| Scenario | Voltage (V) | Power (W) | Efficiency | Duty | Surge | Continuous (A) | Average (A) | Peak (A) |
|---|---|---|---|---|---|---|---|---|
| Wi‑Fi router | 12 | 18 | 90% | 100% | 1.2× | 1.667 | 1.667 | 2.000 |
| DC motor (intermittent) | 24 | 120 | 85% | 40% | 3.0× | 5.882 | 2.353 | 17.647 |
| LED strip | 5 | 10 | 92% | 60% | 1.0× | 2.174 | 1.304 | 2.174 |
Average current: Iavg = Icont · D
Peak current: Ipeak = Icont · S
Battery-side current: Icont = Iload / η
Inferred load power: P = V · Iload
Continuous equivalent: Icont = Iavg / D
Peak current: Ipeak = Icont · S
Average current: Iavg = Pavg / (V · η)
Nominal capacity: CAh ≈ Wh / V
- Pick a method that matches what you know: power, resistance, capacity, or energy.
- Enter values with consistent units, then set efficiency and duty cycle.
- Add a surge factor for startups, inrush, or transient loads.
- Review continuous, average, and peak current results above the form.
- Export CSV for spreadsheets or PDF for design notes and reviews.
Design inputs and measurement discipline
Accurate current draw starts with clean inputs. Use measured voltage at the load and note sag under peak demand. If the system uses a converter, include its efficiency at the expected load point, not a best-case datasheet value. For resistive elements, confirm resistance at operating temperature; copper and heater elements rise with heat, lowering current over time. Record ambient temperature and cable length because they affect loss and heating.
Continuous, average, and peak planning
This calculator separates continuous, average, and peak current because each drives a different engineering decision. Continuous current informs conductor heating and steady-state thermal limits. Average current aligns with energy use and runtime estimates when duty cycling or sleep modes exist. Peak current captures inrush, motor starts, radio bursts, and actuator stalls. Using a surge factor keeps peak sizing conservative while allowing runtime comparisons across different operating profiles.
Using efficiency, duty cycle, and surge factors
Efficiency reduces delivered load power for a given battery current, so lower efficiency increases current draw. Duty cycle scales average current relative to continuous operation; a 40% duty profile typically draws 0.40× the continuous value on average. Surge factor scales peak current for short events. When uncertain, use a higher surge factor and validate with a clamp meter, shunt, or power analyzer.
Sizing protection and wiring with margin
Fuse selection should tolerate normal peaks but open on sustained faults. The fuse suggestion here multiplies peak current by a safety margin to create a starting point. Compare that value against load startup behavior, expected fault currents, and manufacturer time-current curves. Wiring must handle both steady current and transient peaks without excessive voltage drop. Shorter runs, larger conductors, and connectors reduce heat and improve stability during bursts.
Interpreting outputs for battery life and reliability
Use average current to estimate runtime from capacity, but adjust for real discharge conditions. Many chemistries deliver less usable capacity at high currents, low temperatures, or near end-of-life. Peak current helps validate that the battery, BMS, and connectors can supply bursts without undervoltage resets. If peak draw is close to limits, consider soft-start, larger capacitance, staged loads, or a higher voltage bus to reduce current.
1) Which method should I choose?
Use Voltage + Power for known load watts, Voltage + Resistance for resistive loads, Capacity + Runtime for field data, and Energy + Voltage + Runtime when battery Wh is known.
2) What does efficiency represent?
Efficiency models converter, regulator, and wiring losses. Enter the expected efficiency at your operating point. Lower efficiency increases battery-side current for the same load requirement.
3) Why are continuous and average different?
Continuous assumes the load is always on. Average applies duty cycle, reflecting sleep, pulsing, or intermittent operation. Average is best for runtime estimates and energy budgeting.
4) How should I set the surge factor?
Use measured inrush when available. For motors, 2× to 5× is common; for radios and compute bursts, 1.2× to 2× may fit. Validate against worst-case startup and stall behavior.
5) Is the fuse suggestion always correct?
No. It is a starting point. Confirm with the fuse’s time-current curve, ambient temperature, wiring limits, and expected fault conditions. Safety-critical designs require standards-based review.
6) Can I estimate runtime directly from results?
Yes, using average current and capacity, but real runtime depends on temperature, aging, and discharge rate. Measure under representative conditions to confirm the estimate.