Karlsruhe tram-train trials Ni-Cd battery

The pioneering Stadtbahn Karlsruhe tram-train network in Germany has completed an extended field trial with a Saft MATRICS MRX battery system to demonstrate the potential reliability, performance and TCO (Total Cost of Ownership) advantages offered by using specialised rechargeable nickel-cadmium (Ni-Cd) batteries in a 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 main-line railway by running urban trams on both networks. The main line 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 750 V DC on the tram tracks to the 15 kV AC used by the main-line trains (or vice-versa) the tram-train has to pass a cut-off section of 50 to 250 metres where no external power is available. This requires the onboard battery system (with a nominal voltage of 24 V), to provide a minimum of 20 V for a cut-off period of between 20 to 25 seconds to support all the electrical loads, so battery reliability is 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, ideally the battery should have a capacity of at least 300 Ah. But in practice this is limited to 230 Ah due to 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 main-line track, where the external power has been turned off for track maintenance but the onboard loads, such as lights, have been inadvertently left on. The current drain can cause a massive voltage drop to 16-18 V, 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 their overall service-life is limited to only two to three years.

Ni-Cd batteries offer a number of advantages for tram-train applications because they can provide: a larger capacity within the same installation footprint; resistance to deep discharges; high performance at high and low temperatures; long predictable service-life of at least 15 years with no risk of sudden death; low maintenance costs due to simple maintenance requirements and extended service intervals.

Saft worked with Stadtbahn Karlsruhe to carry out a TCO (Total Cost of Ownership) 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 then an Ni-Cd battery would actually cost less.

However, while the cost of the battery is 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, replacing failed batteries in the field and so on, also especially as a stranded vehicle might be up to 130 km 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 that comprises 19 MATRICS MRX 200 batteries which fit within the existing battery box, with a nominal capacity of 200 Ah.

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 230 Ah lead-acid battery it has replaced, in practice it delivers superior performance. Furthermore, the low temperature performance of lead-acid batteries is poorer than Ni-Cd. So the gap in performance between the lead-acid battery and the Saft battery is even greater in cold weather.

“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” said Herr Rainer Supper, deputy workshop manager responsible for electrics who is now in his 46th year with Stadtbahn Karlsruhe.“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 test 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 low TCO in rail applications. The batteries provide the low maintenance and long service life benefits of sintered/PBE technology within a slim, light-weight block battery package that shows a major size and volume advantage compared with conventional batteries.

The MATRICS MRX design is reliable, even in temperatures ranging from -30 to +70 degrees C.

The MATRICS MRX also includes an integrated water filling system. The simple to use 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, maximizing the battery’s useful life.

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