Calculator Inputs
Example Data Table
| Setup | Motor KV | Cells | Motors | Current Per Motor | Battery | Expected Use |
|---|---|---|---|---|---|---|
| Quadcopter | 920 | 6 | 4 | 35 A | 5000 mAh 45C | Aerial lift test |
| RC boat | 1800 | 4 | 1 | 80 A | 6200 mAh 60C | High load run |
| Robot drive | 1100 | 5 | 2 | 28 A | 8000 mAh 35C | Mixed duty motion |
Formula Used
Pack voltage: cell count × voltage per cell.
No load RPM: motor KV × pack voltage.
Loaded RPM: motor KV × delivered voltage × throttle fraction × efficiency fraction.
System current: full current per motor × throttle fraction × motor count + auxiliary current.
Input power: pack voltage × system current.
Suggested controller rating: full current per motor × (1 + headroom fraction).
Battery current limit: battery capacity in Ah × C rating.
Runtime: usable battery Ah ÷ system current × 60.
Required C rating: system current ÷ battery capacity in Ah.
Target capacity: system current × target hours ÷ usable capacity fraction.
How To Use This Calculator
Enter motor KV, battery cells, and voltage per cell first. Then add the number of motors and full throttle current per motor. Use measured current when possible. Add average throttle, efficiency, battery capacity, C rating, and usable capacity. Include auxiliary current for receivers, servos, lights, pumps, or flight controllers. Press calculate. Review controller size, battery current margin, runtime, power, and RPM. Use CSV or PDF export to save the result.
Brushless Motor ESC Battery Sizing Guide
Why This Calculator Matters
A brushless power system works best when every part fits the same electrical load. The motor pulls current from the pack. The controller must carry that current without overheating. The battery must supply it without sagging too far. This calculator joins those checks in one place.
Good sizing starts with voltage. Cell count and charged cell voltage set pack voltage. The motor KV then gives a no load speed estimate. Real propellers and drivetrains reduce that speed. The loaded RPM result gives a practical planning value. It is not a dyno result, but it helps compare setups.
Current is the next key value. Enter the expected motor current at full throttle. The tool scales it by throttle demand and motor count. It also adds controller headroom. That gives a suggested controller rating. A higher rating can run cooler. It can also handle short peaks better.
Battery Planning
Battery capacity controls runtime. C rating controls safe current delivery. A high capacity pack can still be weak if its C rating is low. A small pack can deliver strong bursts, but runtime may be short. The calculator compares total draw with safe pack current. It also estimates used energy and reserve capacity.
Reserve matters because lithium packs should not be fully drained. Leaving capacity unused protects cells. It also reduces voltage sag near the end of a run. Enter a usable capacity percentage that matches your chemistry and safety practice. Many hobby packs are planned around partial discharge, not full discharge.
Using The Results
Use the output as a design screen. If total current exceeds battery capability, choose a stronger pack. If controller current is too low, choose a larger controller. If runtime is too short, reduce current, increase capacity, or lower throttle demand.
The calculator is useful for drones, RC cars, boats, fans, pumps, and robotics. It helps compare ideas before buying parts. Final choices should still be checked with measured current, motor temperature, controller temperature, and battery voltage under load. Real propellers, gearing, airflow, and cooling change the result. Always test carefully and leave extra margin for demanding electrical systems. Log pack temperature, voltage, and current after every full power test. Small notes prevent costly mistakes.
FAQs
What does this calculator size?
It estimates brushless motor load, controller rating, battery current limit, runtime, power, and RPM. It helps compare a planned setup before buying parts or running a full load test.
What is motor KV?
Motor KV means RPM per volt under no load. A higher KV motor spins faster at the same voltage. Real load reduces speed, so loaded RPM is always lower.
How much controller headroom should I use?
Many builders use 20% to 30% headroom. Heavy props, poor cooling, long bursts, or hot weather may need more. Measured current is the best guide.
Why is C rating important?
C rating shows how much current a battery can supply. Current limit equals capacity in Ah times C rating. Low C packs may sag, heat, or fail under high draw.
Does runtime include battery reserve?
Yes. The runtime uses the usable capacity percentage. This lets you leave reserve capacity in the pack instead of planning a full discharge.
Should I use nominal or charged voltage?
Use nominal voltage for average planning. Use charged voltage when checking peak RPM or fresh pack stress. The calculator accepts any voltage per cell value.
Can this replace bench testing?
No. It is a planning tool. Final setups should be tested with a wattmeter, temperature checks, and safe load testing. Real propellers and cooling change results.
Why add auxiliary current?
Auxiliary current covers receivers, servos, lights, pumps, sensors, fans, and controllers. Adding it improves battery runtime and current margin estimates.