Currently, the Tadiran HLC combined with an LTC battery is the only smart metering solution with a working life of 20+ years.
At ﬁrst view, choosing a battery technology to meet the lifetime demands of 20+ years in the metering and iIoT market may seem easy. The only choices are Li/SOCL2 in bobbin construction (LTC) providing high energy density and low self-discharge.
Low energy losses are caused by two characteristics: the small lithium anode surface and the passivation layer. This protective layer is formed by the chemical reaction´s product, mainly lithium chloride, and depositing on the anode surface. The downside is limited power capability of the battery. While this was not initially an issue for applications like memory backup or measurement devices, it has become an obstacle since products have become smart. Data transmission needs high current peaks.
A capacitor working as a power booster is needed. But which classic technology is most reliable to cope with these particular lifetime requirements?
Having very short pulses, the solution is easy. There are high quality ceramic or metal capacitors which are able to survive across wide temperature ranges. However, their capacitance is limited to milli Farads. Radio transmission, on the other hand, requires much more power. For low power transmission a few Farads are enough – but GSM/GPRS transmission easily exceeds 25 Farads. Essentially, a supercapacitor is needed which is comprised of an electrolyte to gain capacitance in each form factor.
The following need to be considered for long term applications:
- electrolyte stability
- ageing effects, capacity losses
- impedance increase
- leakage current
- voltage level.
Although the technology has been optimised, battery seals are still a weak point. Since they are not hermetically sealed by welding, there is interaction between the inner components and the atmosphere. This leads to evaporation and oxidation and thus to a decrease in capacity over time, along with an increase in impedance and leakage currents. All this must be considered when calculating the initial nominal capacitance. The ageing effects start slowly, but when the electrolyte is either low or has chemically changed, the system degrades quickly. At this point the power boost effect is gone and the battery will no longer work.
Moreover, a constant low leakage current over the battery lifespan is mandatory in order not to deplete the battery. Unfortunately, it can be observed that the above-mentioned effects increase this critical value, too.
Supercapacitors are usually limited to 2.7V. To cope with the LTC potential of 3.6V two in-series are needed. But since internal resistance can develop differently in each of them, balancing is needed which can be done cost-effectively by resistors and adding leakage current to the system.
As a leading R&D driven company, Tadiran developed the Hybrid Layer Capacitor (HLC) more than 20 years ago. Based on battery technology, the HLC is hermetically tight due to laser-welding, operating up to 3.9V and offers these advantages:
- deﬁned ageing effects connected to LTC potential
- deﬁned impedance
- stable low leakage current
- sufﬁcient voltage level.
Ageing effects and impedance developments are well known. Since no interaction with the atmosphere is possible no random effects exist. The leakage current is low and stable. While having 3 – 5 µA in smaller supercapacitors, it can easily go up to 10 µA+ in larger systems which are needed for a GSM/GPRS radio. Tadiran´s largest HLC (15 x 50mm) offers just 2 – 3 µA and with the smallest versions (10 x 20mm) is only 1 µA. The capacitance of an HLC is much larger. HLC sized (15 x 50 mm) provides 700 Farad – enough to handle a GSM radio for minutes. SEI
About the author
Marc Henn studied mechatronics engineering near Frankfurt/Main and holds a master´s degree in business administration. He joined Tadiran as Manager of Application Engineering in 2016.
About the company
Tadiran Batteries is a leader in the development of lithium batteries for industrial use. Tadiran Batteries are suitable where utility meters require a single long-term standalone power source even if it has to supply high pulse currents for a GSM module.