This is the third in a series of blogs discussing factors to consider when designing modern lift trucks.
Lift trucks consume a substantial amount of energy over time. If the machine is inefficient, it can impact energy costs, the environment, and battery life. Industrial consumers in Europe face higher electricity prices than customers in many other parts of the world.
Some countries, like the United States, require lift truck components (such as the battery and charger) to comply with strict efficiency standards to reduce electric energy consumption. Although the European Union currently does not have similar standards, it is implementing stringent policies to meet climate change targets and become a climate-neutral (net-zero greenhouse gas) economy by 2050. Energy efficiency is a key element in meeting these goals. Lift truck OEMs should consider designing more energy-efficient machines now to help lower their customers’ utility bills and improve equipment performance.
To design the most energy-efficient lift truck, manufacturers should consider total system energy efficiency, including battery round-trip energy efficiency, charger conversion losses, losses in the AC and DC cables, and charge profiles.
Battery round-trip energy efficiency is an important metric to consider when comparing different battery types for an electric lift truck. This efficiency metric is calculated as the ratio of DC discharge energy (the Watt hours taken out of the battery) to DC charge energy (the Watt hours put into the battery).
Battery chemistries have varying levels of efficiency. Typically, flooded lead-acid batteries are 80% efficient, sealed lead-acid batteries – AGM and gel – are 85% efficient, and lithium batteries are 95% efficient. Two components that impact battery efficiency are charge return factor and internal resistance. Lithium batteries deliver increased efficiency compared to lead-acid batteries because they perform better on both factors.
Less energy-efficient batteries cannot be charged as quickly and have limited duty cycles. They also require more power from the wall, and the lost energy produces heat. Battery heating can shorten battery life and causes a need for longer rest times to cool down. For lift trucks used in multi-shift facilities, lithium batteries are a logical choice.
Charger System Efficiency
Charger system efficiency is calculated as the ratio of DC charge energy (Watt hours put into the battery) to AC source energy (Watt hours taken from the electrical grid). Four significant components that can impact charger system efficiency are conduction losses from the wall to the charger (AC cable), conversion losses in the charger, conduction losses from the charger to the battery (DC cable), and charge profiles.
Conduction Losses in the AC Cable
Conduction losses in the AC cable are calculated by multiplying the resistance of the AC cable times the square of the current. Efficiency can be improved by using shorter or thicker (heavier gauge) AC cords.
Conversion Losses in the Charger
Converter topologies, control methods, semiconductor devices, magnetics and circuit boards are all factors that impact conversion losses in the charger. Many low lifters, pallet stackers or narrow aisle reach trucks are powered by line frequency chargers, such as ferroresonant or silicon-controlled rectifier (SCR) chargers. These charging technologies are inherently less efficient than modern high-frequency chargers, also known as switch-mode power supply (SMPS) chargers. High-frequency chargers are 8% to 10% more efficient than SCR and ferroresonant chargers.
Conduction Losses in the DC Cable
Conduction loss calculations for the DC output cable are similar to the AC cable. The DC cable often incurs more losses than the AC cable due to higher currents at lower voltages. The energy efficiency can change if the charger is on-board or off-board the machine. For example, lift trucks with an on-board charger will see better system efficiency if the charger is closer to the battery.
Charge profiles are a set of instructions the charger follows to charge the battery optimally. Each battery chemistry, because of its inherent efficiency, requires a different charge return factor or overcharge. Returning more energy than the battery requires may waste energy, cause battery heating, and shorten battery life. Returning less energy than the battery requires may reduce range and run-time, and also shorten battery life. Charger and battery suppliers should work closely together to optimize the charge profile.
Lift truck operators can see energy savings if the machine they use has a high-frequency charger running a charge profile tailored for the battery pack, with shorter and/or thicker AC and DC cables. Based on a study by PGE, switching from an older ferroresonant or SCR-based charger to a high-frequency charger in a three-shift operation can create savings ranging from 2,900 to 4,800 kWh per year per charger.
Although the European Union does not have any formal efficiency regulations in place for batteries and chargers, OEMs have the power to enhance efficiency. When designing a battery-operated lift truck, make the right cabling choices and source a high-quality, efficient battery and charger. These decisions will help ensure your machine is environmentally friendly, help your customers reduce their energy cost and consumption, and prolong the equipment’s lifespan.