Lithium battery signals end-of-life

Sept. 13, 2007
A new battery alerts users when it nears the end of its usable life.

The notice lets the lithium-based PulsesPlus battery from Tadiran Batteries , Port Washington, N.Y., reach its maximum service life while ensuring continuous service through scheduled replacements.

Lithium-thionyl-chloride batteries typically find use in applications where it's tough to replace or recharge the battery. Automatic meter reading, tracking devices for homeland security, oceanographic devices, and RFID toll tags are typical examples where batteries are not easily accessible or are sealed against consumer tampering. Batteries used in many of these applications must work reliably for up to 20 years.

In normal use lithium cells maintain a stable output voltage over their entire operating life. The problem is that a voltage drop over time is normally used to signal end-of-life. Batteries that lack this characteristic typically get replaced ahead of their estimated lifespan to avoid system down time. But the cells might actually be viable for several more years. For example, engineers typically design automated meter-reading systems for a 20-year battery life. However, batteries are usually replaced after 15 years.

In contrast, PulsesPlus batteries give positive indication as they approach the end of their operating life by a detectable drop in their output voltage. Engineers can choose between two types: a 3.6-V system with a 5% end-of-life indication or a 3.9-V system with a 10% end-of-life indication. The percentage indicates the amount of battery life that remains. For the automated meter-reading system, the end-of-life signal would mean the 3.6-V battery only has a year of life remaining. That gives more than sufficient time to schedule battery replacement.

Available in sizes from ½AA to DD, the new batteries also have a higher A-hr capacity (19 A-hr for the D-size cell versus 7 A-hr for other lithium batteries), a 1% per year self-discharge rate, an operating range from –40 to 85°C, and a low equivalent-series resistance (ESR) due to an inherent hybrid-layer capacitor. A continuous-laser-welded housing and glass-to-metal hermetic seal protects against leakage.

Tadiran U.S. Battery Division, (800) 537-1368,

Unlike sand in an hourglass, the electrical charge in lithium batteries isn't visible as it drains. However, the ½AA to DD-sized PulsesPlus batteries from Tadiran can signal they only have five or 10% of charge remaining, depending upon the model.

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