Supercapacitors in smart meters
Smart meters use long life high energy batteries but these batteries often cannot deliver the peak power required for data transmission. High power CAP-XX supercapacitors complement the battery providing the peak power needed for wireless data transmission.
Supercapacitors deliver peak power to support the battery and enable data transmission.
Water and gas meters are either too expensive or not safe to wire to mains power. They use a very high capacity primary battery, typically a Lithium Thionyl Chloride battery at 3.6V, with long life power the meter for 10 – 15 years. Narrow band cellular IoT, commonly used to report data, requires peak current in the range of 0.2A – 0.5A. Lithium Thionyl Chloride batteries have difficulty supplying this peak current suffering a drop of ~1V or more. This can result in the data transmission module suffering a brown out. This problem is worse at the sub zero temperatures of North American and European winters. CAP-XX supercapacitors have very low Equivalent Series Resistance (ESR) typically < 100mΩ, and high capacitance to store enough energy to supply the power required for the duration of data transmission. Hence, to supply 0.5A, the supercapacitor voltage drop will only be ~50mV. The battery only needs supply average current to the supercapacitor, which will be ~1mA or less. A dual cell supercapacitor module must be used across a 3.6V battery requiring cell balancing for reliable operation. Since the supercapcitor is permanently across the battery, leakage current and balancing current can be a significant drain on the battery. CAP-XX supercapacitor leakage current is very low at ~1µA/F, and the CAP-XX balancing circuit draws ~700nA. 1.7µA leakage + balancing current only draws 15mAh/year, or 0.1% of charge for a 15Ah battery.
APPLICATON BRIEF
AB1019 Rev 1.1 Apr 2020
Supercapacitors in Smart Meters
The adoption of smart meters has enabled utility companies to read the information remotely, providing near real time data and removing the need for meter readers. Water and gas meters are either too expensive or not safe to wire to mains power, so use a battery to power the meter and data transmission. The very long life high energy batteries used cannot deliver the peak power for data transmission, particularly in the sub zero temperatures of northern winters. CAP-XX supercapacitor’s excellent power density complements the battery providing the peak power needed for wireless data transmission.
Supercapacitor in battery powered smart meter
Smart meter utilises low power, low data rate wide area wireless protocol to transmit meter information to the parent utility company. The amount of data and update rate for smart meters is very low by today’s standards. Protocols such as LoRa, NB-IOT are ideal for this task.
Water and gas meters are either too expensive or not safe to wire to mains power. They use a very high capacity primary battery with long life combined with extremely low power electronics to operate the meter over 10 – 15 years.
Fig 1 is the current waveform during transmission of 1 data packet using a CAT-M LTE module. The transmission lasted over 37s as the unit needed to register with the carrier and handshake before transmitting the data. Whether the unit registers before every transmission or remains logged on to the network between transmissions depends on the interval between transmissions.
If it is less than a certain interval it will be more efficient to remain connected to the network. A 7.7Ah LS26500 LiSOCl2 battery contains enough energy for over 11,900 transmission like that shown in Fig1. Ignoring quiescent current that is over 32 years if transmitted once a day. To extract all the available energy from this type of battery the discharge current needs to be kept as low and as constant as possible.
CAP-XX supercapacitors are the ideal power buffer to help enable pulse transmission and significantly extend battery life without sacrificing any features. Adding a GS230 (1.2F 25mΩ) across the battery allows the supercapacitor to provide the pulse power. Fig 1 shows the peak current was 0.2A. Fig2 below shows the voltage drop with the LiSOCl2 battery supplying 0.2A with and without a GS230 supercapacitor supporting the battery.
The voltage drop without supercapacitor support is 0.45V but with the GS230 supporting the battery, the voltage drop is only ~20mV. This is with a new battery – as the battery ages the battery impedance increases and the voltage drop is worse. Also note that the peak current for an NB IoT or CAT-M LTE module could be up to ~0.5A, resulting in a battery voltage droop ~1.1V without a supercapacitor and ~50mV with. With a supercapacitor the only battery supplies average current, if transmission occurs 1/day this = 28μA, or if 1/hr = 670μA. This ensures that all the energy is extracted from the battery and maximises battery life. See CAP-XX’s new range of supercapacitors, DMF low ESR high power and DMT long life high temperature.
For more information please refer to Coupling a Supercapacitor with a Battery.
The other key attribute of CAP-XX supercapacitors that make them ideal to place across a battery is their very low leakage current (IL), ~1μA/F. The GS230 supercapacitor across a LiSOCl2 battery in Fig 2 has a typical IL of ~1μA. A dual cell supercapacitor requires a cell balancing circuit, see Cell Balancing. CAP-XX can provide a true balance circuit that maximises life and only draws ~700nA. Leakage + balancing current is drawn from the battery continuously so it can be a significant drain on energy. The total 1.7μA current drains only ~15mAh/yr or only 0.2% of the charge of the 7.7Ah LS26500 battery.
Supercapacitor in a smart meter with energy harvesting
It is possible to incorporate energy harvesting in smart meter by converting some mechanical energy from the flow of fluid through the meter. A micro turbine used for flow measurement can extract small amount of power without impacting the metering function and store the harvested energy in a supercapacitor to power the radio transmission and other functions. This allows use of a smaller primary battery or a re-chargeable battery to increase meter life.
Sizing your supercapacitor
Supercapacitors, which can deliver high power due with their low ESR and have high C to supply sufficient energy to support the data capture and transmission for its duration, have “unlimited” cycle life, and can be charged at very low current are the perfect power buffer between a low power battery and/or energy harvester and a data transmission module. See Powering Pulse Loads for guidance on supercapacitor sizing and selection. CAP-XX can assist in your supercapacitor circuit design.
Inrush Current Limiting
A CAP-XX supercapacitor, with its very low ESR, will try to draw very high inrush current when initially charging from 0V. In many cases the battery’s internal impedance will act as a sufficient current limit, but if inrush current limiting is required to protect the battery, then see Current Limiting for Supercapacitors.