A 100C battery label means nothing if the pack sags to 3.3V per cell on punch-out. C-ratings are the most exaggerated spec in FPV — and picking the wrong one either starves your motors or adds dead weight. Here is how to calculate what your build actually needs.
How C-Rating Translates to Real Current
The formula is straightforward: Maximum continuous current = Capacity (Ah) × C-rating. A 1300mAh (1.3Ah) pack labeled 100C claims 130A continuous. In reality, most “100C” packs deliver 40-50C before voltage sag exceeds 0.5V per cell. The label is marketing — your charger’s internal resistance reading tells the real story.
Step 1: Calculate Your Build’s Current Demand
Your motors draw current, not the battery label. A typical 5-inch freestyle quad on 2207 1900KV motors with 5-inch tri-blade props draws 25-35A per motor at full throttle — 100-140A total. A 7-inch long-range build on 2807 1300KV motors might draw 15-20A per motor, totaling 60-80A. A 65mm whoop on 0802 motors draws 3-5A per motor — under 20A total.
Measure your actual draw: hover at 50% throttle in the OSD and note current, then punch to 100% for 2 seconds. The punch-out number is your peak demand. Add 20% headroom for aging packs and cold weather.
Step 2: Calculate Required C-Rating
Required C = Peak Current ÷ Capacity. If your 5-inch quad draws 120A peak and you use a 1300mAh pack: 120 ÷ 1.3 = 92C. A “100C” labeled pack is theoretically adequate, but real-world C is typically half the label. The fix: either size up to 1500-1550mAh (120 ÷ 1.5 = 80C — more achievable) or accept that “130C” packs exist precisely because 100C labels are inflated.
Step 3: Read Internal Resistance, Not Labels
A $30 cell checker that measures per-cell IR is the only honest C-rating indicator. For a 1300mAh pack: IR below 3mΩ per cell is excellent (genuine 60C+), 3-6mΩ is average (40-50C real), above 8mΩ indicates an aging or poor-quality pack regardless of label. As covered in our FPV battery IR testing guide, track IR over time — a rising trend means the pack is degrading.
Step 4: Match C-Rating to Flying Style
Freestyle pilots need burst current for punch-outs and split-S recoveries — prioritize high real C-rating even at a weight penalty. Racers on a tight course where throttle rarely exceeds 70% can trade C-rating for lighter weight. Long-range cruisers flying at constant 40-60% throttle need sustained current capacity, not burst — a 30-40C real rating with higher capacity (3000mAh+) often outperforms a small “high-C” pack.
LiPo C-Rating vs Build Type Reference
| Build Type | Typical Peak Current | Recommended Pack Size | Label C-Rating Needed | Real C-Rating Needed | IR Target |
|---|---|---|---|---|---|
| 65mm Whoop (1S) | 12-18A | 300-450mAh 1S | 60C+ | 30C+ | <15mΩ |
| 3-inch Toothpick | 30-45A | 450-650mAh 4S | 80C+ | 40C+ | <10mΩ |
| 5-inch Freestyle | 100-140A | 1300-1550mAh 6S | 100C+ | 50C+ | <4mΩ |
| 5-inch Racing | 80-100A | 1100-1300mAh 6S | 120C+ | 60C+ | <3mΩ |
| 7-inch Long Range | 60-80A | 2800-4000mAh 6S | 45C+ | 25C+ | <6mΩ |
| Cinewhoop (3.5-inch) | 50-70A | 850-1100mAh 6S | 80C+ | 40C+ | <8mΩ |
What Most Pilots Get Wrong About C-Ratings
Mistake 1: Buying the highest C-rating available without checking weight
The consequence: A 1550mAh “150C” pack weighs 250g versus a 1300mAh “100C” at 210g. The 40g difference shifts your CG forward and costs you more in flight time than the extra capacity provides. The fix: Weigh every battery. If two packs have identical IR and capacity, take the lighter one — the C-rating difference is likely marketing.
Mistake 2: Assuming all brands use the same C-rating scale
The consequence: Brand A’s “100C” delivers 50C real; Brand B’s “75C” delivers 45C real. You pay extra for a number that means nothing. The fix: Check independent IR test databases (RCGroups has per-brand threads) or buy one pack to test before committing to a fleet. Consistency matters more than peak numbers.
Mistake 3: Ignoring voltage sag as the real performance metric
The consequence: A pack that holds 3.7V per cell at 80% throttle delivers more power than a pack that sags to 3.4V — even if both are labeled 100C. Voltage determines motor RPM, and sag costs you thrust exactly when you need it. The fix: Monitor per-cell voltage in your OSD during punch-outs. If cell voltage drops below 3.5V at full throttle, your pack is underrated for the load — size up or accept the performance loss.
Mistake 4: Running packs at their claimed maximum continuously
The consequence: A 1300mAh “100C” pack discharged continuously at 100C (130A) heats to 60°C+ internally, permanently increasing IR. Do this 10 times and the pack degrades to 70C real, then 50C. The fix: Size your pack so peak current never exceeds 70% of the claimed continuous rating. If you need 120A, use a pack rated for at least 170A continuous — or accept that packs are consumables.
⚠️ Regulatory Notice: All LiPo battery handling should comply with 2026 transportation and storage regulations in your jurisdiction. IATA and national aviation authorities enforce limits on Watt-hour ratings for air travel. Never charge LiPo batteries unattended, and always use a fireproof charging bag. Regulations regarding battery disposal vary — check local requirements before discarding damaged or end-of-life packs.
As discussed in our FPV voltage sag troubleshooting guide, sag is the canary in the coal mine — if your OSD shows voltage collapsing under load, your C-rating math was wrong regardless of what the label says.
For pilots building their first 5-inch quad, the motor sizing decisions in our FPV motor sizing guide directly affect current draw — pair the wrong stator volume with a marginal C-rating and you will chase tuning gremlins forever.
For pilots tired of guessing C-ratings, our 1300mAh 6S 120C LiPo packs ship with factory-measured IR values on every unit — you know the real performance before you solder a connector. Available at uavmodel.com.