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The last resort

Niclas Wetterstrand - Program manager Megger Sweden

Standby battery installations provide electricity to key elements of power generation, transmission and distribution systems, such as circuit breakers and protective relays, computers, control panels and telecommunication equipment, when other power sources have failed. The batteries can provide power instantly, either directly as DC or via an inverter as AC, thereby ensuring that critical systems continue to operate until emergency generators are ready to take over, or the main electricity supply is restored.

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In some instances, the batteries may also be required to support “black start capability” for a power station. This capability is provided for selected power stations to enable them to start operating after a national power cut without relying on power from the external transmission network. Once the power stations with black start capability have resumed operation, they can supply other power stations with energy so that they can also restart. Black start capability is therefore essential for the security of the country’s power supplies.

As batteries play such an important role, it is essential for battery systems to be planned, installed and monitored in line with relevant standards, such as European Standard EN 50272-2, which corresponds with German Standard VDE0510. Battery capacity must be calculated from the number of users that need battery power, and the amount of power each user requires. Power station operators must also provide evidence that the emergency power systems can support essential functions in case of emergency.

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Battery Capacity

A battery must be capable of continuously supplying electricity over a specified period of time. This ability is defined as its capacity and is calculated by multiplying current supplied by time, with the result expressed in amp hours (Ah). Battery manufacturers specify the nominal capacity of their products, but this does not always correspond with reality. Even when they are new, for example, batteries must be charged for a time before they reach their maximum capacity. And over the working life of a battery, its capacity can be expected to decrease significantly as it ages, until it eventually reaches the point where it is unable to supply power for the period originally intended.

Under favourable conditions, a battery can have a service life of 20 years, but this is the exception rather than the rule. Batteries store energy chemically, and the efficiency of the chemical processes can deteriorate rapidly if the battery is poorly managed and maintained. Insufficient charge, uneven load levels, corroded clamping bolts or internal connections, unfavourable ambient temperatures and bad ventilation can all rapidly and drastically reduce the life of a battery. 

Furthermore, it’s important to remember that a battery is made up of a number of cells connected in series. A single defective cell can prevent the whole battery from working correctly, possibly shutting down an entire facility, causing the whole of a city district to be without power, or allowing industrial processes to collapse. A battery installation is only as good as its weakest cell.

The Capacity Test 

A battery can be tested in a number of ways. For example, the cell voltages can be measured using a voltmeter, or the internal impedance of each cell can be measured. It is also possible to measure the specific gravity and temperature of the electrolyte. But none of these measurements can be used to draw completely reliable conclusions about the capacity of the overall installation, which is why discharge testing – otherwise known as capacity testing – is so important. 

The new TORKEL 900 from Megger embodies the latest developments in discharge testing. Essentially, it is nothing more than a load resistor; but it is an intelligent and electronically controlled load resistor that can, for example, ensure that the discharge current remains constant throughout a capacity test. Alternatively, the instrument can be configured for constant power delivery throughout the test or for constant load resistance. If at any point it becomes impossible to maintain the specified conditions, the test is stopped immediately.

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Connecting the TORKEL test set to the battery is easy

Simulating load resistance

Capacity tests usually require a constant discharge current from the battery throughout. This is achieved by loading the entire battery with a resistance, simulated by the test set. With TORKEL test sets, if one test set on its own cannot provide a high enough discharge current, additional test sets or load units can be connected. In this way, even the highest discharge currents can be accommodated.

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Additional TORKEL test sets or load units can be connected to increase discharge capacity.

By carrying out a discharge test, it is possible to determine whether the battery is capable of delivering power continuously over the required period, which is often several hours. A practical problem is that the battery voltage decreases continuously during the test, which means that the power delivered by the battery also decreases. In many cases it is desirable for the power delivered to remain constant throughout the test and to ensure this, the test set can be configured to continuously adjust the load resistance.

The test set also monitors the battery voltage throughout the test, to ensure that this does not fall below the end-point voltage specified by the battery manufacturer. If the end-point voltage is reached, the test is automatically terminated. This is important because further discharge could damage the battery.

At the end of the capacity test it is possible to say, with a high degree of confidence, whether the battery installation is functioning in line with the manufacturer’s specifications and whether it is capable of meeting the requirements of the application in which it is being used.

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The BVM data logger system can be attached to each cell simply and quickly using a clamp

Battery Voltage Monitoring

As already mentioned, a battery installation is only as good as its weakest cell but, until now, it has been difficult to evaluate the condition of individual cells. This means that there could be weak cells in the battery that are just managing to do their job, but will soon have to be replaced. The BVM system from Megger makes it possible to find these cells. It is a data logger that monitors and records the terminal voltage of each individual cell during the capacity test. The BVM system uses clamps so that fast and easy connections can be made to each cell.

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By using the BVM system in conjunction with a TORKEL test set, individual cell voltages can be recorded throughout the entire discharging process

If the curve recording the voltage of a cell shows any unusual activity, the critical cell is immediately identified and evaluated. The capacity test is stopped immediately if any cell reaches the point where it could potentially explode. The BVM system also quickly determines whether it is worth carrying out a lengthy capacity test. If individual cells show unusual activity right at the beginning of a capacity test, there is no value in proceeding, as the test results will not be meaningful. The BVM system, working in conjunction with a TORKEL test set, also monitors the recharging process when the battery is reconnected to the charging rectifier after a capacity test has been performed. This ensures that charging is carried out correctly.

Protecting Test Personnel 

The BVM system has another very important function: It protects test personnel against serious hazards, because they do not need to enter the battery room while the capacity test is being carried out. The cells in a battery have a very high energy density, which is why they can, under certain conditions, explode. If that happens, cells can fly through the battery room like deadly bullets. A cell that’s about to explode does not give any advance warning – it doesn’t smoke, it doesn’t spark, it makes no noise and it doesn’t smell. Without the BVM, test personnel must enter the battery room during the test and check the terminal voltage of every single cell, exposing themselves to the explosion risk.

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Devastating battery cell explosions are not uncommon  

What’s New?

Megger’s new TORKEL 900 offers a number of advantages over its predecessor, the TORKEL 800. But, like the TORKEL 800, it can be used fully independently. All test results are stored on the device and can be used for comparisons with later tests, or with data from structurally similar battery installation. This makes it very easy to spot adverse trends and anomalies. All key results are saved in real time and are presented as discharge curves.

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The new TORKEL 900 was launched in 2017   

Regular capacity tests are the most reliable way of determining the condition of a battery installation, but they can also be complicated. The following maintenance routines are therefore recommended to check the battery status between discharge tests: 

Ideally carried out once a month:

  • Visual inspection of the electrolyte levels in lead-acid batteries, and of ridges and levels in NiCd batteries
  • Visual check for corroded connections
  • Checks on ventilation and room temperature

Ideally check twice a year:

  • Overall battery voltage 
  • Cells and block voltage
  • Charge current in the charged state in order to recognise thermal runaways
  • Superimposed AC current and voltage
  • Specific gravity of acid in the cells (not possible for sealed lead batteries)
  • Temperature of acid in the cells (not possible for sealed lead batteries)
  • Impedance (comparative measurement)
  • Load function test (30–60 min) with original load 

Ideally check once a year:

  • Screw connections 
  • Auto battery test where a drop in the DC voltage on the battery string is observed

The worst possible time to discover a problem on a standby battery is when it is called upon to provide support during a power failure. But, as this article has explained, this nightmare scenario can be avoided by good battery management and regular testing. And, with the latest test equipment, these tasks are far less onerous than they used to be.

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