Lithium battery technologies compared

Wireless technologies, the Internet of Things and data collection in general have changed the duty require-ments for dedicated back-up batteries in recent years. At the same time, low voltage operation has made it possible for many applications to go wireless, increasing the requirements for battery capacity.

There are three principal lithium battery technologies: are Lithium Manganese Dioxide (CR); Lithium Poly-Carbon-monofluoride (BR); and Lithium Thionyl Chloride (ER). Let’s look at some of the key characteristics of each.

CR technology is typically rated with a self discharge of 1% per year at +25°C, while at +65°C it rises to 16%. Common experience shows that indicative self-discharge rates are only one aspect to take into consideration when estimating the remaining capacity. Deterioration of capacity is also fuelled by Li2CO3 deposits on the anode surface caused by humidity level inside the battery.

These deposits are responsible for a rise of the impedance and thus reducing the CCV (closed-circuit-voltage) under load. Being unpredictable in time and scale this impacts on the operational ability of the application.

To suit CR technology to industrial needs, where a predictable service life and full capacity usage range are needed, Panasonic developed industrial grade CR. The mechanism of impedance increase is suppressed through an improved cathode material recipe and electrolyte additives.

BR technology has stable impedance throughout its useful life-span. Self-discharge is rated 1% for coin models and 0.5% for the cylindrical line-up. There is also BR-A technology, with improve-ments made in the area of electrolyte composition, cathode material recipe and sealing methods, improving self-discharge capabilities at higher temperatures.

ER technology has a self-discharge rate that varies according to temperature, the applied loads and their frequencies as well, making it quite difficult to predict in an actual application simulation. Pulse usage has a direct effect on the self-discharge rate, making it unique in this comparison of chemistries.

Manufacturer-published temperatures guarantee a safe operation range with no incidents, such as leakage, rupture or fire, and are usually valid for the entire life of the battery. However, some manufacturers apply restrictions from this assumption, such as temperature cycling

For shorter periods all chemistries are capable of enduring higher or lower temperatures than published.

The magnitude of power drained from a battery over time compromises its capacity. Load quantity and load frequency influence the operating voltage of the battery. The operating voltage of a battery under load degrades by its state of charge, which can also be labelled as remaining capacity.

Load limitations depend on chemistry, size and manufacturer. When UL 1642 certification has been granted, the battery is considered to be a safe product even under some abusive conditions with no risk of fire or explosion. Apart from the absolute load a battery can take there is another distinct difference between ER and BR/CR batteries if the load exceeds the specification limits. BR/CR will return to its nominal performance envelope when being treated within specification limits at a later stage, providing the excursion was of non-destructive nature. On the other hand, ER batteries may suffer permanently from loads outside the specification parameters.

By design, the Li-content of most primary lithium batteries is approximately 2.5 times higher in comparison to secondary Li-Ion batteries, because the anode is made of metallic lithium and not a lithium substrate. In case of temperatures exceeding approximately +180°C lithium metal shifts phase to a liquid state and an exothermal reaction with the cathode material occurs.

To date there have been no reports with BR/CR coin type batteries of fire or explosion at extreme temperatures. Generally even the most abusive conditions result in failure modes below 100°C surface temperature, causing cell deformation and/or leakage. But in the case of cylindrical batteries, abusive tests and market incidents alike confirm that BR/CR chemistry is not immune to fire/explosion in case of abuse.

ER technology is certainly not immune, but there are some key differences to point out. ER comprises laser weld/glass-to-metal sealings which (when no case vent is installed) makes an eventual overpressure release much more violent than a counterpart with plastic sealing. And ER gas release in case of cell case rupture is highly toxic, while BR/CR gas release is not.

Panasonic has produced a white paper offering wide-ranging information about the various aspects of lithium battery technology. It is freely downloadable from the company’s website.