Charging Toward Optimal Battery Backup in 2017
Maintaining continuity of service to their subscribers is the number one consideration for telecom network operators. It’s vital for them to maximize customer revenue in the “always on” world of Cloud computing and high-bandwidth data streaming, especially when low network availability can be a major factor in customer churn. It’s equally important for safety and security.
This demand for continuity is placing an ever-increasing emphasis on the design and management of backup power systems that support a wide variety of telecom installations: wireless or wireline, indoor or outdoor remote terminals (RT), on-grid, or standalone.
Typically, these backup batteries will be 48 volts, and can deliver a wide range of power capabilities from 100s of Watts to 10s of kilowatts. They offer backup periods from 8 hours in the case of landlines, to possibly less than an hour for a small mobile cell. At sites served by a high-quality grid, the battery may have to support approximately 10 to 30 power outages per year. Where the grid is “weak”, in developing countries for example, the battery might be subjected to several deep discharges every day.
But whatever the size or application, when the main power is interrupted the battery absolutely must perform its duty. Reliability is also a given factor. Now there is another factor that keeps operators up at night. How can they maintain this reliability while reducing the need for maintenance, often at remote and hard to access sites?
Currently, valve regulated lead-acid (VRLA) batteries represent more than 80% of the global telecoms market. This is despite well-reported concerns about their shorter life expectancy at higher temperature, shorter cycling life, the necessity to over-size the capacity, low reliability, high weight, and low-energy density.
Thanks to their much lower initial purchase cost, compared to Ni-Cd batteries, VRLA batteries will always dominate when specification decisions are based solely on CapEx considerations. It is important to look at the bigger picture, however. A less-expensive battery can, in fact, prove very expensive if it fails to function when required, causing loss of subscriber revenues. In contrast, an initially more-expensive specialized telecom battery that delivers in terms of reliability, performance, and life, will provide a superior return on investment (ROI) as well as peace of mind.
The Achilles heel of VRLA technology is its dramatic shortening of life when operating in ambient temperatures above +35°C. Think of an outdoor installation in Phoenix, Arizona, where summer temperatures can exceed +40°C. There, VRLA batteries are expected to last less than 2 years. Under the same conditions a Ni-Cd battery will last 15 years. This battery might cost between 3 to 5 times as much. However, when you factor in the full cost of battery replacement in this hard-to-access location, then the Ni-Cd alternative becomes the cheaper option when the VRLA battery is replaced for the second time.
VRLA manufacturers are working hard to improve the lifetime of their products. Yet even if they are able
to increase their life at higher temperatures by a factor of 3, then Ni-Cd technology may still offer a significant advantage.
The need for telecom companies to focus on Total Cost of Ownership (TCO) has driven a significant trend from VRLA batteries to Ni-Cd in recent years.
Ni-Cd batteries offer high energy-density, inherently long standby lifetime, resilience to harsh and uncontrolled environments (especially high temperatures), and ability to operate with little or no maintenance throughout their lifetime. It is this reduced maintenance that is particularly attractive for telecom operators.
In order to get a clear, accurate picture of Ni-Cd battery performance over time, Saft has conducted several field studies of a number of Ni-Cd battery strings over the past 2 decades. The most recent of these, reported at INTELEC, examined the performance of Ni-Cd batteries after 15 years of continuous operation at RT sites across the US.
The studies were based on industry-standard tests for physical assessment, discharge performance, electrolyte levels, voltage characteristics, and float current characteristics.
When Ni-Cd batteries were first deployed in telecom RT applications, the performance expectations were: to maintain no less than 80% capacity after 14 years of life; a minimum 5 years watering interval; no thermal runaway; and no sudden death. However, the studies have shown that Ni-Cd batteries can operate for more than 15 years in an uncontrolled RT environment with no need for maintenance or replacement at all. Interestingly, in southern Texas, the oldest Ni-Cd string installed is still in operation today after more than 16 years of operation.
There is an important growing trend for telecom networks to cover a greater surface area in towns, sports arenas, and campuses. Telecom operators are therefore creating smaller cells, which are likely to drive the use of Li-ion batteries.
Li-ion technology offers significant advantages in terms of performance at extreme temperatures, cycling capability, energy density, and zero-maintenance. It is still in its early stage of deployment for telecoms, but is finding favor in installations where a small footprint is vital. It is also around one third the size and weight of the equivalent VRLA battery, so it is well-suited for concealed applications and for pole-mounted equipment.
A further key trend is for telecom batteries to be integrated within the Internet of Things (IoT) for remote monitoring and operation. VRLA and Ni-Cd batteries may need additional equipment to provide this functionality. In contrast, Li-ion batteries already integrate monitoring of vital indicators such as state of charge (SOC) and state of health (SOH). All of these are important considerations when moving toward IoT.