Lift truck designers must understand the range of usage patterns their vehicles will face. Warehouse operations vary greatly, and their needs for material handling equipment vary as well. As discussed in our first blog in this series, there is a trend towards much larger and more sophisticated distribution centers, with ever taller racking heights, great physical travel distances, and expectations for rapid throughput that require constant 24/7 operations. Smaller warehouse operations exist, often with far less stress placed on a small fleet of material handling equipment. Lift truck OEMs offer a range of vehicles – and should provide for configuration of these vehicles to meet the warehouse operator’s requirements.
The capabilities and purpose of a specific lift truck is not necessarily aligned with the duty cycle it will face. High reaching lift trucks with high weight capacity won’t always have a high use duty cycle, and short pallet stackers won’t always be used occasionally. The velocity and throughput of a warehouse will define the duty cycle and usage pattern of each piece of equipment deployed.
Light Duty Lift Trucks
Small warehouses (perhaps operated by a specialized manufacturer) managing primarily palletized shipments of higher value goods out to a distribution network can achieve their profitability targets with lower throughputs – these warehouses might carry high levels of inventory with lower turns expectations because the cost of stocking out is high. As a result, the business may be able to manage all of its daily fulfillment requirements within a single shift. Within that shift, individual vehicles may be operated as little as 5-15% of the day. A low lifter, pallet stacker and narrow aisle reach truck may all be deployed in such a warehouse, but a small staff of one to five may be able to operate that warehouse.
Light use, single shift warehouses will place little demand on the battery and charge systems of their material handling equipment. Energy storage capacity can be sized appropriately to keep the acquisition costs of the lift trucks lower. Investments in higher cost lithium battery systems may not provide the appropriate payback, so lead-acid battery systems can be deployed. In single shift facilities, there is ample time for a lead acid battery pack, even if fully depleted, to recharge and cool overnight for the next day’s operation.
Higher Duty Cycles
As warehouse size and throughput expectations increase, so too do the demands placed on material handling equipment. Increases in operations productivity are needed where space is expensive, margins are thinner, and delivery expectations are measured in hours, not days. Larger staffs operate machines on a more regular basis, increasing duty cycles to 30-50% of the day. More than one of several types of lift truck may be deployed in such facilities, and it is possible that a second shift may be regularly scheduled.
Medium use, two shift warehouses are more dependent on their material handling equipment – and require increased capacity from each vehicle. Warehouse operators should analyze the expected usage patterns of each vehicle and determine the energy capacity and recharge requirements for each. Lift truck OEMs offer standard and larger capacity lead-acid battery packs, and many now offer lithium battery packs with even greater capacity. Of course, higher capacity packs require more energy to recharge, so sizing the battery charger to the battery pack is important. It is also important to consider charging efficiency and recharge schedules. It may be appropriate to have on-board chargers to allow for some intermittent opportunity charging throughout the day, either when loading or unloading a pallet, or when scheduled breaks occur. This opportunity charging can help reduce the necessary battery capacity compared to a system where you only charge at the end of the day. Lithium battery systems charge more efficiently, and due to their higher power density, they can provide more capacity in less space – leaving room for an onboard opportunity charger. In many cases, this on-board charger can be the only charger the vehicle needs – it is simply plugged in overnight after the second shift.
Warehouses with high demands for productivity and throughput will often operate on a 24/7 schedule to maximize the output from their space and ensure that rapid delivery can be achieved. These are often e-commerce fulfillment centers, with tens of thousands of SKUs being picked, packed, and shipped at tremendous velocity. Generally larger facilities, they are often populated by an army of material handling vehicles, many of them operating at very high duty cycles approaching 75%.
Around-the-clock operations place tremendous stress on the batteries and charging systems of these vehicles. Warehouse operators should be encouraged to deploy vehicles with large battery capacity – today, the recommendation would be to deploy lithium batteries. Lithium batteries offer more than three times the power density of lead-acid, allowing increased capacity in the same footprint (while often allowing space for an on-board battery charger). Lithium batteries can be charged at a higher rate, are inherently much more efficient, and require very little maintenance. With a sufficient sized battery charger on-board, regular opportunity charging may be sufficient for many of these vehicles. The alternative lead-acid battery system architecture for the 24/7 warehouse involves multiple battery packs per vehicle (generally about 2:1), and a costly battery changing system that occupies valuable space. Lead-acid batteries take longer to charge and are left to cool for several hours before they can be put back into service. Lead-acid batteries require more maintenance and have shorter expected lifetimes.
Lift truck designers should design each type of vehicle to allow configuration to meet the operational needs of the end customer. Energy storage and recharge represents a significant component of a vehicle’s capital cost – providing options to match duty cycles and acquisition costs provides customers with tremendous operational flexibility.