Saft Ni-Cd batterieshelp keep tram-trainsrunning in Karlsruhe

Saft nickel-cadmium (Ni-Cd)

batteries help ensure non-stop

tram-train service in the

German city of Karlsruhe,

with a balance of reliability,

performance and low

maintenance.

The city of Karlsruhe was the first inGermany to link its street tramway andmainline railway by running urban trams onboth networks. These tram-trains have dualequipment that enables them to run on bothtram and train networks, using multiplesupply voltages.

Changing between the 750 V DC tram trackpower supply and the 15 kV AC main linesupply, requires the tram-train to pass acut-off section of up to 250 metres whereno external power is available.

The onboard battery system – with a nominalvoltage of 24 V – must provide a minimumof 20 V for 20–25 seconds to support all theelectrical loads. This means battery reliabilityis vital to ensuring the Stadtbahn Karlsruheservices run continuously and on time.

Battery challengesPreviously, most of the Karlsruhe tram-trainswere fitted with flooded lead-acid batteries,which had a number of reliability,maintenance and service-life issues.

For example, ideally the battery should havea capacity of at least 300 Ah. But in practicethis is limited to 230 Ah with lead-acidbatteries due to the need to fit them in thespace offered by the under-floor battery box.This meant there was a risk of the tram-trainrunning out of power, and stopping beforeit could pass across the cut-off section.

Further battery problems were experiencedduring the winter months, when lowtemperatures adversely affected batteryperformance. There had also been at leastone case of ‘sudden death’ – in which theopen circuit failure of one or more cellscaused the whole battery to fail. Overallservice-life of the lead-acid batteries waslimited to only two to three years.

Ni-Cd advantagesNi-Cd batteries offer a number of advantagesin tram-train applications. They can providea larger capacity within the same footprint;resistance to deep discharges; highperformance at extreme temperatures; long,predictable service-life (at least 15 years),with no risk of sudden death; simplifiedmaintenance and extended service intervals.

Saft worked with Stadtbahn Karlsruhe tocarry out a Total Cost of Ownership (TCO)analysis. This showed that, while the initialcost of acquisition of the Ni-Cd batterysystem would be higher than for lead-acid,when operation, maintenance andreplacement costs over a 12-year periodwere considered, the Ni-Cd battery wouldactually cost less.

Successful trialAfter proving the case for Ni-Cd batteriesin principle, Stadtbahn Karlsruhe decided tocarry out a practical evaluation through allthe seasons of the year and asked Saft toprovide a trial battery system. This comprised19 MATRICS MRX 200 batteries that fittedwithin the existing battery box, with a nominalcapacity of 200 Ah.

SaftIndustrial Battery Group12, rue Sadi Carnot93710 Bagnolet, FranceTel: +33 (0)1 49 93 19 18Fax: +33 (0)1 49 93 19 69

www.saftbatteries.com

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Information in this document is subject tochange without notice and becomes contractualonly after written confirmation by Saft.

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C A S E S T U D Y

The Ni-Cd battery’s high current performanceis crucial in supporting the passage of thetram-train through the cut-off section. Eventhough the Saft Ni-Cd battery had a lowerrated capacity than the 230 Ah lead-acidbattery it replaced, in practice it deliveredsuperior performance, especially at lowtemperatures.

“Saft’s Ni-Cd battery performed very wellduring the field test,” says Rainer Supper,deputy workshop manager responsible forelectrics at Stadtbahn Karlsruhe. “Therewere no problems or outages, whichis more important than anything else –including cost – since fewer outages meanmore passenger availability.

“We didn’t need to touch the Saft batteryfor a whole year and it only needed toppingup with three litres of water,” Suppercontinues. “And, thanks to the centralizedwater filling system incorporated into theMRX design, topping up only took 15minutes. With our lead-acid batteries, eachcell had to be opened individually for refilling,and this had to be done three times. Withthree major maintenance sessions during theyear, at least one hour of time could besaved per tram-train.”

The success of the first field test promptedStadtbahn Karlsruhe to order two furthertest Ni-Cd test batteries, and the first ofthese was installed in December 2008.