The rise of warehouse AGVs: what role do batteries play?

As warehouses move toward full automation, AGVs are taking automated materials handling beyond fixed conveyor and sortation machinery and enabling free movement through sites. Unlike static installations, most AGVs are not permanently wired to the mains electricity supply but instead rely on motive batteries for power. Investment in AGVs to increase productivity, efficiency and economy must be complemented by selection of battery equipment that will get the most out of them.

AGV benefits

An AGV is a type of mobile robot, or transport robot, whose navigation is guided by technology of varying sophistication. Within warehouses, AGVs take the place of lift trucks in transporting, storing and retrieving goods. They are also used widely in factories.

AGVs can be designed with smaller dimensions than conventional lift trucks, to fit into narrower aisles and enable higher storage density. Their automated operations lower the risk of errors and product damage, and when working in human-free zones they reduce the need for lighting and heating.

They can take on tasks that are uncomfortable or dangerous for workers. AGVs also make the workplace safer for employees in another way, thanks to their predictable behaviour, precisely controlled movements and inbuilt safety systems. On sensing human presence, they are built to sound an alarm, slow down and, if necessary, come to a complete halt. 

AGV categories

The variety of AGVs is expanding and each vehicle needs careful consideration to identify its optimum choice of battery, charging method and battery management approach. The simplest AGVs are just automated carts – with minimal features – into which goods are placed for transport. Tugger AGVs, in which a powered unit pulls a trailer, or a train of trailers, are an advance on this.

A unit load or ‘top carrier’ AGV typically carries a discrete item – or a container such as a pallet, bin or tote – on its deck. The larger ones will transport heavy goods like engines, paper rolls and steel coils. Smaller designs include thin, box-shaped robots which drive under a load container then lift and transport it.

Forklift AGVs are specialised for activities within the range covered by manned warehouse lift trucks. Most fulfil the roles of power pallets and order pickers, but there are also AGV equivalents of stackers, reach trucks, narrow and very narrow aisle trucks, and counterbalance forklift trucks.

Navigation and control

AGV activities are directed by a combination of software and sensor-based systems. The earliest AGVs followed a path set by inductive wires embedded in the warehouse floor. This still works well today but makes layout modification difficult. Magnetic strips, bars or dots on the floor are less inflexible. Laser scanning of reflectors placed throughout the warehouse is a versatile alternative. A further option is inertial guidance, using a computer and a range of sensors – motion, rotation (gyroscopic) and sometimes magnetic – to calculate position. Then, of course, there is GPS navigation.

While these technologies essentially enable AGVs to be programmed to carry out repetitive tasks, addition of artificial intelligence promises to make them more autonomous. Some advanced AGVs can now explore new environments, create their own maps and calculate ideal routes, for example.

AGVs which can learn and make decisions give us a glimpse of Industry 4.0, with its smart factories and warehouses. The Internet of Things – a vital enabler of Industry 4.0 – is used extensively in controlling and monitoring AGVs. Through wireless communication, via the IoT, data is collected from the AGVs, for processing, and instructions are transmitted.

Battery choices

To maintain non-stop AGV productivity, and maximise efficiency, one obvious target is to minimise time and effort spent on maintaining and charging their batteries. Traditional flooded lead-acid batteries have the highest maintenance needs, including the requirement for a ventilated, cooled room for water top-ups, recharging and storage. 

On the other hand, this is the least expensive battery type to buy. Every battery-related decision must be based on full knowledge of the application, and in some cases flooded lead-acid may provide a good business case despite its maintenance demands.

Sealed lead-acid batteries, including gel and AGM (absorbent glass mat) types, although maintenance-free, typically require a full recharge after every shift, which takes several hours. However, advanced AGM batteries based on TPPL (Thin Plate Pure Lead) technology can be opportunity-charged – like lithium-ion batteries – during short breaks in activity. In many applications this offers an optimum solution for recharging within the required duty pattern of the AGV. 

Due to its features – fast charge, high energy density, PSoC (Partial State of Charge) capability and minimum maintenance required- TPPL is typically the product of choice of most of the AGV’s applications. With increasing energy demand, Lithium-ion batteries can offer a more viable solution thanks to their higher energy density and ultra-fast charge capabilities. Today Li-ion’s major downside is its much higher price but with expectation of cost reduction in the future.

Again, the user’s specific needs and circumstances must be assessed in detail before choosing the power solution. This includes the application, environment, shift pattern, load weights and AGV specifications. In some vehicles, like small, flat unit load AGVs, the high energy density and compact size of an Li-ion battery is ideal. In all cases, batteries must be specified with enough capacity to prevent runouts during shifts. Ultimately, the right battery choice depends on accurate calculation of TCO (total cost of ownership) and ROI (return on investment) for each option.

Charging and power management

For some operations and working patterns with little stoppage time, opportunity charging is not enough to meet an AGV’s power demands. Replacing discharged batteries with fully charged packs may then be a more efficient and productive option. This can be achieved, manually or, in most of the cases by installing an automated station in which batteries are swapped by a machine.

Another effective way to run the AGVs, when time is available between tasks and shifts, is with fast opportunity charging. Chargers are installed at strategic points on the AGV’s path to provide replenishment during any short break between tasks. The process can be fully automated, with AGVs seeking a charging station whenever their battery’s charge drops below a certain level.

The AGV will find the first available docking station and it will drive there in order to perform fast opportunity charging for the next mission. The connection can be made with physical hot shoes or by means of the most modern wireless chargers. The performance and intelligence of charging equipment varies greatly between products, so another careful choice is needed to optimise charge rates, efficiency and battery life. 

Increases in digitisation, collection and exchange of data have been essential to the development of automated warehouses. Fitted with sensors and communication electronics, AGVs become very useful as data-gathering and transmitting devices. If suitably equipped, they are also capable of acting upon received data. These functions create a good basis for fleet management systems. 

Users of AGVs will naturally wish to automate battery management as well. Ideally, all data from AGVs, chargers and battery handling devices should be drawn together by a single integrated platform for analysis and production of actionable outputs.