1 Introduction of the Smart Meter

Smart meters, including gas and water meters need to record information such as gas or water consumption and then communicate this information to a data center. Wireless communication commonly uses methods such as, NB-IoT, LoRa and ZigBee^®. Take NB-IoT as an example, the typical range of input voltage of a NB-IoT module (such as the ZTE ZM8300G module) is from 3 V to 4.2 V, with a typical voltage of 3.6 V (reference 1 ). The current consumption is similar to Figure 1-1. The typical peak current is about 250 mA.

Figure 1-1 Typical Current Consumption of NB-IoT

Most smart meters are powered by LiMnO₂ (lithium manganese dioxide) or LiSOCl₂ batteries and need to support 10 years or more of operation. Because the voltage of a LiSOCl₂ battery (about 3.6 V) is higher than a LiMnO₂ battery (about 2-3 V), a LiSOCl₂ battery can better support a 3-V electromagnetic valve, which why it is a more popular choice for smart meter applications. The weakness of a LiSOCl₂ battery is that the maximum continuous current and pulse current capability is limited. Take an 8.5 Ah LiSOCl₂ battery (Tadiran TL-4920(ER26500)) as an example, the maximum recommended continuous current is 75 mAand the maximum 1 sec. pulse capability is 200 mA (reference 2). Because of this, it is common to parallel the hybrid layer capacitor (HLC) or the supercapacitor with the LiSOCl₂ battery to support the high pulse current for data transmission. Another characteristic of the LiSOCl₂ battery is that the battery capacity is related to the discharging current and the working temperature as shown in Figure 1-2 (reference 2). The battery capacity is about 8.5 Ah with 3-mA discharging current at 25 °C, which is shown in the ER26500 data sheet. However, the capacity drops to 2 Ah (a 76% reduction) with a 100 mA load. It is better to control the discharge current of a LiSOCl₂ battery to get a higher capacity and thus, increase the working lifetime (reference 3).

Figure 1-2 Capacity vs. Discharging Current and Temperature