Power Electronics

Lithium Batteries Are Not Created Equal

Contrary to popular misconceptions, all lithium batteries are not essentially the same. Within the lithium family of batteries there are numerous types of chemistries and manufacturing methods, with each offering unique advantages and disadvantages. The choices include poly carbon monoflouride (Li/CFX), manganese dioxide (Li/MNO2) and lithium thionyl chloride (Li/SOCL2).

Unlike consumer batteries such as alkaline, zinc-carbon-ammonium chloride (Leclanche cell), zinc-carbon-zinc chloride and zinc-air, lithium is better suited for remote applications because of its intrinsic negative potential, which exceeds that of all other metals. Lithium is the lightest nongaseous metal and offers the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of all available battery chemistries. Lithium cells, all of which use a nonaqueous electrolyte, have normal open-circuit voltages (OCVs) between 2.7 V and 3.6 V. The absence of water also allows certain lithium batteries to operate in extreme temperatures (-55°C to 150°C).

Primary lithium batteries are typically found in remote locations or hard-to-access sites where hardwiring to an ac power source is impossible or not cost effective, and the use of photovoltaic cells with rechargeable lithium batteries is impractical.

Choosing the optimum lithium battery begins with a careful evaluation of your power and performance requirements. Then, you can develop a prioritized checklist of desired attributes such as voltage, capacity, size, weight and/or special packaging requirements, expected service life, temperature and/or environmental issues and cost. You also should note special requirements such as the need for high-current pulses or high discharge rate.

Poly carbon monoflouride and manganese dioxide are best suited for applications that do not require long operating life or possible exposure to extreme temperatures. However, when extremely long battery life, extended temperature range and reduced battery size and weight are important considerations, the lithium battery of choice is Li/SOCL2, which is available in two styles: bobbin or spirally wound construction.

Spirally wound Li/SOCL2 cells have an energy density of 800 Wh/l, a temperature range of -55°C to 85°C and a maximum service life of approximately 10 years. Bobbin-type Li/SOCL2 cells are especially well suited for long-term remote-monitoring applications because of their high energy density (1420 Wh/l), high capacity, ability to withstand extreme temperatures (-55°C to 150°C) and extremely long service life of 20-plus years, which is due to very low self-discharge.

While not designed to handle high-current pulses, Bobbin-type Li/SOCL2 cells have been effectively modified to deliver high-current pulses through the development of the PulsesPlus technology, whereby a standard bobbin-type Li/SOCL2 cell is combined with a high-rate, low-impedance hybrid layer capacitor that stores and releases energy.

Devices that use high-current pulses must be specially designed to ensure extended battery life. To conserve energy, design engineers are programming remote devices to operate in multiple modes including: a sleep or standby mode, where power consumption is nil or a low background current; a measurement or interrogation mode, where the unit requires a few hundred milliamps of energy; and a transmission mode that requires high-current pulses for a period of seconds up to 20 minutes before returning to sleep or standby status.

Once the ideal primary lithium battery chemistry has been selected, your next step is to evaluate battery suppliers. With the rapid growth in knockoff products now flooding the marketplace, design engineers must be increasingly vigilant in terms of verifying product quality and authenticity, which requires 100% product traceability back to the original manufacturer.

It also makes sense to require your battery supplier to provide customer references along with fully documented and verifiable test results for parameters such as battery pulse, low-temperature pulses, discharge and repeatability. Faithfully adhering to these and other due-diligence practices during your vendor-selection process will help ensure years of trouble-free battery performance.

Sol Jacobs has more than 25 years of experience in developing solutions for powering remote devices. His educational background includes a BSME degree and a master's degree in business administration.

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