LiPo C-Rating & Voltage Sag Checker
Verifies whether a LiPo pack can actually sustain your drone's continuous and burst current draw. Computes voltage sag from per-cell internal resistance and warns when manufacturer C-ratings are overstated.
Cheap LiPo packs lie about their C-rating. A 100C label on a pack that physically can't deliver 50C continuous is the most common reason a build that should fly for five minutes folds at three with the voltage on the floor. This checker takes the pack spec and your motors' actual current draw and tells you (a) whether the pack survives hover, (b) whether it survives full-throttle punch-outs, and (c) how far the voltage will sag at burst — the number that decides whether the FC's low-voltage cut-off saves you or strands you.
curl -X POST https://toolsamurai.com/api/v1/drones-uav/lipo-c-rating-checker \ -H "Authorization: Bearer sk_live_•••••••••••••••" \ -H "Content-Type: application/json" \ -d '{ "pack_capacity_mah": 1500, "pack_cells_s": "4", "pack_c_continuous": 95, "pack_c_burst": 190, "num_motors": "4", "motor_hover_current_a": 2.1, "motor_max_current_a": 26, "cell_ir_mohm": 4 }'
The method behind the numbers
Continuous current capability is pack capacity (in amps) times the continuous C-rating: I_max = (mAh ÷ 1000) × C. Burst capability uses the burst C-rating the same way. Total hover and burst current sum the per-motor draw across all motors (the FC and VTX draw is a rounding error at quad scale).
Voltage sag under load is governed by the pack's series internal resistance. For S cells in series each with resistance R_cell, total IR is S × R_cell. Sag in volts is then Ohm's law: V_sag = I_burst × (S × R_cell) ÷ 1000 (because R is in milliohms). The per-cell voltage at burst is the nominal cell voltage (3.7 V for LiPo) minus the per-cell share of the sag. Below 3.2 V/cell under load most FC low-voltage alarms scream; below 3.0 V/cell sustained, the cells take permanent capacity damage.
The minimum recommended C-rating is the rating needed to cover burst draw with 30% headroom, accounting for the well-documented fact that label C-ratings are optimistic. Buying to this number leaves you a margin for cold packs, ageing IR, and the difference between marketing C and laboratory C.
See it in practice
Four 2207 motors drawing 2.1 A hover / 26 A burst on 4S, healthy 4 mΩ/cell IR.
- pack_capacity_mah
- 1500
- pack_cells_s
- 4
- pack_c_continuous
- 95
- pack_c_burst
- 190
- num_motors
- 4
- motor_hover_current_a
- 2.1
- motor_max_current_a
- 26
- cell_ir_mohm
- 4
Larger, lower-C pack sized for endurance — verify it actually survives a burst.
- pack_capacity_mah
- 2200
- pack_cells_s
- 6
- pack_c_continuous
- 35
- pack_c_burst
- 70
- num_motors
- 4
- motor_hover_current_a
- 2.3
- motor_max_current_a
- 22
- cell_ir_mohm
- 6
Frequently asked questions
How accurate are printed C-ratings?
Generally optimistic. Independent testing (RCGroups, FPV community lab tests) consistently shows printed C-ratings beat real sustained delivery by 30–60% on race packs. Use the printed number as a relative comparison between packs from the same brand, not as an absolute spec. The minimum C output here already builds in headroom for this gap.
What's a healthy internal resistance?
Per cell, on a quality fresh race LiPo: 2–4 mΩ. After 30–50 cycles or in cold weather: 5–8 mΩ. Above 10 mΩ per cell, the pack is end-of-life — sag will be severe and capacity is fading fast. A good IR-capable charger (iSDT, ToolkitRC, ISDT) reads this directly.
Why does voltage sag matter so much?
ESCs and motors are voltage-driven — less voltage means less RPM and less thrust at the same throttle. Worse, the FC's low-voltage cut-off triggers on sagged voltage during a punch-out, not the unloaded resting voltage. A pack that reads 22 V at rest may sag to 18 V at burst and trigger an auto-land mid-trick.
Where do I get motor current numbers?
Every reputable motor maker (T-Motor, EMAX, BrotherHobby, iFlight) publishes a thrust table showing thrust, current and power across throttle settings for each prop + voltage combo. Use the hover row (≈ AUW ÷ motors thrust value) for hover current and the 100% row for max.
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