When designing a power management system for a wireless remote sensor, it is important to choose a primary battery that last for decades under extreme operating conditions. Lithium thionyl chloride chemistry is the preferred choice due to its proven ability to deliver 25-plus years of service life.
Optimizing battery life and long-term reliability involves numerous variables, including the chemistry, the cell design, the quality of mechanical components, the purity of raw materials and the manufacturing processes employed. Shortcuts in quality can negatively impact service life.
The total amount of active chemical ingredients and the ratio of each ingredient determine the cell's nominal capacity. Predicting expected operating life solely on the cell's nominal capacity can be misleading however, as the cell's capacity is affected by the active components, the internal self-discharge, the application power profile and environmental factors. Since the volume of active ingredients is limited by the size of the cell, nominal capacity values often do not vary substantially. So the key differentiator often involves the inner structure of the cell and the ratio of active ingredients. For this reason, design engineers should evaluate the battery's Equivalent Operating Capacity (EOC) to properly calculate its expected operating life, taking into account the cell's self-discharge rate, application current profile and environmental conditions.
For example, a standard AA-size lithium cell with a nominal 2.4Ah capacity and 10΅A self-discharge, will lose about 1.75 Ah in 20 years, or 73 percent of its initial capacity, due to annual self-discharge. In long-life applications of 10-plus years where current demand is low, higher EOCs are preferred.
By contrast, Tadiran XOL series lithium thionyl chloride (LiSOCL2) cells feature a self-discharge rate of less than 1percent per year, which reduces operating capacity losses during long-term operation at low average current with moderate high-current pulses. In a typical long-life application, an AA-size 2.4 Ah XOL Series LiSOCL2 cell will retain more initial capacity after 10 years of operation than a larger standard A-size LiSOCL2 3.2 Ah cell.
For applications that require extended life and high current-pulses, PulsesPlus lithium thionyl chloride cells were developed that combine LiSOCL2 chemistry with a hybrid layer capacitor (HLC), enabling the battery to deliver continuous nominal current with intermittent high current pulses when the device collects or transmits data. Once these cycles are completed, the device returns to a low energy sleep state where energy usage is nil or nominal.
Since lithium batteries are not created equal, design engineers need to conduct thorough due diligence, first to determine the ideal battery chemistry, and then to compare competing brands to identify the optimal power management system capable of delivering 25-plus years of maintenance-free service life.