Choosing the right battery for use in harsh environments.
Some recent examples include sensors that monitor seismic activity in the arctic, medical devices that must be autoclave sterilized, electronic toll tags that endure rapid temperature cycling on automotive windshields, engine-mounted controllers, and sensors used throughout the medical cold chain.
Harsh environments present complex design challenges that influence the choice of power supply, requiring carefully evaluated solutions based on application-specific requirements. Important attributes to look for include:
* Lower total cost of ownership--taking into account the cost of future battery replacements over time.
* Longer operating life--battery-powered industrial devices may have to operate for decades without battery replacement, and the annual self-discharge of the battery can far exceed the energy consumed by the battery, demanding a high capacity battery.
* Maximum reliability -data integrity and communications cannot be compromised by dead batteries.
* Smaller size--battery-powered instruments often have to be miniaturized, requiring a battery that offers the highest energy density.
* Higher Voltage--the higher the voltage, the fewer cells are needed, which saves space and expense.
* Wider temperature range--industrial devices are often situated in extremely hot or cold places where ordinary alkaline batteries may fail or prematurely self-discharge.
Match the Power Supply to the Operating Environment
Where a battery is deployed greatly influences the choice of power supply. For example, if the battery is easily accessible and a short life is acceptable, then consumer alkaline batteries may suffice despite their low voltage (1.5 V), narrow temperature range (-0[degrees]C to 60[degrees]C), high self-discharge rate, and crimped seals that may leak. Having to replace alkaline batteries every few months also increases the total cost of ownership.
If the aplication requires long life, then primary, nonrechargeable lithium batteries are preferred. Lithium is the lightest non-gaseous metal, with a negative potential that exceeds all other metals, offering the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of all commercially available battery chemistries. Lithium cells have normal OCVs ranging from 2.7 to 3.6 V. The electrolyte is also non-aqueous, which extends the temperature range.
Choices among primary (non-rechargeable) lithium chemistries include: iron disulfate (LiFeS2); lithium manganese dioxide (LiMN02); lithium thionyl chloride (LiSOCL2); and lithium metal oxide chemistry.
Bobbin-type lithium thionyl chloride (LiSOCL2) chemistry is often preferred for remote wireless applications because it offers a high capacity and energy density, along with an annual self-discharge rate of less than 1 % per year to permit up to 40-year operating life. Bobbin-type LiSOCL2 batteries also feature a wide temperature range (-80[degrees] to 125[degrees]C), and are hermetically sealed. Typical examples involving bobbin-type LiSOCL2 batteries, include:
* The medical cold chain uses specially modified LiSOCL2 batteries that can operate continuously at -80[degrees]C to provide an uninterrupted flow of data while monitoring the transport of frozen pharmaceuticals, tissue samples, and transplant organs. Certain cells have operated successfully under prolonged test conditions at -100[degrees]C.
* RFID tracking devices that monitor the location and status of medical equipment in hospitals and other sterile environments use bobbin-type LiSOCL2 batteries to remain operational even during sterilization procedures, whereas ordinary batteries need to be removed prior to sterilization to prevent battery failure or premature self-discharge.
* Electronic toll tags speed motorists through toll booths, using bobbin-type LiSOCL2 batteries to operate maintenance-free for decades while handling the severe temperature cycles that characterize car interiors, where heat soak can hit 113[degrees]C (according to SAE) when parked, cooling down rapidly to room temperature.
Conversely, in frigid weather, these batteries must endure cold soak and a rapid temperature rise.
* AMR/AMI utility meters require long-life batteries that operate mainly in a dormant state requiring little or no energy, yet periodically require high pulses to power advanced two-way communications and remote shut-off capabilities. Due to their low rate design, standard bobbin-type USOCI2 batteries may experience a temporary drop in voltage when first subjected to this type of pulsed load: a phenomenon known as transient minimum voltage (TMV).
One way to minimize TMV is to use supercapacitors in tandem with lithium batteries. Supercapacitors have major drawbacks, including a high self-discharge rate, a limited temperature range, and solutions involving multiple supercapacitors require balancing circuits.
Another alternative is to combine a standard bobbin-type LiSOCI2 cell with a patented Hybrid Layer Capacitor (HLC).
The battery and HLC work in parallel: the battery supplies long-term low-current power in the 3.6 to 3.9 V nominal range, while the single-unit HLC delivers high pulses, thus avoiding the balancing and current leakage problems associated with supercapacitors. These hybrid LiSOCL2 batteries also feature a distinctive end-of-life performance curve, allowing devices to be programmed to provide low battery status alerts.
Specialized Requirements for High Rate Current
Applications that draw continuous high rate power (i.e. surgical power tools, automatic external defibrillators (AEDs), emergency beacons, and smart munitions) could require a different type of battery that uses lithium metal oxide chemistry.
Constructed with a carbon-based anode, a multi metal oxide cathode, and an organic electrolyte, lithium metal oxide batteries can deliver up to a 20-year operating life with an annual self-discharge rate of less than 1 % per year. These small but powerful cells feature a nominal voltage of 4 V and up to 2 Wh of energy, with a discharge capacity of 135 to 500 mAh, capable of handling 5A continuous loads and 15A maximum pulses. These batteries also offer a wider operating temperature range (-40[degrees] to 85[degrees]C.), and a hermetic seal. When used in smart munitions, a ruggedized version of the lithium metal oxide battery is capable of withstanding extreme shock, vibration, and high spin rates.
Industrial Grade Li-Ion Rechargeable Batteries
Energy harvesting devices can be considered for use in certain harsh environments, depending upon the reliability of the energy source, the expected operating life of the device, environmental requirements, size and weight considerations, and total cost of ownership. Rechargeable lithium batteries or supercapacitors are then used to store the harvested energy.
Consumer grade rechargeable lithium batteries are generally unsuited for harsh environments. To address this need, an industrial grade rechargeable lithium-ion (Li-ion) battery was recently developed which delivers up to a 20-year operating life and 5,000 full recharge cycles, along with an extended temperature range of -40[degrees]C to 85[degrees]C. These ruggedized batteries can deliver high pulses (5 A for a AA-size cell), and feature a glass-to-metal hermetic seal.
How a lithium battery is manufactured greatly affects its performance. For example, a bobbi-type LiSOCL2 battery can feature a self-discharge rate of just 0.7[degrees]/o per year, enabling a 40-year operating life. By contrast, an inferior quality LiSOCL2 battery may only provide a 10-year operating life with an annual self-discharge rate of 2 to 3% per year.
Specifying a superior grade battery can greatly reduce your total cost of ownership by eliminating the expense for future battery replacements, as the cost of battery replacement far exceeds the cost of the initial battery. With extreme environments, it is especially important to verify the accuracy of all battery manufacturer claims and to confirm that the battery is UL-approved.
By Michael LaBossiere Applications Specialist, Acadia Lithium