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A perfect power partnership

Retailer John Lewis has introduced a combination of PowerNet battery and power management systems from EnerSys Motive Power to control the energy used at its newest distribution centre.

The installation automatically manages battery allocation to ensure balanced utilisation, minimises energy consumption and prevents battery systems from drawing too much power when overall demand in the warehouse is above pre-determined limits. This ensures other services are not disrupted and avoids the need for costly additional electrical infrastructure to keep the installation within budget.

"We're getting better utilisation, better discipline, and better use of batteries," says Ted Weager, Operations Manager MHE for the John Lewis Partnership. "This installation has fulfilled our expectations."

The Magna Park distribution centre opened in 2009 and is the latest investment by the John Lewis Partnership to support sustained growth. The site covers around 800,000 sq ft (74,300 sq metres) with separate zones allocated to different operations that serve the partnership's 30 retail stores and 20 distribution centres around the UK. The latest materials handling techniques and equipment create the optimum storage and picking environment with the 50 warehouse trucks on site complemented by automated systems including conveyors and picking cranes.

Jungheinrich was selected as the truck supplier and recommended EnerSys Motive Power for the battery systems. The fleet comprises of very narrow aisle combination pallet stacker/order pickers, reach trucks, ride-on pallet transporters, pedestrian powered pallet trucks and electric counterbalance trucks. By using detailed projections of the handling operations, the parties worked together to assess the battery management requirement. One of the key considerations was the provision of change and charge facilities for the warehouse trucks.

"We wanted a system that avoided the need for 100% spare batteries and offered balanced use to prolong their operational lifespan," says Weager. "We also wanted to make the best use of the power." As the batteries need to support intervals of up to eight hours between charging, the project team specified the highest capacity batteries available for the warehouse trucks to help meet this objective. The traditional approach for an intensive operation like this is to select a spare battery for each truck so that one can be charged while the other is being used.

In practice, the number of spare batteries needed will depend on factors such as the recharge time and the frequency and number of changes. Utilisation patterns at Magna Park predicted a 60% spare battery capacity; six spares for every ten trucks. The reduction in battery requirement has considerable benefits including lower capital costs and reduced space for the charging facilities.

Another consideration was the amount of power consumed. The electrical infrastructure for the building was finalised early in the design process and while provision to the charging area was more than adequate, subsequent changes to meet evolving requirements of the operation meant that additional power supplies would be required. One estimate put the cost of providing this additional power supply to the charging area at around £100,000. Instead, the project team looked at ways of limiting the amount of power consumed by the battery chargers without compromising the ability to maintain warehouse truck availability.

EnerSys recommended a novel implementation of its PowerNet battery and power management system to oversee all charging and changing operations. A typical PowerNet installation allows operators to manage their charging patterns to take advantage of off-peak rates for lower price electricity. However, as the system runs on a standard PC and is completely configurable in software, it can be set-up with a wide range of user-defined parameters to provide a unique set of performance characteristics.

Under normal operations, all of the chargers can draw the power they need to top-up the battery to which they are connected. However, if the available power is low the system automatically shuts down the chargers supplying the batteries requiring the greatest amount of charge. Power is then allocated to the remaining batteries to ensure that they are fully charged as soon as possible and ready when a truck requires a change. Batteries are brought back online for charging whenever power is available.

"The unique aspect of this installation is the way we are using PowerNet to manage the consumption," says Mike Gosling, Enersys UK Sales Manager. "It also provides ideal rotation of batteries."

The batteries are charged with EnerSys Powertech HF chargers. They automatically detect the battery's level of discharge and will supply only the amount of power needed to restore full charge. This means that less mains power is required to charge a battery and the energy costs are reduced.

The intelligent diagnostic software in the chargers transmits data via the system's PC to a large display screen installed in the changing area. The display clearly indicates the order in which batteries should be retrieved for each type of truck, ensuring only fully-charged units are taken. The system makes certain that all batteries receive the correct amount of charge before being used and that they are systematically rotated, overcoming the potential problem of operators simply taking the nearest available battery, regardless of its true state of charge. There is no longer any need to dedicate batteries to specific trucks.

The reach truck and pallet transporter batteries are arranged along one side of the facility on roller beds with the chargers mounted above for easy access and identification. Batteries are changed using an EnerSys Pro Series Tugger mounted on a modified powered pallet transporter. When a truck enters the changing room, the Tugger moves alongside and pulls the battery onto its own platform using a magnetic arm. The battery is taken to a free charging position and a fully charged unit, as indicated by the management system, is transferred to the awaiting truck.

Alongside the main battery bank is a smaller installation with one spare battery for the six VNA combination trucks. These are normally charged using an in-aisle system to minimise truck movements and reduce the time spent off-duty. The spare battery is used for periodic rotation to ensure balanced utilisation and as emergency back-up.

PowerNet also generates reports covering areas such as battery utilisation patterns, charging history, minimum/maximum batteries available, depths of discharge, power consumption, number of automatic charger off/on events, and alarm events to show when a battery was selected incorrectly. Performance and efficiency of the operation can be assessed accurately to support informed decisions about future requirements.

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