Challenges in 24/7 Operations
An AGV in three-shift operation has no overnight charging window. The energy demand per vehicle depends on payload, travel speed, start-stop frequency, and drive concept. For typical industrial AGVs with lithium battery packs this results in an energy throughput that draws on the pack's nominal capacity several times each day. Simply oversizing capacity shifts the problem: an oversized pack increases vehicle weight, reduces payload, and extends charging time.
Three factors define the technical boundary of every charging concept: usable capacity (a function of depth of discharge, DoD), permissible charge and discharge rate (C-rate), and the cycle durability of the cell chemistry in use. LiFePO4 cells deliver very high cycle counts at moderate DoD (typically 80 %) and C-rates below 1C — a key argument for their use in high-availability fleets. Li-NMC cells offer higher energy density in the same footprint but are more sensitive to high charge rates and deep discharge. LTO (lithium titanate) cells permit very high C-rates and extreme cycle durability but offer lower energy density and are used mainly where short charge pulses are mandatory.
The battery management system (BMS) is the control centre in all three concepts: it monitors cell voltages, temperatures and state of charge (SoC), regulates the charge output, protects against over- and under-voltage, and supplies the vehicle controller with status data for predictive mission planning. A BMS that does not deliver precise SoC values in multi-shift operation endangers both cell longevity and fleet schedulability.
Three Charging Concepts Compared
The battery swap system decouples vehicle availability and charging completely. A depleted pack is exchanged at a dedicated swap station for a fully charged one — manually, semi-automatically or fully automatically in 20 to 90 seconds. Packs charge in a buffer rack at a controlled rate, without time pressure. The vehicle starts every mission with a known, full energy level. The price: a pool of spare packs (typically 1.5 to 2 times fleet size), a swap station, defined mechanical interfaces on the pack, and a BMS that completes its initialisation handshake in a few seconds.
Opportunity charging uses the vehicle's unavoidable dwell times — loading and unloading points, transfer stations, waiting positions — for short top-up pulses via floor contacts or a docking connector. It requires no spare packs and no swap infrastructure. The prerequisite is that dwell times are frequent and predictable enough to cumulatively cover the energy demand. LiFePO4 is particularly well suited because its flat discharge curve enables accurate SoC estimation and the cell chemistry tolerates high-frequency partial cycles without significant accelerated ageing. Critical is charge rate control by the BMS: if partial charge pulses are initiated at too high a C-rate, cell temperature rises and service life drops.
Static charging with oversized capacity is the simplest concept: a pack with sufficient nominal capacity covers an entire shift's energy demand; between shifts or in a defined break window it is fully recharged. This concept works in single-shift and to a limited extent in two-shift operations. In three-shift operation the necessary charging window is absent unless a sufficiently long shift changeover is built in. The oversized capacity increases pack weight and acquisition cost but reduces cycle stress per unit of capacity, which benefits longevity.
Sizing Parameters: Capacity, C-Rate and Cycles at DoD
Sizing starts with the energy balance per shift: drive power × travel fraction + load handling + auxiliary consumers, summed over shift length. The result is the minimum usable energy the pack must deliver per shift. From the chosen DoD the required nominal capacity follows. For LiFePO4 an 80 % DoD is typical to keep cycle count in the economical range. A smaller DoD (e.g. 70 %) extends service life further but requires more nominal capacity for the same usable energy.
C-rate is the second critical parameter. C-rate is the ratio of charge or discharge current to nominal capacity: 1C means a pack charges or discharges fully in one hour. For opportunity charging with short pulses, C-rates of 0.5C to 2C are used, depending on available dwell time and BMS protection regime. High charge rates (> 1C) accelerate cell degradation, especially at elevated cell temperature. The BMS must limit charge rate as a function of temperature — a function frequently absent or unconfigured in basic BMS solutions.
Cell chemistry cycle durability determines overall pack service life. Indicative values for LiFePO4 are several thousand full cycles at 80 % DoD and room temperature; the exact value depends on cell quality, C-rate, and temperature management. Li-NMC achieves fewer full cycles under the same conditions but offers higher energy density. In multi-shift operations with several full cycles daily, cycle stress is the dominant cost driver for the battery strategy — not the pack acquisition price.
BMS Requirements in Multi-Shift Operations
A BMS for multi-shift operations must do more than basic protection. First: precise SoC estimation across the entire operating range. Coulomb counting alone is insufficient because measurement errors accumulate across many partial cycles. A good BMS algorithm combines coulomb counting with a model correction (e.g. via open-circuit voltage during dwell times) to compensate drift. Second: cell-individual balancing. In multi-shift operations, capacity imbalances between cells intensify rapidly. Passive balancing dissipates excess charge as heat; active balancing transfers it to weaker cells and is more efficient but more expensive. The choice depends on cell spread in the pack and the thermal boundary conditions.
Third: communication with the vehicle controller. In multi-shift operations the fleet software must know the battery status of every vehicle to schedule missions and charging pauses intelligently. CAN bus (CANopen CiA profiles or manufacturer-specific PDO mapping), SMBus, or proprietary protocols are common interfaces. The BMS must transmit SoC, state of health (SoH), fault codes, and temperature values cyclically. A BMS without a documented communication protocol cannot be integrated into automated fleet control.
Fourth: thermal management. In continuous operation the battery pack heats up from charge and discharge losses. If cell temperature exceeds the permissible range, the BMS must limit current and charge rate or interrupt charging. Passive cooling (housing conduction, ventilation slots) is sufficient for moderate loads; active cooling (liquid or air) is required at high C-rates or in confined installation situations. The BMS must control the cooling circuit or at minimum provide a temperature signal for an external cooling controller.
Decision Guide by Shift Model
Single-shift operation (8 h running, 16 h pause): static charging with adequately sized capacity is the simplest and lowest-maintenance solution. The long charging pause permits low C-rates (< 0.3C) that maximise cell life. Swap systems and opportunity charging rarely pay off for small fleets.
Two-shift operation (16 h running, 8 h pause): for many fleet sizes opportunity charging is the most economical choice if the hall layout allows floor contacts or docking points. The 8-hour pause can alternatively be used for a full static charge, provided capacity covers 16 hours of operation. At very high utilisation with little dwell time during the shift, battery swapping becomes relevant.
Three-shift operation (24 h running, no fixed charging window): opportunity charging or battery swapping are the only practical concepts. Opportunity charging requires sufficiently frequent and plannable dwell times — in highly dynamic AGV applications with irregular order patterns this is not always the case. Battery swapping guarantees availability independent of the driving cycle but requires investment in infrastructure and spare packs. Hellpower Energy supports the sizing of both concepts — from initial capacity calculation through BMS parametrisation to the mechanical interface definition of swappable packs.
Checklist: Sizing Battery Strategy for Multi-Shift Operations
- ✓Energy balance per shift created — drive power, load handling, auxiliary consumers, shift length?
- ✓DoD and capacity defined — usable energy calculated, nominal capacity derived from DoD (typically 80 % for LiFePO4)?
- ✓C-rate for charge and discharge checked — opportunity charging pulses and dwell times matched, BMS protection limits defined?
- ✓Cell chemistry selected — LiFePO4 (high cycles, robust), Li-NMC (higher energy density), LTO (extreme C-rate) per requirement?
- ✓BMS communication specified — protocol (CAN/CANopen/SMBus), PDO mapping, SoC/SoH transmission agreed with fleet controller?
- ✓Thermal management evaluated — passive or active cooling, temperature limits parametrised in BMS?
- ✓Charging infrastructure planned — swap station/buffer rack or floor contacts/docking connector per concept?
The first three points define the energy framework and are a prerequisite for any concept decision. Points four and five determine technical feasibility. The last two secure the operation. We accompany the sizing from the first specification through to series delivery.