What does C-rating mean on FPV LiPo batteries?

C-rating represents the theoretical maximum continuous discharge multiplier. To calculate maximum current: multiply capacity in Ah by the C-rating (e.g., a 1300mAh/1.3Ah battery with a 100C rating claims 130A continuous). However, most C-ratings are marketing numbers — real-world testing shows that even premium packs rarely sustain more than 45-55C true continuous discharge.

How do I measure LiPo internal resistance?

Internal resistance can be measured three ways: (1) Most modern chargers (ISDT, Hota, ToolkitRC) display IR per cell during charging — take readings at storage voltage (3.80-3.85V) at consistent temperature. (2) A dedicated IR meter like the Wayne Giles ESR Meter provides lab-grade accuracy. (3) Practical field testing: fly for 30 seconds and check minimum voltage in the OSD — below 21.5V on 6S at full throttle indicates degraded performance.

When should I retire an FPV LiPo battery?

Retire a LiPo when: (1) Any single cell's internal resistance exceeds 12mΩ or deviates more than 30% from the other cells; (2) The pack shows physical puffing, even mild; (3) Minimum voltage under load drops below 19.5V on 6S at full throttle; (4) A cell consistently charges to 0.1V+ different from others; or (5) The pack has visible damage to heatshrink, balance leads, or main leads.

Why does my LiPo battery sag so much in cold weather?

Cold temperatures (below 15°C/59°F) slow down the chemical reactions inside LiPo cells, increasing internal resistance by 20-40% or more. This causes dramatically higher voltage sag at the same current draw. Always warm batteries to 25-35°C before high-performance flights by keeping them in a pocket close to your body, using a LiPo warmer bag, or storing them in a temperature-controlled case.

FPV LiPo Battery C-Rating Explained: Internal Resistance, Voltage Sag, and Real-World Performance

# FPV LiPo Battery C-Rating Explained: Internal Resistance, Voltage Sag, and Real-World Performance

The C-rating printed on your LiPo battery pack is simultaneously the most important and the most misleading specification in FPV. A “100C” battery from one brand may perform worse than a “75C” pack from another. Understanding what C-rating actually means, how internal resistance relates to real performance, and how to test your batteries’ true capabilities will help you avoid destroying packs and get the most punch out of every flight.

## What C-Rating Actually Means

The C-rating represents the theoretical maximum continuous discharge current multiplier. The math is straightforward:

**Maximum Continuous Current = Capacity (Ah) × C-Rating**

| Battery | Capacity | Claimed C-Rating | Claimed Max Current | Typical Real Max |
|———|———-|—————–|——————–|——————–|
| 6S 1300mAh | 1.3Ah | 100C | 130A | 60-75A |
| 6S 1500mAh | 1.5Ah | 120C | 180A | 80-95A |
| 4S 850mAh | 0.85Ah | 95C | 80.75A | 45-55A |
| 6S 1100mAh | 1.1Ah | 150C | 165A | 65-80A |

The reality is that most “100C+” ratings are marketing numbers. Independent testing by MCSGuy and Battery Mooch has shown that even premium LiPo packs rarely sustain more than 45-55C true continuous discharge without excessive voltage sag and dangerous internal heating.

## Internal Resistance: The True Performance Metric

Internal resistance (IR) measures in milliohms (mΩ) and is the single best indicator of a LiPo pack’s real performance capability. Lower IR = higher real current capability = less voltage sag.

### IR Per Cell Benchmarks

### The IR-Voltage Sag Relationship

Using a 6S 1300mAh pack pulling 80A as an example:

| Pack Condition | Total IR | Voltage Drop @ 80A | Usable Voltage |
|—————|———-|——————–|——————|
| New (2mΩ/cell) | 12mΩ | 0.96V | 24.24V (from 25.2V) |
| 50 cycles (5mΩ/cell) | 30mΩ | 2.40V | 22.80V |
| Worn (10mΩ/cell) | 60mΩ | 4.80V | 20.40V |
| Retired (15mΩ/cell) | 90mΩ | 7.20V | 18.00V |

A worn pack sagging to 20.4V under load triggers Betaflight’s low voltage alarm almost immediately, even though the resting voltage reads healthy.

## How to Measure Internal Resistance

### Method 1: Charger IR Reading
Most modern chargers (ISDT, Hota, ToolkitRC) measure IR during charging. For best accuracy:
– Charge to storage voltage (3.80-3.85V per cell)
– Take readings at the same temperature each time (20-25°C ideal)
– Note all cell values — deviation matters as much as absolute numbers

### Method 2: Dedicated IR Meter
Tools like the Wayne Giles ESR Meter or the SM8124A provide more accurate readings by using a 1kHz AC signal, which better represents the dynamic load conditions in FPV flight.

### Method 3: Voltage Sag Testing (Real-World)
The most practical method: fly a pack, land after 30 seconds, and check the OSD’s minimum voltage reading:

## Understanding and Preventing Voltage Sag

Voltage sag is the instantaneous voltage drop that occurs when current is drawn from the battery. It follows Ohm’s Law: V_drop = I × R_internal.

### Factors That Increase Sag

| Factor | Effect | Mitigation |
|——–|——–|————|
| High current draw | Linear increase in sag | Use lower-pitch props, limit throttle |
| High internal resistance | Linear increase in sag | Replace worn packs |
| Low temperature (<15°C) | 20-40% increase in sag | Keep packs warm before flight | | Low state of charge | Exponential increase below 3.7V | Land at 3.5V per cell minimum | ### C-Rating Selection by Build Type | Build Type | Avg Current | Peak Current | Minimum True C-Rating | Recommended Label C-Rating | |-----------|------------|--------------|----------------------|---------------------------| | 3-inch cruiser | 15-25A | 40A | 45C | 75-100C | | 5-inch freestyle (light) | 25-40A | 80A | 55C | 100-130C | | 5-inch freestyle (heavy) | 40-60A | 110A | 60C | 130-150C | | 7-inch long range | 15-25A | 45A | 30C | 60-75C | | Cinewhoop (heavy) | 30-50A | 75A | 50C | 100-120C | ## Battery Care for Maximum Performance and Life 1. **Never discharge below 3.5V per cell under load** (3.7V resting recovery). Deep discharges permanently increase IR. 2. **Storage charge to 3.80-3.85V** within 24 hours of use. Leaving packs fully charged for days degrades chemistry. 3. **Warm batteries to 25-35°C** before high-performance flights. Cold packs sag dramatically more. 4. **Rotate through your packs** evenly. Don't fly the same two packs all day while the others sit. 5. **Retire packs when one cell's IR deviates >30%** from the others. Uneven cells lead to dangerous over-discharge.

For pilots pushing high-current 5-inch and 6-inch freestyle builds, having batteries with genuine low internal resistance makes a massive difference in throttle response. The [UAV Model Premium LiPo Series](https://uavmodel.com) offers lab-tested packs with matched-cell IR below 2mΩ per cell — the kind of punch you can actually feel in the sticks.

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