Powering instrumentation: Batteries, wireless
04 August 2008
The variety of applications that demand self-contained power supplies is growing with the wider deployment of remote and wireless devices. While many alternative sources of energy are emerging, batteries still represent a reliable and inexpensive method.
Like many products, there is a variety of battery types with characteristics that make them suited for specific types of applications. Understanding how to apply various offerings can make your job easier. While there are many interesting rechargeable battery technologies, for purposes of this brief discussion, we will zero in on disposable designs.
Industrial applications generally focus on longevity first. Few companies have large enough maintenance staffs to change batteries regularly, so a lifetime that is both long and predictable is very desirable. With this in mind, battery manufacturers have achieved high levels of consistency in their products, so performance is quantified and dependable. With some research, you should be able to find data that will help you evaluate performance and make a wise choice.
There are many types of batteries that exhibit specific discharge characteristics that should be matched with your devices consumption characteristics to optimise power flow.
For example, some devices drain power constantly at the same rate. The manufacturer should be able to give you current consumption vs. lifetime data, which makes predicting battery life very simple (see graph). On the other hand, some devices, such as wireless instruments, consume short pulses of relatively high draw with long periods of inactivity in between. You can average this to an extent, but the battery may not see it as perfectly linear. Consult with your prospective supplier and make sure the battery is suited to the peak load. You can mitigate this effect by adding a capacitor that will draw current at a consistent low level from the battery and then discharge to power the transmission pulse.
High-performance batteries cost more than those at your local supermarket, but should be compared against maintenance costs to change.
‘For long-life industrial applications where temperature and long maintenance intervals are important, the cost of the solution is not really that important,’ says Lou Adams, application engineer for Tadiran Battery Co. ‘On the other hand, if the battery is easily changed and the temperature rarely gets very hot or cold, a low-cost alkaline solution quite easily fills the bill.’
Remember that predictable life depends on predictable power consumption. If a device changes its power usage due to new operational parameters, settings, or other reconfiguration, battery life will change proportionally. Understanding how batteries work often begins with understanding the devices they power.
Peter Welander, Control Engineering
Figure 1: This graph shows load vs. life curves for two types of "D"cells. The blue curves apply to lithium thionyl chloride cells, which are used frequently to power wireless instrumentation devices. They provide high voltage, stable voltage, wide operating temperature range, and very long life; however, they have relatively low maximum discharge rates. They are not suitable for a high discharge device such as a flashlight, although they can operate for five, ten, or more years when power is drawn at a low rate. The green curves apply to a common alkaline battery. Note that output voltage begins to decline immediately and continues until exhaustion. It is capable of putting out more current, but for a shorter period of time.
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