LiPo Battery Discharge Curves: Understanding Voltage Sag Under Load
Voltage sag is the silent performance killer that turns a punchy quad into a sluggish liability halfway through a pack. This deep dive into LiPo discharge physics explains the relationship between C-rate and internal resistance, how to measure and interpret sag, the real-world performance implications at the sticks, and which battery brands deliver the flattest discharge curves when it counts.
The Electrochemistry of Voltage Sag
Every LiPo cell has an internal resistance (IR) — the sum of ionic resistance through the electrolyte, electronic resistance through the current collectors and electrodes, and contact resistance at the interfaces. When current flows, Ohm’s law dictates that voltage drops across this internal resistance: V_drop = I × IR. A fully charged LiPo cell at 4.20V resting voltage might sag to 3.65V under a 100A load if the total pack IR is 5.5 milliohms. That lost 0.55V per cell — 3.3V across a 6S pack — represents energy dissipated as heat inside the battery rather than delivered to the motors.
The practical consequence is that your quad’s actual voltage under load is significantly lower than what your OSD displays at idle. A pilot who lands at 3.5V per cell resting voltage may have been flying at 3.0-3.2V per cell under throttle, operating dangerously close to the point where the cell’s voltage collapses and the quad falls out of the sky. Understanding where your specific battery sits on the sag curve — and how that sag changes with temperature, state of charge, and pack age — is essential knowledge for anyone who pushes their power system hard.
C-Rate Reality Check
The C-rate printed on LiPo labels has become a marketing farce. A 1300mAh pack labeled “150C” claims to deliver 195 amps continuously (1.3Ah × 150C) — a physically impossible figure for a connector rated at 60A continuous, wire gauge that would melt at half that current, and cell chemistry that can’t maintain terminal voltage under that load for more than a few seconds before permanent damage. Independent testing by the FPV community has repeatedly demonstrated that no LiPo on the market sustains more than 45-50 true C under realistic flight conditions.
The useful metric is sustained C-rate: the current at which the cell maintains above 3.5V per cell for the majority of its discharge capacity. Top-tier packs from manufacturers like CNHL (Black Series), Tattu (R-Line), and GNB (HV series) consistently deliver 35-40 true C in this definition. Budget packs often collapse to 25-30 true C, regardless of what the label claims. The gap between label C and true C has widened so far that many experienced pilots have stopped paying attention to C-ratings entirely, relying instead on community testing data and personal experience with specific pack models.
Internal Resistance: The Number That Actually Matters
Internal resistance is the single most predictive number for LiPo performance. It’s measurable with a charger that supports IR testing (most ISDT, HOTA, and ToolkitRC chargers do), and it tells you more about a pack’s health and capability than any label specification.
For a 6S 1300mAh pack, here’s what IR values mean in practice:
| Per-Cell IR (mΩ) | Pack Condition | Usable True C-Rate | Voltage Sag at 100A |
|---|---|---|---|
| 1.0-1.5 | Excellent — new premium pack | 40-50C | ~0.6-0.9V (entire pack) |
| 1.5-2.5 | Good — broken in, still strong | 30-40C | ~0.9-1.5V |
| 2.5-4.0 | Acceptable — showing age | 20-30C | ~1.5-2.4V |
| 4.0-6.0 | Marginal — significant sag, reduced flight time | 15-20C | ~2.4-3.6V |
| 6.0+ | Retire the pack — dangerous under load | <15C | 3.6V+, risk of cell collapse |
IR increases with every cycle as the solid electrolyte interphase (SEI) layer thickens and the electrode materials mechanically degrade. A pack that starts at 1.5mΩ per cell might drift to 3.0mΩ after 100 cycles and 4.5mΩ after 200 cycles. The degradation isn’t linear — it accelerates once the pack crosses roughly 4.0mΩ per cell, at which point flight performance becomes noticeably sluggish and the pack is best relegated to cruising duty or retired.
Temperature dramatically affects IR. A LiPo at 5°C has roughly 3-4 times the internal resistance of the same pack at 35°C. This is why cold-weather flying feels so anemic — the battery simply can’t deliver current efficiently until it self-heats under moderate load. Pre-warming packs to 30-35°C (using a LiPo warmer bag or simply keeping them in an inside pocket) is standard practice for pilots flying in temperatures below 15°C.
Sag Comparison Across Popular Brands
Community testing data consistently ranks the following 6S 1300-1400mAh packs by voltage sag under standardized 80A load:
- Tattu R-Line V5: 3.52V/cell at 80A after 30 seconds. The gold standard for flat discharge; IR typically 1.0-1.3mΩ per cell new. Weight penalty at ~215g reflects higher electrode density.
- CNHL Black Series: 3.48V/cell. Slightly more sag than R-Line but $15-20 cheaper per pack. IR typically 1.2-1.5mΩ. Excellent value for freestyle pilots.
- GNB 1300mAh HV: 3.45V/cell (at 4.35V charge). HV chemistry provides higher starting voltage, partially offsetting sag. IR typically 1.3-1.8mΩ. The 4.35V charge voltage requires an HV-compatible charger.
- Ovonic 1300mAh: 3.40V/cell. Budget option with respectable performance. IR typically 1.8-2.5mΩ new. Sag is noticeably worse but price is 40-50% below R-Line.
- Generic/Unbranded: 3.25-3.35V/cell. IR often 2.5-4.0mΩ even when new. The false economy — these packs degrade rapidly and the sag makes flying unenjoyable.
How to Measure Your Own Sag
The most practical method uses your OSD data. Enable voltage logging in Betaflight (or your flight controller firmware of choice) and fly a pack with aggressive throttle usage. After landing, review the blackbox log or DVR recording. Note the minimum voltage reached during sustained full-throttle punches — this is your sag floor for that pack under your specific motor and prop combination. A healthy system should show sag of no more than 0.7-0.9V per cell at maximum throttle. If you’re seeing 1.0V+ sag per cell with a pack showing under 3.0mΩ per cell IR, the problem is likely in the power delivery path: undersized XT60 connectors, poor solder joints, or excessively long battery leads.
For a bench measurement, a constant-current electronic load provides the most precise data. Set the load to your quad’s approximate hover current (typically 5-8A for a 5-inch build) and measure voltage after 30 seconds. Then increase to full-throttle-equivalent current (80-100A for a 5-inch 6S build with aggressive props) and measure again after 5 seconds. The voltage difference divided by the current difference gives your pack’s dynamic internal resistance: R_internal = (V_hover – V_full) / (I_full – I_hover).
The Discharge Curve Shape
LiPo discharge curves have a characteristic shape: a sharp initial voltage drop from the fully-charged 4.20V to roughly 3.85-3.90V within the first 10-15% of capacity, followed by a long, relatively flat plateau from 3.85V down to 3.60V (spanning roughly 70% of the total capacity), then a steep dive from 3.60V to the cutoff around 3.30-3.40V in the final 10-15%.
The flatness of the plateau region distinguishes good packs from great ones. A premium pack holds voltage within a 0.05V band across the entire 15-85% state-of-charge range. Budget packs exhibit a continuous downward slope, losing 0.1-0.15V across the same range. This slope means the quad progressively loses power throughout the flight rather than maintaining consistent performance until the pack is nearly depleted.
For pilots who fly aggressively, the landing decision should be driven by voltage under load, not resting voltage. A pack that bounces back to 3.70V after landing may have been sagging to 3.30V during the final punch-out — dangerously low. Setting your OSD to display lowest cell voltage rather than average, and landing when that lowest cell dips below 3.3V under moderate throttle, will dramatically extend the lifespan of your packs.
Impact on Flight Performance
Voltage sag manifests at the sticks as a loss of top-end RPM. Motor speed is directly proportional to voltage: at 25.2V (fully charged 6S), a 1900KV motor spins at roughly 47,880 RPM unloaded. At 21.0V (3.5V per cell under load), the same motor can only reach 39,900 RPM — a 17% reduction. Thrust scales roughly with the square of RPM, so that 17% RPM loss translates to approximately 31% less thrust at the top end. This is why a quad feels dramatically punchier in the first 30 seconds of a fresh pack compared to the last minute before landing.
Beyond raw thrust, sag affects PID controller performance. Betaflight’s dynamic idle and thrust linearization features are designed to compensate for voltage-dependent thrust curves, but they can only do so much. As voltage sags, the PID loop must output higher motor commands to achieve the same angular rates, consuming headroom that isn’t available at the bottom of the pack. A tune that feels locked-in on a fresh pack can feel loose and overshoot-prone as voltage drops, particularly on yaw where the available torque margin is smallest.
Understanding your battery’s discharge curve isn’t just academic — it’s the difference between flying confidently to the last 10% of a pack and limping home early because you don’t trust what your OSD is telling you.