The Power of Metering: How Long-life Batteries Affect AMI Profitability

battery illustration

By Sol Jacobs

Economics ultimately dictate the decision to upgrade from traditional water meters to a smart metering solution that combines robust meter construction with advanced metering infrastructure (AMI), automated meter reading (AMR) and two-way wireless communications.

The newest automated meters are incredibly robust, thus creating the potential for extended return on investment (ROI) over a 20 to 30-year period. These devices deliver numerous benefits, including reduced labor, improved cash flow and more efficient customer service. Additional benefits include enhanced water conservation, the ability to pinpoint leaks and theft of services, and numerous analytic and reporting functions.

Innovation does come at a cost, however, as smart meters are roughly twice as expensive as mechanical meters. Bridging this cost differential requires the long-term benefits to far outweigh the implementation costs and ongoing operational expenses.

Dr. Hans Allender authored a study entitled “Determining the economical optimum life of residential water meters” in which he analyzed non-commercial water usage in Arundel County, MD. The study proved his theory that mechanical water meters operate reliably for years but became increasingly inaccurate over time, reaching a point where the accrued savings do not match the growing water losses. The data identified a point of optimization where “replacement at the end of year 16 will guarantee a minimum annual cost…” In other words, after 16 years it makes sense to change out mechanical meters.

Automated smart meters cost about twice as much as mechanical meters, so ideally, they should last up to twice as long as those being replaced.

The weak link in a long-life meter design is the battery, which needs to operate as long as the meter can operate reliably. New ultrasonic meters, with no moving parts, can last indefinitely and are limited only by the life of their battery.

To extend operating life, all leading meter transmitter units (MTUs) are powered by long-life bobbin-type lithium thionyl chloride (LiSOCl2) cells that feature the highest capacity, highest energy density, the widest possible temperature range, and unrivaled battery life. These batteries represent only a small fraction of the overall AMI implementation cost, but remain top-of-mind among utility managers.

For example, when the California Energy Commission (CEC-2010-08) surveyed members of the Association of California Water Agencies (AWCA) about their plans for AMR/AMI implementation, 62 percent identified ‘meter battery life’ as a major issue, even greater than concerns over cost (60 percent).

Long-Life Batteries Result in Greater ROI

Pairing a 30-year MTU with a 10-year battery is hard to cost justify since it could result in high volume meter change-outs.

Due to low confidence in meter batteries, the City of Springfield, MA chose to preemptively replace thousands of batteries each year in order to avoid the chaos of disrupted billing systems, poor cash flow, and loss of valuable data. Preemptively replacing thousands of batteries cost local taxpayers millions and slashed total ROI: an outcome that may have been avoided with the choice of a different battery.

LiSOCl2 Batteries Are Not Created Equal

Bobbin-type LiSOClbatteries are overwhelmingly preferred for low-power wireless applications that draw current measurable in micro-amps, featuring the highest capacity, highest energy density, and extremely low annual self-discharge (less than 1 percent per year). Bobbin-type LiSOClcells also deliver the widest possible temperature range (-80°C to 125°C), with a glass-to-metal hermetic seal that resists battery leakage.

All batteries experience some form self-discharge, with cell capacity being exhausted even when the battery is not connected to an external load. Bobbin-type LiSOClbatteries are less affected by annual self-discharge, the result of controlled passivation.

Passivation occurs when a thin film of lithium chloride (LiCl) forms on the surface of the lithium anode, thus impeding the chemical reactions that result in battery self-discharge. When a load is placed on the cell, the passivation layer causes high initial resistance, resulting in a temporary drop in cell voltage until the discharge reaction slowly removes the passivation layer: a process that repeats itself every time the load is removed.

Several factors influence passivation, including: the current capacity of the cell, length of storage, storage temperature, discharge temperature, and prior discharge conditions, as partially discharging a cell and then removing the load increases the amount of passivation relative to when the cell was new. Higher passivation reduces a battery’s self-discharge rate, but too much of it can block energy flow.

Battery self-discharge is also affected by the quality of the raw materials and the way the battery is manufactured. For example, an inferior quality bobbin-type LiSOClbattery can lose up to 3 percent of its normal capacity each year due to self-discharge, exhausting 30 percent of its initial capacity every 10 years, making 20-year battery life unachievable.

By contrast, a superior quality bobbin-type LiSOClbattery can feature a self-discharge rate as low as 0.7 percent per year, retaining 86 percent of its original capacity after 20 years. This fact was confirmed by Aclara (formerly Hexagram) when they began replacing older AMR meters originally installed in the mid-1980s. Samples of these older batteries were lab tested and shown to have a significant amount of unused capacity after 28-plus years in the field.

Smart Metering Applications Demand High Pulse Energy

Standard bobbin-type LiSOClbatteries cannot deliver the high pulses required to power two-way wireless communications. This challenge can be easily overcome with the addition of a patented hybrid layer capacitor (HLC). The standard bobbin-type LiSOClcell delivers low daily background current, while the HLC delivers periodic high pulses.

Certain bobbin-type LiSOClbatteries also feature an end-of-life voltage plateau that can be interpreted to deliver low battery status alerts, providing ample time for scheduled battery replacement. This end-of-life alert capability may have allowed city officials in Springfield MA to forego the costly preemptive battery replacement program.

Long-Life AMR/AMI Applications Cannot Be Easily Simulated with Short-Term Tests

Comparing competing batteries can be difficult, as long-term battery performance cannot be easily simulated using short-term tests. Appropriate battery testing methods are required, including long-term laboratory testing, accelerated testing, calorimeter testing, and lithium titration analysis. AMI/AMI meter manufacturers are asked to submit random samples from the field for continuous lab testing that verifies real-life performance. Another useful yardstick is Failures In Time, which quantifies the number of battery failures per billion based on total in-field operating hours. Tadiran batteries, for example, continue to achieve FIT rates that range between 5 and 20 batteries per billion, which is extremely low compared to the industry average.

Do Your Due Diligence and Investigate Your Batteries

Short-term tests often under-represent the true impact that passivation and long-term exposure to extreme temperatures can have on batteries. As a result, utility managers need to perform added due diligence to verify the claims of battery manufacturers, as this knowledge helps to ensure they maximize ROI and reduce the risk of system-wide battery failures. Start by demanding fully documented long-term battery test results, in-field performance data, and numerous customer references.

Sol Jacobs is the vice president and general manager of Tadiran Batteries. Jacobs has more than 30 years of experience in developing battery-powered solutions for remote wireless devices. His educational background includes a BS in Engineering and an MBA.

Leave a Reply

Your email address will not be published. Required fields are marked *