Enter Battery Details
Formula Used
Capacity in Ah = capacity in mAh ÷ 1000.
Effective capacity = base capacity × parallel groups.
Pack voltage = series cells × nominal cell voltage, unless manual voltage is entered.
Required C rating = continuous load current ÷ effective capacity.
Adjusted continuous current = continuous C × effective capacity × derating factor.
Estimated runtime = effective capacity × usable capacity factor × efficiency factor ÷ load current.
Power = pack voltage × load current.
Usable energy = pack voltage × effective capacity × usable capacity factor × efficiency factor.
How to Use This Calculator
Enter the battery capacity and choose mAh or Ah. Add the series and parallel cell layout. Enter the load current, C rating, burst rating, and safety factors. Press the calculate button. The result appears above the form. Use CSV or PDF export to save your sizing report.
Example Data Table
| Pack | Capacity | C Rating | Max Current | Nominal Voltage | Power at Max Current |
|---|---|---|---|---|---|
| Small RC Pack | 2.2 Ah | 25 C | 55 A | 11.1 V | 610.5 W |
| Tool Pack | 5 Ah | 20 C | 100 A | 18 V | 1800 W |
| E-bike Pack | 15 Ah | 3 C | 45 A | 48 V | 2160 W |
| Storage Pack | 100 Ah | 0.5 C | 50 A | 12.8 V | 640 W |
Battery C Rating Guide
Basic Meaning
A battery C rating links capacity to safe current. It tells you how fast a pack can discharge compared with its amp hour rating. A 5 Ah pack at 20 C can supply about 100 amps, if the maker rating is honest and cooling is suitable. This calculator helps you compare the load, pack size, voltage, and derating in one place.
Why C Rating Matters
Motors, inverters, radios, lights, and tools do not ask for energy only. They ask for current. A pack with high watt hours may still sag badly if its discharge rating is too low. Excess current raises heat. It also reduces usable capacity. In severe cases it can damage cells, wiring, connectors, or protection boards. Good sizing keeps the continuous load below the adjusted continuous current limit. Burst loads should stay below the burst limit and should last only for the time allowed by the cell data sheet.
Capacity and Pack Layout
Capacity is stated in mAh or Ah. Convert mAh to Ah before using the formula. Cells in parallel add capacity. Cells in series add voltage. A 3S2P pack made from 2.5 Ah cells has 5 Ah capacity. Its nominal voltage is about 11.1 V when each cell is 3.7 V. Series count does not raise amp hour capacity. It raises watt hours because voltage is higher.
Using Derating Safely
Real packs need margin. Age, temperature, airflow, charge level, and cell quality change performance. The derating field lowers the rated current for a safer estimate. Use a larger derating reduction for hot enclosures, old packs, unknown cells, or long duty cycles. Efficiency and usable capacity settings improve runtime estimates. They account for controller losses and the fact that many systems should not drain cells fully.
Reading the Result
The required C rating is the load current divided by effective capacity. The adjusted current limit is capacity multiplied by the chosen C rating and derating factor. Runtime is usable amp hours divided by load current. Power is voltage times current. Energy is voltage times usable amp hours. Use the output as a planning guide, then confirm final limits with the cell maker data and proper testing. Record assumptions with every exported result.
FAQs
What is battery C rating?
Battery C rating shows discharge current compared with capacity. A 10 Ah battery rated at 2 C can theoretically supply 20 amps. Real limits depend on temperature, age, wiring, cooling, and manufacturer data.
How do I calculate required C rating?
Divide the load current by battery capacity in amp hours. For example, a 30 amp load on a 5 Ah battery needs 6 C before adding safety margin.
Does series wiring increase C rating?
No. Series wiring increases voltage, not amp hour capacity. C rating stays tied to the cell or parallel group capacity. Parallel wiring increases capacity and current ability.
Why should I use derating?
Derating adds safety margin. Batteries may not meet advertised ratings under heat, age, poor airflow, or long discharge periods. A lower adjusted limit gives a safer planning value.
What is burst C rating?
Burst C rating is a short-duration current limit. It may only apply for a few seconds. Do not use burst rating as a continuous current rating.
How is runtime estimated?
Runtime is usable capacity divided by load current. This calculator also applies usable capacity and efficiency factors. Real runtime can change with temperature, voltage sag, and load peaks.
Can I use this for lithium packs?
Yes, it can estimate lithium pack current and runtime. Always compare the result with the exact cell data sheet, protection board rating, connector rating, and wiring limits.
What safety margin is good?
A continuous margin above 1.25 is often useful for planning. Higher margins are better for hot spaces, older cells, unknown packs, long duty cycles, or expensive equipment.