Calculating LiPo C-Ratings: Real-World Current, Internal Resistance, and Max Continuous Limits

An engineering guide on decoding Lithium Polymer (LiPo) battery discharge ratings, calculating continuous vs. burst amperage limits, evaluating marketing inflations, and determining true performance from cell internal resistance (IR).

1.

What is a LiPo C-Rating?

In rotary wing and high-performance RC flight, the C-rating is a dimensionless multiplier defining a Lithium Polymer battery pack's safe maximum discharge rate relative to its total capacity. A 1C rate drains a battery's total rated energy in exactly 60 minutes (1 hour).

Calculating the maximum continuous current output in Amperes (A) uses the foundational relationship:

Imax = ( Capacity in mAh ÷ 1,000 ) × C-Rating
  • Imax is the maximum continuous current output in Amperes (A).
  • Capacity converted to Ampere-hours (Ah) where 1 Ah = 1,000 mAh.
  • C-Rating is the discharge multiplier stamped on the pack (e.g., 45C, 70C, 120C).
2.

Baseline Theoretical Calculations & Examples

The matrix below details standard theoretical current outputs across common RC helicopter and drone battery capacities:

Pack Capacity Stamped C-Rating Theoretical Max Continuous Current Discharge Time at Max Current
850 mAh (0.85 Ah) 45C 0.85 Ah × 45 = 38.25 A 80 seconds
2,200 mAh (2.20 Ah) 50C 2.20 Ah × 50 = 110.0 A 72 seconds
5,000 mAh (5.00 Ah) 70C 5.00 Ah × 70 = 350.0 A 51 seconds
5,000 mAh (5.00 Ah) 120C 5.00 Ah × 120 = 600.0 A 30 seconds
3.

The Reality of Marketing Inflation vs. Internal Resistance (IR)

In the commercial hobby industry, manufacturer continuous C-ratings are frequently inflated for marketing purposes. Claiming a 5,000 mAh pack can sustain 120C continuous (600 Amperes) implies discharging the battery to empty in 30 seconds while generating massive thermal heat that would melt solder joints and destroy cell chemistry.

Engineering Fact: A pack's TRUE continuous current limit is restricted by thermal dissipation and internal cell resistance (IR measured in milliohms, mΩ). Excessive IR converts electrical power into thermal waste heat (P = I2R).
4.

Calculating True C-Rating from Cell Internal Resistance

To determine a battery's true physical capability without relying on label claims, measure individual cell internal resistance using a calibrated charger or IR meter at nominal room temperature (20°C–25°C / 68°F–77°F).

  1. Step 1: Calculate Maximum Thermal Power Dissipation

    A high-quality LiPo cell can safely dissipate approximately 6 watts of heat per Ampere-hour of capacity before experiencing thermal runaway or permanent degradation.

    Pmax (Watts) = Capacity (Ah) × 6 W/Ah

  2. Step 2: Solve for True Maximum Continuous Current

    Using the power dissipation formula P = I2 × R (where R is cell internal resistance in Ohms):

    Itrue = √( Pmax ÷ Rcell )

  3. Step 3: Calculate True Continuous C-Rating

    Divide the calculated true current limit by the pack capacity in Ah:

    True C-Rating = Itrue ÷ Capacity (Ah)
5.

Worked Engineering Example

Consider a 6S 5,000 mAh (5.0 Ah) pack rated by the manufacturer at 100C (claiming 500 A continuous capability). Measuring the pack shows an average cell IR of 1.8 mΩ (0.0018 Ohms).

1. Maximum allowable thermal power loss: Pmax = 5.0 Ah × 6 W/Ah = 30 Watts per cell.
2. True current capability: Itrue = √( 30 W ÷ 0.0018 Ω ) = √( 16,666.67 ) ≈ 129.1 Amperes.
3. True continuous C-Rating: 129.1 A ÷ 5.0 Ah = 25.8C.

While the manufacturer stamped 100C on the label, the physical internal resistance limits safe continuous output to approximately 26C (129 Amperes). Peak 2–3 second 3D pitch pumps may briefly reach 180 A–220 A, but sustained high-load flying beyond 130 A will overheat the cells.

6.

Summary & Practical Guidelines

  • Continuous vs. Burst: Continuous rating represents safe load over the full discharge curve; burst ratings (typically 2–3 seconds) allow higher transients during pitch pumps or collective maneuvers.
  • Temperature Sensitivity: Internal resistance increases sharply in cold weather. Always warm battery packs to ~30°C (86°F) before high-load 3D flights to lower IR and prevent severe voltage sag.
  • Connector Bottlenecks: Stamped C-ratings are meaningless if battery connectors cannot handle the load. Ensure XT60 (60 A continuous), EC5/XT90 (90 A continuous), or QS8/AS150 (150 A+ continuous) connectors match your power train demands.