The engineering decision
Define normal operation, allowed transients and faults separately. Size continuous capability from the mission and validate peaks against voltage, thermal and protection limits.
Have these inputs ready
- Motor and controller data
- Battery-side current trace
- Controller current limits and stall response
- Simultaneous actions and minimum input voltage
Write the requirement as current against time
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Measure on the battery side of the controller
For a simple brushed motor, winding voltage approximately equals back EMF plus current times winding resistance, with an inductive term during transients. At zero speed the back EMF falls away. Actual current still depends on the applied voltage, winding resistance, drive switching and current regulation.
Motor phase current is not automatically battery current. PWM recirculation, motor speed and conversion losses change the relationship. Summing four motor nameplate stall currents can describe a hypothetical simultaneous locked-rotor condition; it does not establish the required continuous battery current.
Use controller settings to define permitted torque and fault response. A BMS protection trip is a system disconnect, not a substitute for normal motor current regulation. The robot must also reach a safe state when power is removed.
Separate four load classes
The example below starts with a 32 A continuous operating requirement. The two planned maneuvers are allowed events with explicit duration and repetition. The fault event instead requires a defined detection and clearance response.
In a hypothetical 60-second segment containing six 2-second acceleration events and one 1-second pivot, the remaining 47 seconds are at 32 A. The average is about 41.7 A and the RMS current about 45.6 A. Those numbers answer different questions: average charge demand versus constant-resistance heating. Neither replaces the 95 A loaded-voltage check.
| Requirement | Current | Duration | Condition |
|---|---|---|---|
| Normal operation | 32 A | Continuous | Declared operating envelope |
| Acceleration | 75 A | 2 s | Up to 6 events/min |
| Loaded pivot | 95 A | 1 s | Occasional |
| Fault transient | 125 A | 250 ms | Protection case; clearance required |
| Minimum terminal voltage | To define | All allowed events | Equipment input limit |
Put the event inside a coordinated protection envelope
Overlay the worst allowed operating event on the actual overcurrent threshold and delay tolerances. Include current-sensor error, startup blanking, temperature dependence and firmware configuration. Then check the fault case against cable, connector, switch and fuse withstand behavior.
Repetition matters. A sequence can remain below a single-pulse limit while accumulating heat. Conversely, a short circuit needs fast interruption even when the thermal average looks small. Assess these as separate design cases.
Regenerative braking returns energy to the DC bus and can charge the battery. Net battery current depends on regenerated power, other DC-bus loads and the available charge path. Verify charge acceptance at high SOC and low temperature, maximum controller bus voltage and the equipment response when the battery cannot accept energy.
Use the shutdown timing to narrow the investigation
| When it fails | Check first | Required evidence |
|---|---|---|
| At every startup | Inrush, blanking and peak current | High-bandwidth current trace |
| Only with simultaneous axes | Coincident load and voltage margin | Controller timestamps and bus voltage |
| After repeated turns | Heating and pulse repetition | Thermal trend across the mission |
| During braking at full charge | Regeneration and charge blocking | Bus overvoltage and charge-current log |
Measurement & validation procedure
| Test / review | Record | Acceptance condition |
|---|---|---|
| Capture real maneuvers | Battery current, bus voltage, controller state | All intended maneuvers fit the approved envelope |
| Repeat the worst sequence | Hotspot temperatures and minimum voltage | No cumulative thermal or undervoltage limit |
| Validate fault response in a controlled setup | Detection, disable and protection timing | Equipment reaches its specified safe state |
Continue this engineering task
Related engineering guidance
Prepare the evidence for review
- Motor/controller models and settings
- Simultaneous motion sequence
- Battery-side current capture
- Stall detection and safe-state requirement
Technical sources & claim support
References support the stated engineering principles. Worked examples use their explicitly stated assumptions.
- 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 BQ76952 Technical Reference Manual, SLUUBY2B ↗
Protections; overcurrent in discharge; short circuit; FET control
Threshold, delay, recovery and configuration behavior must be checked for the actual device revision.
- Infineon: Designing with power MOSFETs ↗
Current ratings, on-resistance, thermal impedance and safe operating area
Component design constraints; does not establish a complete pack current rating.
Key claims and evidence
| Claim | Basis | Reference |
|---|---|---|
| Stall detection and startup blanking depend on motor/application | documentation | Texas Instruments: Integrated Stall Detection for Brushed DC Motors, SLVAFQ3 |
| RMS heating differs from average charge demand | calculation | calculation |
| Protection delays must be coordinated with allowed events | inference | Texas Instruments BQ76952 Technical Reference Manual, SLUUBY2B |