The engineering decision
Use delivered energy for runtime, minimum loaded voltage for peak current, and the actual docking window for recharge feasibility.
Have these inputs ready
- Mission states and duration
- Battery-side or load-side power boundary
- Voltage window, payload and route
- Docking schedule and service environment
Make each mission state visible
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Start with non-overlapping mission states
Define travel, handling, idle, docking and exceptional events with durations that sum to the mission time. Capture concurrent functions explicitly. If an acceleration interval is already included in a travel average, adding its full energy again double-counts it.
State where power is measured. If it is equipment-side electrical power, include converter losses to obtain battery-terminal demand. If it is already battery-terminal power, do not apply that same efficiency penalty again. Mechanical output power additionally requires drive efficiency.
| State | Duration | Battery power | Energy |
|---|---|---|---|
| Travel including maneuvers | 4 h | 0.60 kW average | 2.40 kWh |
| Handling | 1 h | 1.20 kW average | 1.20 kWh |
| Idle electronics | 3 h | 0.10 kW | 0.30 kWh |
| Total | 8 h | Variable | 3.90 kWh |
Keep usable energy, reserve and aging separate
Assume the mission requires 3.90 kWh at the battery terminals. If only 80% of validated usable energy is allocated to the mission, the battery must provide 4.875 kWh of usable energy under the specified service condition. This is division by 0.8, not a 20% markup.
If the required end-of-life energy retention is hypothetically 80%, the corresponding beginning-of-life usable-energy target is 6.094 kWh before any separate temperature allowance. Avoid stacking generic margins that cover the same effect twice. Define each factor, its evidence and the operating corner where it applies.
At a 2.4 kW battery-side peak and 40 V minimum loaded input, current is 60 A. Calculating at 48 V nominal would give 50 A and understate this peak-current requirement.
Check the turnaround constraint
Dividing the 3.90 kWh delivered during the mission by the two-hour docking window gives an ideal average energy-replacement rate of 1.95 kW. Determine charger power and recharge time from the measured charge profile and cycle energy efficiency, including conversion losses and any docked load. Constant-voltage taper may prevent two-hour recovery even when rated charger power exceeds this average.
If the battery cannot recharge in the available window, the decision is architectural: more charging time, a higher permitted charge rate, a changed SOC window, an exchangeable pack or a revised mission. Increasing Ah alone can increase mass without solving the charge bottleneck.
Close the requirement across disciplines
| Input | Affects | Validation evidence |
|---|---|---|
| Minimum/maximum input voltage | Chemistry and series count | Full-charge and end-of-mission bus voltage |
| Peak current and repetition | Cells, switches and conductors | Loaded voltage and thermal traces |
| Envelope and mass | Parallel count and cooling | Mechanical design and payload mission |
| Temperature and ingress | Cell limits, enclosure and charging | Environmental test plan |
| Docking window | Charger and power path | Repeated mission/charge cycles |
Separate chemistry, cell format and power capability
LFP is a lithium-ion chemistry; pouch, cylindrical and prismatic describe cell formats. A 200 A pack requirement does not select either chemistry or format: parallel count, loaded voltage, event duration and heat remain decisive.
Compare exact cell candidates against the robot mission, mass, charging and lifetime requirements. Pouch, cylindrical and prismatic formats each require a defined electrical, mechanical and thermal design. Use measured duty-cycle performance to choose the cell and pack architecture.
Measurement & validation procedure
| Test / review | Record | Acceptance condition |
|---|---|---|
| Log a representative mission | State duration, power, payload and route | Time and energy accounting are complete |
| Evaluate service corners | Peak current and usable Wh at stated temperature/aging | Mission and voltage margins are met |
| Repeat mission and charge | SOC recovery and thermal trend across shifts | No accumulating energy deficit |
Continue this engineering task
Related engineering guidance
Prepare the evidence for review
- Mission trace and payload
- Voltage window and power boundary
- Docking/charging schedule
- Envelope, temperature and lifetime target
Technical sources & claim support
References support the stated engineering principles. Worked examples use their explicitly stated assumptions.
- Molicel INR-21700-P45B product data sheet, revision 1.2 ↗
Cell specifications; discharge-rate and temperature characteristics
Cell-specific capacity, impedance and rate/temperature data; worked examples state their own input values.
- Texas Instruments: Integrated Stall Detection for Brushed DC Motors, SLVAFQ3 ↗
Discrete stall detection; startup blanking; current regulation
Brushed DC example; controller settings and motor topology must be verified for the equipment.
- Texas Instruments BQ25798 data sheet ↗
Power-path management; battery supplement mode; charge termination
Low-voltage power-path example covering load sharing, charge termination and battery supplement mode; apply the device voltage and current limits.
Key claims and evidence
| Claim | Basis | Reference |
|---|---|---|
| Current at fixed power increases as voltage falls | calculation | calculation |
| Mission reserve uses allocated-energy division | calculation | calculation |
| Charge behavior and load sharing affect docking feasibility | inference | Texas Instruments BQ25798 data sheet |
| Duty states are a proposed requirements method | workflow | engineering workflow |