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A Saft Matrics MRX battery system has completed a successful extended field trial with the Stadtbahn Karlsruhe tram-train network in Germany.

The aim of the test installation was to demonstrate the potential reliability, performance and total cost of ownership (TCO) advantages offered by using specialised rechargeable nickel-cadmium (Ni-Cd) batteries in this demanding light rail application.

The trial has now entered a second phase with the installation of two further test batteries.

The city of Karlsruhe was the first in Germany to link its street tramway and the mainline railway by running urban trams on both networks.

The mainline remains open at all times to all types of train – including local trains, heavy freight and high-speed express trains – and this model has led to the creation of tram trains featuring dual equipment to suit the needs of both tram and train networks, such as support for multiple voltages.

In order to change the power supply from 750VDC on the tram tracks to the 15kVAC used by the mainline trains (or vice versa), the tram train has to pass a cut-off section of 50m to 250m where no external power is available.

This requires the onboard battery system (with a nominal voltage of 24V) to provide a minimum of 20V for a cut-off period of 20-25 seconds to support all the electrical loads, so battery reliability is said to be a vital factor in ensuring that the Stadtbahn Karlsruhe services run on time.

Currently, the majority of the Karlsruhe tram trains are fitted with flooded lead-acid batteries that can give rise to a number of reliability, maintenance and service-life issues.

For example, the battery should, ideally, have a capacity of at least 300Ah.

In practice, however, this is limited to 230Ah because of the need to fit within the space available within the under-floor battery box.

This means that the tram train often runs out of power, stopping before it can complete its passage across the cut-off section.

In addition, these lead-acid batteries can not withstand deep discharges without irreversible capacity loss.

There have been a number of cases where a tram train has been parked overnight on the mainline track, where the external power has been turned off for track maintenance but the onboard loads, such as lights, have been inadvertently left on.

This current drain can cause a voltage drop to 16-18V, resulting in a reduced battery lifespan or even instant failure.

Further battery problems are experienced during the winter months, when low temperatures cause a substantial decrease in battery performance.

At least one battery has also experienced ‘sudden death’, in which the open-circuit failure of one or more cells has caused the whole battery to fail and the overall service life is limited to two to three years.

Potentially, Ni-Cd batteries offer a number of advantages for tram-train applications.

They can provide: a larger capacity within the same installation footprint; resistance to deep discharges; high performance at high and low temperatures; a service life of at least 15 years with no risk of sudden death; low maintenance costs because of simple maintenance requirements; and extended service intervals.

Saft worked with Stadtbahn Karlsruhe to carry out a TCO analysis to confirm that Ni-Cd could be a cost-effective alternative to lead-acid batteries.

This showed that, although the initial cost of acquisition of the Ni-Cd battery system would be higher than lead-acid, when operation, maintenance and replacement costs over a 12-year period were considered an Ni-Cd battery would actually cost less.

However, while the cost of the battery is naturally important to the operator, it is reliability that is crucial.

As poor reliability is not only damaging to the reputation of the tram-train service, it also incurs considerable extra costs in towing vehicles and replacing failed batteries in the field, particularly as a stranded vehicle might be up to 130km from the service depot.

After proving the case for Ni-Cd batteries in principle, Stadtbahn Karlsruhe decided to carry out a practical evaluation over all the seasons of the year and asked Saft to provide a trial battery system.

This comprises 19 Matrics MRX 200 batteries that fit within the existing battery box, with a nominal capacity of 200Ah.

The battery’s current performance is the crucial factor in supporting the passage of the tram train through the cut-off section.

In general, lead-acid batteries, at the same rated capacity, are not capable of delivering such high currents as an Ni-Cd battery.

This means that, even though the Saft Ni-Cd battery might appear to have a lower rated capacity than the 230Ah lead-acid battery that it has replaced, in practice it delivers good performance.

Furthermore, the low-temperature performance of lead-acid batteries is claimed to be significantly poorer than Ni-Cd.

The gap in performance between the lead-acid battery and the Saft battery is even greater in cold weather.

Herr Rainer Supper, deputy workshop manager responsible for electrics at Stadtbahn Karlsruhe, said: ‘Saft’s Ni-Cd battery performed very well during the field test, with no problems or outages, which is more important than anything else, including cost, since fewer outages mean more passenger availability.

‘We didn’t need to touch the Saft battery for a whole year and it only needed topping up with three litres of water and, thanks to the centralised water filling system incorporated in the MRX design, topping up only took 15 minutes.

‘With our lead-acid batteries, each cell has to be opened individually for refilling and this has to be done three times.

‘With three major maintenance sessions during the year, at least one hour of time could be saved per tram train.’ The success of the first field test prompted Stadtbahn Karlsruhe to order two further Ni-Cd test batteries and the first of these was installed in December 2008.

Saft Matrics MRX batteries have been purpose designed to deliver maximum performance, reliability and a low TCO in rail applications.

They provide the low maintenance and long service life benefits of sintered/PBE technology within a slim, lightweight block battery package that shows a major size and volume advantage compared with conventional batteries.

The Matrics MRX design is said to be reliable, even in extreme temperatures ranging from -30C to +70C.

It also includes an integrated water filling system.

This feature fills all the cells from one central point – without any moving parts – and reduces the battery maintenance requirements by enabling cells to be topped up quickly, safely and accurately, maximising the battery’s useful life.

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