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Posted on 01 December 2019

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Continuous monitoring is important in backup battery applications

We increasingly rely on technology to provide us with a feeling of security; cameras, emergency telephones and even safety lighting have a reassuringly high profile, letting us know that, if they are ever needed, they’re available. Ensuring their availability in times of emergency can often demand an infallible power supply, which in turn falls to the venerable backup battery. But how do you know your battery backup really is infallible?

by Alan Denny, LEM

 

This is a problem that has beleaguered the manufacturers of equipment that relies on batteries for emergency power; when you really need it most, how will you know it will work? It is a particularly relevant question for manufacturers of uninterruptible power supplies (UPS), whose sole purpose is to supply electricity, to computer systems or medical equipment, in case of mains power failure. In these circumstances, it is not only imperative that the power is available but that it is delivered within definite time and supply tolerances.

Most battery backup systems are constructed from a number of valve regulated lead acid (VRLA) cells to create monobloc batteries. Although described as ‘maintenance free’, the technology has well known weaknesses, any one of which can render the battery inefficient or even completely inactive.

Weak, aged or otherwise ‘unhealthy’ batteries therefore represent a serious hazard in these systems, so it is commonplace to carry out regular maintenance checks on their state of health (SOH) and state of charge (SOC). However regularly these are done, there is still a risk of a battery failure occurring between maintenance checks and to combat this some companies are turning towards systems that offer constant SOH and SOC monitoring, in-situ.

Continuous monitoring

It may sound like a simple solution but in reality it comes down to economics. Continuous monitoring solutions can typically add 50% to the cost of the battery, even reaching as much as 70% when installation and operation is factored in. With such a high cost, it can prove cheaper to replace the batteries on a regular basis, before the Mean Time Before Failure (MTBF) would suggest they have reached their end-of-life. However, like routine maintenance this too is fraught with uncertainty, because environmental conditions can play a large role in the MTBF of a battery.

The answer many manufacturers seek is a low cost, continuous monitoring system that provides comprehensive diagnostics of a battery’s SOH and SOC, under all conditions. In March 2007 LEM, a specialist in this class of intelligent transducer, teamed up with RWTH Aachen University, one of the world’s leading authorities on sealed and vented lead acid batteries diagnostics and management, to set a roadmap for the development of advanced low cost battery monitoring management.

While other manufacturers have pursued more ‘fashionable’ battery technologies, RWTH Aachen University has maintained and furthered a centre of excellence that concentrates on the most established and widely selling battery chemistry. The LEM-Aachen partnership is a long term co-operation to research the failure modes of VRLA ,(Valve Regulated Lead Acid ) flooded and gel batteries and to look at the next generation of monitoring and analysis systems, including SOH and SOC.

Through this partnership and by listening to the needs of users, LEM has continued the development of its solution for continuous monitoring, known as Sentinel. Capable of measuring cell voltage, internal temperature and internal impedance, Sentinel offers a level of diagnostic measuring comparable to that offered by much more sophisticated – and expensive – laboratory equipment, but at a cost that does not prohibit its application as a continuous monitoring solution.

In order to develop Sentinel, LEM conducted extensive R&D using the aforementioned lab-based equipment, as shown in Figure 1, employing a wide selection of battery makes and brands. In this case, the method being applied and replicated in Sentinel is electrochemical impedance spectroscopy, which can also be implemented in hand-held devices and is commonly used during routine maintenance.

Test set-up for the evaluation of the monitoring devices

Before explaining how this sophisticated methodology is replicated in a cost-effective, single-chip solution, it is worth outlining exactly what level of diagnostics it achieves, and how that can help protect the integrity of a battery-based UPS.

An ageing problem

The majority of systems in this class use lead-acid battery technology, a technology that is well known to suffer from degradation in capacity and increase of internal resistance, due to ageing. However because the technology is so well established, the ageing condition is also well understood and can, therefore, be identified through the detection of several phenomena.

One effect that is particularly common is loss in capacity, due largely to the use-model of the batteries. In a UPS, the batteries are discharged with a high current, which can lead to the growth of large crystals on the electrodes. It is a condition that can be partly controlled through proper battery conditioning but in severe causes it can prove irreversible. It can also lead to growth of small crystals – or “dendrites”- which, if left undetected, can grow together and create short-circuits within the battery.

A short-circuit may also result from internal corrosion, where flakes from the terminals drop on to the electrode. Significant contributors to corrosion include temperature, voltage and local acid concentration, normally affecting the positive terminal. Any of these age related effects will lead to a loss of battery capacity, or power, and so any kind of diagnostic must be capable of identifying them, in order for the appropriate action to be taken before catastrophic failure results.

The above effects lead to a decreasing battery’s capacity or power. Any type of diagnostic shall be aiming for an identification of these ageing effects.

In the tests conducted, an electrochemical impedance spectroscope (EISmeter by RWTH Aachen Universaty ) was used to employ full spectrum measurement; a series of sinusoidal waveforms are applied to the battery and the resulting impedance measured across a spectrum of frequencies, between a few mHz and 7.5kHz. The results are derived through a Fourier analysis to calculate the real and imaginary part of the voltage response for a given frequency. The result, the complex impedance, can be obtained by analysing the relationship between the voltage response and the excitation current, in amplitude and phase angle.

For the Sentinel solution, this was impractical, as the processing power needed to achieve this would render any solution commercially non-viable for a continuous monitoring system. The challenge, therefore, was to develop a methodology where only a single frequency could be used for measurement, but was capable of achieving comparable results to the EISmeter. Figure 2 shows a comparison between the impedance measurements made using the EISmeter (full spectrum) and the Sentinel.

Comparison between impedance modulus given by the EISmeter and impedance values given by the Sentinel

Trend analysis

As the results in Figure 2 show, the two values are very consistent. Although a slightly higher value was returned using the Sentinel, this could easily be compensated for through calibration. However, for the purposes of battery diagnosis, it is the relative – not absolute – differences that are of interest. As the measurements are carried out on a continuous basis, it is the trend data that is important, which can be clearly seen in the results. This, coupled with temperature and voltage measurement all carried out using a single integrated circuit, constitutes the intelligence in the Sentinel solution.

Sentinel is the first single integrated circuit (system-on chip) monitoring for VRLA and flooded cells that provides measurement for individual cells and monoblocs for internal temperature, voltage and impedance as standard. Each module monitors an individual cell or monobloc, from 2 to 12 volts nominal, reporting over a proprietary communications bus to a Battery Data Logger (BDL). The function of the Sentinel is to derive key electrical parameters under test to determine the ability of the battery to perform in the event of a mains failure. Up to 250 Sentinel modules can be accessed via a single serial bus, making installation extremely easy as pushing plugs into sockets using pre-terminated data bus cables. Each Sentinel has an integrated temperature sensor for the continuous measurement of individual cell skin temperature. This is essential in the detection of potential thermal runaway and also enables intelligent temperature controlled charging profiles. Not subject to the restrictions of a single ambient sensor, cell skin temperature is more accurate and reliable. The measurement of individual cell temperature enables thermal mapping of the battery, which until now has only been available as an expensive additional service cost. LEMs unique True Energy Layer method of impedance measurement, together with a more robust test current ensures accurate and repeatable results every time. The impedance is measured by performing many “short-duration, mini discharge” of the bloc using a square-ware signal at a set frequency for a duration of 4.5 seconds as shown in Figure 3.

Impedance wave form

This action of the single longer pre-conditioning pulse at the start brings the cell into the right “energy layer” state before starting to draw measuring pulses. The latter creates a varying cell voltage response which, combined with the reference pulsed current, provides an impedance value.

The Sentinel’s impedance test method perturbs only the cell under test. High currents through sections of the battery are not required and DC links is not disturbed by any oscillations.

Sentinel

This is the first time that temperature, impedance and voltage have been combined in a module for single cell, or monobloc, monitoring. Combining accurate temperature, discharge (dynamic) and floating (static), with accurate ripple current measurements, the Sentinel system represents the most comprehensive battery monitoring system available today.

It is also designed for simple installation, requiring around a quarter of the time it takes to install less comprehensive systems. This is achieved through its monolithic design and simplified communications system. Using a proprietary communications bus, which LEM has termed the S-bus, each self-contained unit operates autonomously yet can be directly controlled from a central intelligence unit, called the Battery Data Logger (BDL); a monitor and data-logger with comprehensive alarm parameters and data storage facilities (see Figure 5).

MicroGuard, the battery data logger

Only this intelligent combination of the single or monobloc measurement units with the accurate information on temperature, voltage and impedance with the central intelligence given by the Microguard (BDL) including current measurement that allows an intelligent analysis of the state of health of the battery.

Configurations allow up to 250 Sentinels in eight strings, with up to 8 float/discharge currents being monitored, with all data available via a network interface.

Because the Sentinel is itself powered by the cell being monitored, it is designed to remain in ‘sleep’ mode for the majority of the time, only ‘waking up’ to take measurements. The wake cycle takes less than 100mS and is conducted approximately once every 5–10 minutes, which means for the vast majority of time the Sentinel is consuming minimal power from the host cell.

The use of lead-acid batteries in UPS systems is likely to increase, given our growing dependence on evermore sophisticated electronic devices. While the failure of an individual cell could spell catastrophe for any system employing a UPS as an emergency power source, using LEM’s Sentinel, that failure can be predicted, averted and, therefore, cost-effectively rectified long before any collateral damage occurs.

LEM firmly believes that continuous monitoring is important in these applications, but that it should cost no more than 15% of the cost of the battery. Because impedance is known to change in most modes of failure, it remains to date the most effective method of detecting deterioration of failure in a cell. In order to achieve true readings it is necessary to test a cell at a current level sufficient to penetrate the ‘surface’ charge present and for this reason the Sentinel has also been developed to automatically optimise the impedance signal test level, for any voltage between 1.5 and 15V.

The Sentinel system is a single-chip solution capable of operating completely automatically, providing extremely cost-effective and reliable monitoring for safety and mission critical applications. The operation of an entire system could be based on the integrity of a single cell. However, Sentinel maintains that integrity thus avoiding what could be a potential catastrophic failure.

 

 

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