ion) is a popular rechargeable system that performs well in many critical applications due to its light weight, high energy density, and long cycle life as compared to other rechargeable batteries (potentially double the cycle life as compared to lead acid). It is quickly becoming the battery of choice in many small, portable applications, supported by its high availability driven by the consumer segment. Liion is available in a limited number of form factors and is not particularly durable in terms of withstanding extraordinary shock and vibration, making it more suitable for applications where shock and vibration conditions are relatively benign. Small handheld devices with ready access to recharging systems are particularly well suited to this chemistry.
Li-ion’s limited upper temperature range also restricts use in certain extreme environment applications, such as downhole drilling and aerospace devices. This system is, however, the fastest growing of all rechargeables in both the consumer and commercial segment, and as such will see continued improvements in terms of performance envelope and safety, with increased adoption in a variety of applications.
Of available primary battery types, the familiar alkaline cell is the most popular. Its lower cost, availability, long shelf life, and low internal resistance make it a fit for many mission-critical activities, particularly those with unpredictable mission schedules - alkaline batteries will hold up to 80% of charge for several years if stored in standard warehouse conditions. Alkaline cells also are easy to transport and do not require hazardous material certifications for shipment. Alkaline’s relatively low energy density, however, must be taken into consideration when designing a battery. In applications where space, weight, and replacement schedule are not critical, alkaline cells offer a reliable, low-cost solution for primary battery power.