Don’t Overdesign Your Battery

Design engineers must strike a balance between two inherently competing goals: long-term product performance versus price.

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Tadiran Batteries

When developing an industrial grade wireless device intended for long-term deployment, design engineers must strike a balance between two inherently competing goals: long-term product performance versus price.

For remote battery-powered devices, this can lead to compromise solutions involving unnecessarily large and heavy batteries that carry unforeseen expenses, including more frequent battery replacements and the cost of transporting these oversized batteries to remote, hard-to-access locations.

In order to make a more informed product specification decision, the following parameters should be considered:

Operating voltage affects number of cells

Basic math tells you that it takes more than twice as many 1.5v cells to deliver the same voltage as 3.6v cells. Selecting the battery with a higher voltage could reduce size and weight while also saving money by requiring fewer cells.

Extreme temperatures affect voltage

Exposure to extreme temperatures reduces battery voltage under pulse. If a battery with a limited temperature range is deployed in a harsh environment, oversized batteries may be required in order to compensate for an expected voltage drop under pulsed load. One solution may be to utilize a specially modified bobbin-type lithium thionyl chloride (LiSOCl2) battery that features extremely high energy density along with the ability to handle high pulses at extreme temperatures, thus eliminating the need for all that extra capacity and/or voltage.

Self-discharge rate affects capacity

Certain battery technologies suffer from high self-discharge rates of up to 8% per month, thus requiring a larger battery to compensate for the expected capacity losses. Choosing a battery with a low annual self-discharge rate could enable the use of a smaller battery while possibly eliminating the need for future battery replacements over the life of the device.

For example, superior quality bobbin-type LiSOCl2 batteries feature a self-discharge rate of 0.7% per year, able to retain over 70% of their original capacity after 40 years. By contrast, a lesser quality battery made with the exact same chemistry could have a much higher self-discharge rate of 3% per year, thus exhausting 30% of its original capacity every 10 years, making it impossible to achieve 40-year battery life.

Power or energy

Commonly confused are the need for power (a measure of short-term energy consumed) and the total amount of energy required (battery capacity). Certain wireless devices are designed for infrequent use, requiring high pulses for short bursts without exhausting a large amount of energy. Prime examples include surgical power tools, which may operate for a few minutes, and guided munitions, which may remain airborne for seconds. For example, a surgical power drill powered by four AA-size lithium metal oxide batteries can replace a much bulkier device powered by 12 alkaline cells, resulting in a significantly lighter and ergonomic device for use by surgeons.

Another illustrative example is a missile guidance system, where a small pack of lithium metal oxide batteries were able to replace a much larger and costlier custom battery pack made with silver zinc batteries.

Be aware that most commercially available battery technologies are not designed to deliver a high power-per-energy ratio, thus demanding the use of a large number of cells in order to compensate for their low pulse design, resulting in compromise solutions that require added space and unneeded battery capacity.

Pulse size

Remote wireless devices increasingly require high pulses to power advanced two-way communications and remote shut-off capabilities.

Alkaline batteries are ideal for delivering high rate energy, but have major limitations, including low voltage (1.5v), a limited temperature range (0°C to 60°C), a high self-discharge rate that reduces life expectancy, the inability to deliver high pulses, and crimped seals that may leak. Alkaline batteries may also need be replaced every few months, causing long-term maintenance costs to skyrocket, especially for devices located in remote, hard-to-access locations.

A 3.0v LiMnO2 battery such as the popular CR123A can deliver twice the voltage of an alkaline cell, potentially reducing the total number of batteries required. However, CR123A batteries can only deliver moderate pulses, making them ill-suited for powering two-way wireless communications.

Standard bobbin-type LiSOCl2 batteries are not designed to handle periodic high pulses as they can experience a temporary drop in voltage when first subjected to a pulsed load: a phenomenon known as transient minimum voltage (TMV). One way to minimize TMV is to use supercapacitors in tandem with lithium batteries. While popular for consumer applications, supercapacitors have major drawbacks for industrial grade applications, including bulkiness, a high annual self-discharge rate, and an extremely limited temperature range. Solutions involving multiple supercapacitors also require the use of expensive balancing circuits that draw additional current.

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An alternative solution is to combine a standard bobbin-type LiSOCl2 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 acts like a rechargeable battery to deliver periodic high pulses, thus avoiding the need for supercapacitors. This hybrid LiSOCl2 battery design also features a unique end-of-life voltage curve plateau that can be interpreted to deliver low battery status alerts.

Rechargeable battery cycle life

If the application calls for rechargeable batteries, then the design engineer must be mindful that consumer grade rechargeable Lithium-ion (Li-ion) cells have a limited life of approximately 5 years and 500 full recharge cycles. If the rechargeable device is intended to operate for more than 500 recharge cycles, then extra cells may need to be incorporated to reduce the average depth of discharge per cell.

Choosing a battery with a higher cycle life could reduce the total number of cells needed. Industrial grade rechargeable Li-ion batteries are available that can operate for up to 20 years and up to 5,000 recharge cycles. Unlike consumer batteries, these industrial grade cells can also deliver the high pulses (15 A pulses and 5 A continuous current) while also featuring an extended temperature range (-40°C to 85°C).

Cheaper is often more expensive

Application-specific requirements dictate the need to think long-term, comparing the total cost of ownership over the lifetime of the wireless device versus achieving low initial cost. If a wireless device is intended for long-term deployment in a highly remote and inaccessible location, then you need to factor in all expenses associated with frequent battery replacement, which invariably will eat up any initial savings achieved by specifying a less expensive battery.

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The costs associated with excessive size and weight can also be important considerations. For example, a compact and lightweight power supply could be especially valuable to research scientists conducting experiments in extremely frigid conditions. Battery size and weight also factor into transportation costs, especially to remote places. For remote industrial applications, it pays to think long-term, and assess the total cost of ownership when specifying a battery-powered solution.

Sol Jacobs is vice president and general manager at Tadiran Batteries.

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