This is the seventh and final in a series of blogs discussing factors to consider when designing modern lift trucks.
In this blog series, we have discussed many factors in the design of a lift truck. We’ve discussed warehouse space, weight and balance, environmental standards, operating conditions, usage patterns, battery chemistry and storage capacity. Many of these factors are intertwined in the design and purchase process.
Current and foreseeable duty cycles will drive your need for battery capacity. Once you have selected your battery chemistry and storage capacity, choosing the output power level and location (or locations) for the battery charger is really a final step in electrical system design and integration.
There is usually a recommended range of charge rates for each battery chemistry. Flooded lead-acid, sealed or valve-regulated lead-acid, and lithium batteries can all accept charge at different maximum rates. Flooded lead-acid batteries should be charged at no more than C/10, where C represents the capacity of the battery pack in Ah. A 200 Ah battery pack should be charged at no more than 20A. It is also important to note that lead-acid batteries charge in multiple stages, where a constant voltage stage reduces charger current to finish the charging – and this extends charge time. Sealed lead-acid batteries can accept charge at approximately 3 x that rate, or C/3.3. In the same example, they could accept 60A of charge – sealed lead-acid batteries also charge slower in their second stage. Lithium batteries can accept charge rates up to C, the battery’s capacity – 200A in our example.
There is less agreement on minimum charge rates for each chemistry, but too low charge rates will never allow your battery pack to become fully charged. Different battery chemistries charge differently. Lithium batteries accept charge more efficiently, but the charge must be tightly controlled. Where time is available, lithium batteries will have longer life if the charge rate is reduced, since higher charge rates create more heat inside batteries. As noted above, lead-acid batteries require more gentle charging during their second stage that will make each charge cycle take longer than dividing battery capacity by maximum charge current. In addition, lead-acid batteries should cool after a full charge cycle before returning to operation.
Aligning Cost to Duty Cycle
In low duty cycle environments, lift trucks should be designed or configured with a smaller battery pack to reduce capital cost. With plenty of time to recharge, the battery charger can be relatively low in power – also reducing system capital cost. The location of the charger for these vehicles is not critical, but if room permits on the lift truck, an on-board charger will provide convenience, allowing operators to plug the lift truck into any available AC power source and not occupying warehouse space with an off-board charger. Several suppliers of DIN or BS standard trays can offer versions with integrated on-board chargers.
As per-shift usage and duty cycles increase, lift trucks will need larger battery capacity. With more time spent in operation, there is less time available to recharge the battery packs. Combining larger capacity and higher recharge requirements with less available charge time requires higher power charging capability.
In medium duty cycle environments with one or two shifts, it may be possible to fully recharge the battery pack overnight with a large enough battery charger. Since lithium packs offer more energy in less space, and accept charge more quickly and efficiently, they are often the best choice for locations with these duty cycles. Space constraints may make it difficult to fit a large enough battery charger on-board the vehicle in this configuration, and with an overnight period to charge, it may be best to return the lift truck to a higher power charging station.
In highest duty cycle environments and warehouse with 24/7 operations, battery capacity and charger output power must be carefully matched to maximize lift truck uptime. Lithium battery packs of sufficient capacity to operate at a high rate and higher power on-board battery chargers can be combined to create a highly productive system. These vehicles should be plugged in to any available power source, maximize the power available from the AC circuit, and quickly add energy back to the battery pack during any period where the lift truck is not in use. Opportunity charging during breaks, or a pallet loading or unloading period, will regularly return energy to the battery pack.
Where usage levels prevent the on-board charger from keeping up with the lift truck’s energy demands, a dedicated high power charging location can be used. These can take two forms. The traditional battery swapping system can be used, with a reduced ratio of batteries to vehicles because the lithium packs can be recharged more quickly and returned to service. The alternative is a hybrid model where lift trucks are opportunity charged on a regular basis with an on-board charger, but the lift truck returns to the higher power charging station when needed for a full charge.
Where possible, charger power levels should be selected to most cost-effectively return energy to the chosen battery pack facing a current and expected duty cycle. Chargers can be mounted on-board or off-board depending on system requirements – hybrid charging solutions may offer the ideal solution.
To learn more about trends in material handling and Delta-Q’s charging solutions for lift trucks, visit delta-q.com today.